erlang(3)

MODULE
erlang - The Erlang BIFs

DESCRIPTION

By convention, most built-in functions (BIFs) are seen as being in the module erlang. A number of the BIFs are viewed more or less as part of the Erlang programming language and are auto-imported. Thus, it is not necessary to specify the module name and both the calls atom_to_list(Erlang) and erlang:atom_to_list(Erlang) are identical.

In the text, auto-imported BIFs are listed without module prefix. BIFs listed with module prefix are not auto-imported.

BIFs may fail for a variety of reasons. All BIFs fail with reason badarg if they are called with arguments of an incorrect type. The other reasons that may make BIFs fail are described in connection with the description of each individual BIF.

Some BIFs may be used in guard tests, these are marked with "Allowed in guard tests".

DATA TYPES
ext_binary()
  a binary data object,
  structured according to the Erlang external term format
iodata() = iolist() | binary()
iolist() = [char() | binary() | iolist()]
  a binary is allowed as the tail of the list

EXPORTS

abs(Number) -> int() | float()

    Types
        Number = number()

    Returns an integer or float which is the arithmetical absolute value of Number.
    > abs(-3.33).
    3.33
    > abs(-3).
    3

    Allowed in guard tests.

adler32(Data) -> int()

    Types
        Data = iodata()

    Computes and returns the adler32 checksum for Data.

adler32(OldAdler, Data) -> int()

    Types
        OldAdler = int()
        Data = iodata()

    Continue computing the adler32 checksum by combining the previous checksum, OldAdler, with the checksum of Data.

    The following code:
            X = adler32(Data1),
            Y = adler32(X,Data2).

    - would assign the same value to Y as this would:
            Y = adler32([Data1,Data2]).

adler32_combine(FirstAdler, SecondAdler, SecondSize) -> int()

    Types
        FirstAdler = SecondAdler = int()
        SecondSize = int()

    Combines two previously computed adler32 checksums. This computation requires the size of the data object for the second checksum to be known.

    The following code:
            Y = adler32(Data1),
            Z = adler32(Y,Data2).

    - would assign the same value to Z as this would:
            X = adler32(Data1),
            Y = adler32(Data2),
            Z = adler32_combine(X,Y,iolist_size(Data2)).

erlang:append_element(Tuple1, Term) -> Tuple2

    Types
        Tuple1 = Tuple2 = tuple()
        Term = term()

    Returns a new tuple which has one element more than Tuple1, and contains the elements in Tuple1 followed by Term as the last element. Semantically equivalent to list_to_tuple(tuple_to_list(Tuple ++ [Term]), but much faster.
    > erlang:append_element({one, two}, three).
    {one,two,three}

apply(Fun, Args) -> term() | empty()

    Types
        Fun = fun()
        Args = [term()]

    Call a fun, passing the elements in Args as arguments.

    Note: If the number of elements in the arguments are known at compile-time, the call is better written as Fun(Arg1, Arg2, ... ArgN).

  Warning:

Earlier, Fun could also be given as {Module, Function}, equivalent to apply(Module, Function, Args). This usage is deprecated and will stop working in a future release of Erlang/OTP.

apply(Module, Function, Args) -> term() | empty()

    Types
        Module = Function = atom()
        Args = [term()]

    Returns the result of applying Function in Module to Args. The applied function must be exported from Module. The arity of the function is the length of Args.
    > apply(lists, reverse, [[a, b, c]]).
    [c,b,a]

    apply can be used to evaluate BIFs by using the module name erlang.
    > apply(erlang, atom_to_list, ['Erlang']).
    "Erlang"

    Note: If the number of arguments are known at compile-time, the call is better written as Module:Function(Arg1, Arg2, ..., ArgN).

    Failure: error_handler:undefined_function/3 is called if the applied function is not exported. The error handler can be redefined (see process_flag/2). If the error_handler is undefined, or if the user has redefined the default error_handler so the replacement module is undefined, an error with the reason undef is generated.

atom_to_list(Atom) -> string()

    Types
        Atom = atom()

    Returns a string which corresponds to the text representation of Atom.
    > atom_to_list('Erlang').
    "Erlang"

binary_to_list(Binary) -> [char()]

    Types
        Binary = binary()

    Returns a list of integers which correspond to the bytes of Binary.

binary_to_list(Binary, Start, Stop) -> [char()]

    Types
        Binary = binary()
        Start = Stop = 1..byte_size(Binary)

    As binary_to_list/1, but returns a list of integers corresponding to the bytes from position Start to position Stop in Binary. Positions in the binary are numbered starting from 1.

bitstring_to_list(Bitstring) -> [char()|bitstring()]

    Types
        Bitstring = bitstring()

    Returns a list of integers which correspond to the bytes of Bitstring. If the number of bits in the binary is not divisible by 8, the last element of the list will be a bitstring containing the remaining bits (1 up to 7 bits).

binary_to_term(Binary) -> term()

    Types
        Binary = ext_binary()

    Returns an Erlang term which is the result of decoding the binary object Binary, which must be encoded according to the Erlang external term format. See also term_to_binary/1.

bit_size(Bitstring) -> int()

    Types
        Bitstring = bitstring()

    Returns an integer which is the size in bits of Bitstring.
    > bit_size(<<433:16,3:3>>).
    19
    > bit_size(<<1,2,3>>).
    24

    Allowed in guard tests.

erlang:bump_reductions(Reductions) -> void()

    Types
        Reductions = int()

    This implementation-dependent function increments the reduction counter for the calling process. In the Beam emulator, the reduction counter is normally incremented by one for each function and BIF call, and a context switch is forced when the counter reaches 1000.

  Warning:

This BIF might be removed in a future version of the Beam machine without prior warning. It is unlikely to be implemented in other Erlang implementations.

byte_size(Bitstring) -> int()

    Types
        Bitstring = bitstring()

    Returns an integer which is the number of bytes needed to contain Bitstring. (That is, if the number of bits in Bitstring is not divisible by 8, the resulting number of bytes will be rounded up.)
    > byte_size(<<433:16,3:3>>).
    3
    > byte_size(<<1,2,3>>).
    3

    Allowed in guard tests.

erlang:cancel_timer(TimerRef) -> Time | false

    Types
        TimerRef = ref()
        Time = int()

    Cancels a timer, where TimerRef was returned by either erlang:send_after/3 or erlang:start_timer/3. If the timer is there to be removed, the function returns the time in milliseconds left until the timer would have expired, otherwise false (which means that TimerRef was never a timer, that it has already been cancelled, or that it has already delivered its message).

    See also erlang:send_after/3, erlang:start_timer/3, and erlang:read_timer/1.

    Note: Cancelling a timer does not guarantee that the message has not already been delivered to the message queue.

check_process_code(Pid, Module) -> bool()

    Types
        Pid = pid()
        Module = atom()

    Returns true if the process Pid is executing old code for Module. That is, if the current call of the process executes old code for this module, or if the process has references to old code for this module, or if the process contains funs that references old code for this module. Otherwise, it returns false.
    > check_process_code(Pid, lists).
    false

    See also code(3).

concat_binary(ListOfBinaries)

    Do not use; use list_to_binary/1 instead.

crc32(Data) -> int()

    Types
        Data = iodata()

    Computes and returns the crc32 (IEEE 802.3 style) checksum for Data.

crc32(OldCrc, Data) -> int()

    Types
        OldCrc = int()
        Data = iodata()

    Continue computing the crc32 checksum by combining the previous checksum, OldCrc, with the checksum of Data.

    The following code:
            X = crc32(Data1),
            Y = crc32(X,Data2).

    - would assign the same value to Y as this would:
            Y = crc32([Data1,Data2]).

crc32_combine(FirstCrc, SecondCrc, SecondSize) -> int()

    Types
        FirstCrc = SecondCrc = int()
        SecondSize = int()

    Combines two previously computed crc32 checksums. This computation requires the size of the data object for the second checksum to be known.

    The following code:
            Y = crc32(Data1),
            Z = crc32(Y,Data2).

    - would assign the same value to Z as this would:
            X = crc32(Data1),
            Y = crc32(Data2),
            Z = crc32_combine(X,Y,iolist_size(Data2)).

date() -> {Year, Month, Day}

    Types
        Year = Month = Day = int()

    Returns the current date as {Year, Month, Day}.

    The time zone and daylight saving time correction depend on the underlying OS.
    > date().
    {1995,2,19}

decode_packet(Type,Bin,Options) -> {ok,Packet,Rest} | {more,Length} | {error,Reason}

    Types
        Bin = binary()
        Options = [Opt]
        Packet = binary() | HttpPacket
        Rest = binary()
        Length = int() | undefined
        Reason = term()
        Type, Opt -- see below
        HttpPacket = HttpRequest | HttpResponse | HttpHeader | http_eoh | HttpError
        HttpRequest = {http_request, HttpMethod, HttpUri, HttpVersion}
        HttpResponse = {http_response, HttpVersion, integer(), string()}
        HttpHeader = {http_header, int(), HttpField, Reserved=term(), Value=string()}
        HttpError = {http_error, string()}
        HttpMethod = HttpMethodAtom | string()
        HttpMethodAtom = 'OPTIONS' | 'GET' | 'HEAD' | 'POST' | 'PUT' | 'DELETE' | 'TRACE'
        HttpUri = '*' | {absoluteURI, http|https, Host=string(), Port=int()|undefined, Path=string()} | {scheme, Scheme=string(), string()} | {abs_path, string} | string()
        HttpVersion = {Major=int(), Minor=int()}
        HttpField = HttpFieldAtom | string()
        HttpFieldAtom = 'Cache-Control' | 'Connection' | 'Date' | 'Pragma' | 'Transfer-Encoding' | 'Upgrade' | 'Via' | 'Accept' | 'Accept-Charset' | 'Accept-Encoding' | 'Accept-Language' | 'Authorization' | 'From' | 'Host' | 'If-Modified-Since' | 'If-Match' | 'If-None-Match' | 'If-Range' | 'If-Unmodified-Since' | 'Max-Forwards' | 'Proxy-Authorization' | 'Range' | 'Referer' | 'User-Agent' | 'Age' | 'Location' | 'Proxy-Authenticate' | 'Public' | 'Retry-After' | 'Server' | 'Vary' | 'Warning' | 'Www-Authenticate' | 'Allow' | 'Content-Base' | 'Content-Encoding' | 'Content-Language' | 'Content-Length' | 'Content-Location' | 'Content-Md5' | 'Content-Range' | 'Content-Type' | 'Etag' | 'Expires' | 'Last-Modified' | 'Accept-Ranges' | 'Set-Cookie' | 'Set-Cookie2' | 'X-Forwarded-For' | 'Cookie' | 'Keep-Alive' | 'Proxy-Connection'

    Decodes the binary Bin according to the packet protocol specified by Type. Very simular to the packet handling done by sockets with the option {packet,Type}.

    If an entire packet is contained in Bin it is returned together with the remainder of the binary as {ok, Packet, Rest}.

    If Bin does not contain the entire packet, {more, Length} is returned. Length is either the expected total size of the packet or undefined if the expected packet size is not known. decode_packet can then be called again with more data added.

    If the packet does not conform to the protocol format {error, Reason} is returned.

    The following values of Type are valid:

        raw | 0:
            No packet handling is done. Entire binary is returned unless it is empty.

        1 | 2 | 4:
            Packets consist of a header specifying the number of bytes in the packet, followed by that number of bytes. The length of header can be one, two, or four bytes; the order of the bytes is big-endian. The header will be stripped off when the packet is returned.

        line:
            A packet is a line terminated with newline. The newline character is included in the returned packet unless the line was truncated according to the option line_length.

        asn1 | cdr | sunrm | fcgi | tpkt:
            The header is not stripped off.

            The meanings of the packet types are as follows:

                asn1 - ASN.1 BER:

                sunrm - Sun's RPC encoding:

                cdr - CORBA (GIOP 1.1):

                fcgi - Fast CGI:

                tpkt - TPKT format [RFC1006]:

        http | httph:
            The Hypertext Transfer Protocol. The packets are returned with the format according to HttpPacket described above. A packet is either a request, a response, a header or an end of header mark. Invalid lines are returned as HttpError.

            Recognized request methods and header fields are returned as atoms. Others are returned as strings.

            The protocol type http should only be used for the first line when a HttpRequest or a HttpResponse is expected. The following calls should use httph to get HttpHeader's until http_eoh is returned that marks the end of the headers and the beginning of any following message body.

    The following options are available:

        {packet_size, int()}:
            Sets the max allowed size of the packet body. If the packet header indicates that the length of the packet is longer than the max allowed length, the packet is considered invalid. Default is 0 which means no size limit.

        {line_length, int()}:
            Applies only to line oriented protocols (line, http). Lines longer than this will be truncated.
    > erlang:decode_packet(1,<<3,"abcd">>,[]).
    {ok,<<"abc">>,<<"d">>}
    > erlang:decode_packet(1,<<5,"abcd">>,[]).
    {more,6}

delete_module(Module) -> true | undefined

    Types
        Module = atom()

    Makes the current code for Module become old code, and deletes all references for this module from the export table. Returns undefined if the module does not exist, otherwise true.

  Warning:

This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

Failure: badarg if there is already an old version of Module.

erlang:demonitor(MonitorRef) -> true

    Types
        MonitorRef = ref()

    If MonitorRef is a reference which the calling process obtained by calling erlang:monitor/2, this monitoring is turned off. If the monitoring is already turned off, nothing happens.

    Once erlang:demonitor(MonitorRef) has returned it is guaranteed that no {'DOWN', MonitorRef, _, _, _} message due to the monitor will be placed in the callers message queue in the future. A {'DOWN', MonitorRef, _, _, _} message might have been placed in the callers message queue prior to the call, though. Therefore, in most cases, it is advisable to remove such a 'DOWN' message from the message queue after monitoring has been stopped. erlang:demonitor(MonitorRef, [flush]) can be used instead of erlang:demonitor(MonitorRef) if this cleanup is wanted.

  Note:

Prior to OTP release R11B (erts version 5.5) erlang:demonitor/1 behaved completely asynchronous, i.e., the monitor was active until the "demonitor signal" reached the monitored entity. This had one undesirable effect, though. You could never know when you were guaranteed not to receive a DOWN message due to the monitor.

Current behavior can be viewed as two combined operations: asynchronously send a "demonitor signal" to the monitored entity and ignore any future results of the monitor.

Failure: It is an error if MonitorRef refers to a monitoring started by another process. Not all such cases are cheap to check; if checking is cheap, the call fails with badarg (for example if MonitorRef is a remote reference).

erlang:demonitor(MonitorRef, OptionList) -> true

    Types
        MonitorRef = ref()
        OptionList = [Option]
        Option = flush

    erlang:demonitor(MonitorRef, []) is equivalent to erlang:demonitor(MonitorRef).

    Currently the following Options are valid:

        flush:
            Remove (one) {_, MonitorRef, _, _, _} message, if there is one, from the callers message queue after monitoring has been stopped.

            Calling erlang:demonitor(MonitorRef, [flush]) is equivalent to:

                erlang:demonitor(MonitorRef),
                receive
                    {_, MonitorRef, _, _, _} ->
                        true
                after 0 ->
                        true
                end

  Note:

More options may be added in the future.

Failure: badarg if OptionList is not a list, or if Option is not a valid option, or the same failure as for erlang:demonitor/1

disconnect_node(Node) -> bool() | ignored

    Types
        Node = atom()

    Forces the disconnection of a node. This will appear to the node Node as if the local node has crashed. This BIF is mainly used in the Erlang network authentication protocols. Returns true if disconnection succeeds, otherwise false. If the local node is not alive, the function returns ignored.

erlang:display(Term) -> true

    Types
        Term = term()

    Prints a text representation of Term on the standard output.

  Warning:

This BIF is intended for debugging only.

element(N, Tuple) -> term()

    Types
        N = 1..tuple_size(Tuple)
        Tuple = tuple()

    Returns the Nth element (numbering from 1) of Tuple.
    > element(2, {a, b, c}).
    b

    Allowed in guard tests.

erase() -> [{Key, Val}]

    Types
        Key = Val = term()

    Returns the process dictionary and deletes it.
    > put(key1, {1, 2, 3}),
    put(key2, [a, b, c]),
    erase().
    [{key1,{1,2,3}},{key2,[a,b,c]}]

erase(Key) -> Val | undefined

    Types
        Key = Val = term()

    Returns the value Val associated with Key and deletes it from the process dictionary. Returns undefined if no value is associated with Key.
    > put(key1, {merry, lambs, are, playing}),
    X = erase(key1),
    {X, erase(key1)}.
    {{merry,lambs,are,playing},undefined}

erlang:error(Reason)

    Types
        Reason = term()

    Stops the execution of the calling process with the reason Reason, where Reason is any term. The actual exit reason will be {Reason, Where}, where Where is a list of the functions most recently called (the current function first). Since evaluating this function causes the process to terminate, it has no return value.
    > catch erlang:error(foobar).
    {'EXIT',{foobar,[{erl_eval,do_apply,5},
                     {erl_eval,expr,5},
                     {shell,exprs,6},
                     {shell,eval_exprs,6},
                     {shell,eval_loop,3}]}}

erlang:error(Reason, Args)

    Types
        Reason = term()
        Args = [term()]

    Stops the execution of the calling process with the reason Reason, where Reason is any term. The actual exit reason will be {Reason, Where}, where Where is a list of the functions most recently called (the current function first). Args is expected to be the list of arguments for the current function; in Beam it will be used to provide the actual arguments for the current function in the Where term. Since evaluating this function causes the process to terminate, it has no return value.

exit(Reason)

    Types
        Reason = term()

    Stops the execution of the calling process with the exit reason Reason, where Reason is any term. Since evaluating this function causes the process to terminate, it has no return value.
    > exit(foobar).
    ** exception exit: foobar
    > catch exit(foobar).
    {'EXIT',foobar}

exit(Pid, Reason) -> true

    Types
        Pid = pid()
        Reason = term()

    Sends an exit signal with exit reason Reason to the process Pid.

    The following behavior apply if Reason is any term except normal or kill:

    If Pid is not trapping exits, Pid itself will exit with exit reason Reason. If Pid is trapping exits, the exit signal is transformed into a message {'EXIT', From, Reason} and delivered to the message queue of Pid. From is the pid of the process which sent the exit signal. See also process_flag/2.

    If Reason is the atom normal, Pid will not exit. If it is trapping exits, the exit signal is transformed into a message {'EXIT', From, normal} and delivered to its message queue.

    If Reason is the atom kill, that is if exit(Pid, kill) is called, an untrappable exit signal is sent to Pid which will unconditionally exit with exit reason killed.

erlang:fault(Reason)

    Types
        Reason = term()

    This function is deprecated and will be removed in the next release. Used erlang:error(Reason) instead.

erlang:fault(Reason, Args)

    Types
        Reason = term()
        Args = [term()]

    This function is deprecated and will be removed in the next release. Use erlang:error(Reason, Args) instead.

float(Number) -> float()

    Types
        Number = number()

    Returns a float by converting Number to a float.
    > float(55).
    55.0

    Allowed in guard tests.

  Note:

Note that if used on the top-level in a guard, it will test whether the argument is a floating point number; for clarity, use is_float/1 instead.

When float/1 is used in an expression in a guard, such as 'float(A) == 4.0', it converts a number as described above.

float_to_list(Float) -> string()

    Types
        Float = float()

    Returns a string which corresponds to the text representation of Float.
    > float_to_list(7.0).
    "7.00000000000000000000e+00"

erlang:fun_info(Fun) -> [{Item, Info}]

    Types
        Fun = fun()
        Item, Info -- see below

    Returns a list containing information about the fun Fun. Each element of the list is a tuple. The order of the tuples is not defined, and more tuples may be added in a future release.

  Warning:

This BIF is mainly intended for debugging, but it can occasionally be useful in library functions that might need to verify, for instance, the arity of a fun.

There are two types of funs with slightly different semantics:

A fun created by fun M:F/A is called an external fun. Calling it will always call the function F with arity A in the latest code for module M. Note that module M does not even need to be loaded when the fun fun M:F/A is created.

All other funs are called local. When a local fun is called, the same version of the code that created the fun will be called (even if newer version of the module has been loaded).

The following elements will always be present in the list for both local and external funs:

    {type, Type}:
        Type is either local or external.

    {module, Module}:
        Module (an atom) is the module name.

        If Fun is a local fun, Module is the module in which the fun is defined.

        If Fun is an external fun, Module is the module that the fun refers to.

    {name, Name}:
        Name (an atom) is a function name.

        If Fun is a local fun, Name is the name of the local function that implements the fun. (This name was generated by the compiler, and is generally only of informational use. As it is a local function, it is not possible to call it directly.) If no code is currently loaded for the fun, [] will be returned instead of an atom.

        If Fun is an external fun, Name is the name of the exported function that the fun refers to.

    {arity, Arity}:
        Arity is the number of arguments that the fun should be called with.

    {env, Env}:
        Env (a list) is the environment or free variables for the fun. (For external funs, the returned list is always empty.)

The following elements will only be present in the list if Fun is local:

    {pid, Pid}:
        Pid is the pid of the process that originally created the fun.

    {index, Index}:
        Index (an integer) is an index into the module's fun table.

    {new_index, Index}:
        Index (an integer) is an index into the module's fun table.

    {new_uniq, Uniq}:
        Uniq (a binary) is a unique value for this fun.

    {uniq, Uniq}:
        Uniq (an integer) is a unique value for this fun.

erlang:fun_info(Fun, Item) -> {Item, Info}

    Types
        Fun = fun()
        Item, Info -- see below

    Returns information about Fun as specified by Item, in the form {Item, Info}.

    For any fun, Item can be any of the atoms module, name, arity, or env.

    For a local fun, Item can also be any of the atoms index, new_index, new_uniq, uniq, and pid. For an external fun, the value of any of these items is always the atom undefined.

    See erlang:fun_info/1.

erlang:fun_to_list(Fun) -> string()

    Types
        Fun = fun()

    Returns a string which corresponds to the text representation of Fun.

erlang:function_exported(Module, Function, Arity) -> bool()

    Types
        Module = Function = atom()
        Arity = int()

    Returns true if the module Module is loaded and contains an exported function Function/Arity; otherwise false.

    Returns false for any BIF (functions implemented in C rather than in Erlang).

garbage_collect() -> true

    Forces an immediate garbage collection of the currently executing process. The function should not be used, unless it has been noticed -- or there are good reasons to suspect -- that the spontaneous garbage collection will occur too late or not at all. Improper use may seriously degrade system performance.

    Compatibility note: In versions of OTP prior to R7, the garbage collection took place at the next context switch, not immediately. To force a context switch after a call to erlang:garbage_collect(), it was sufficient to make any function call.

garbage_collect(Pid) -> bool()

    Types
        Pid = pid()

    Works like erlang:garbage_collect() but on any process. The same caveats apply. Returns false if Pid refers to a dead process; true otherwise.

get() -> [{Key, Val}]

    Types
        Key = Val = term()

    Returns the process dictionary as a list of {Key, Val} tuples.
    > put(key1, merry),
    put(key2, lambs),
    put(key3, {are, playing}),
    get().
    [{key1,merry},{key2,lambs},{key3,{are,playing}}]

get(Key) -> Val | undefined

    Types
        Key = Val = term()

    Returns the value Valassociated with Key in the process dictionary, or undefined if Key does not exist.
    > put(key1, merry),
    put(key2, lambs),
    put({any, [valid, term]}, {are, playing}),
    get({any, [valid, term]}).
    {are,playing}

erlang:get_cookie() -> Cookie | nocookie

    Types
        Cookie = atom()

    Returns the magic cookie of the local node, if the node is alive; otherwise the atom nocookie.

get_keys(Val) -> [Key]

    Types
        Val = Key = term()

    Returns a list of keys which are associated with the value Val in the process dictionary.
    > put(mary, {1, 2}),
    put(had, {1, 2}),
    put(a, {1, 2}),
    put(little, {1, 2}),
    put(dog, {1, 3}),
    put(lamb, {1, 2}),
    get_keys({1, 2}).
    [mary,had,a,little,lamb]

erlang:get_stacktrace() -> [{Module, Function, Arity | Args}]

    Types
        Module = Function = atom()
        Arity = int()
        Args = [term()]

    Get the call stack back-trace (stacktrace) of the last exception in the calling process as a list of {Module, Function, Arity} tuples. The Arity field in the first tuple may be the argument list of that function call instead of an arity integer, depending on the exception.

    If there has not been any exceptions in a process, the stacktrace is []. After a code change for the process, the stacktrace may also be reset to [].

    The stacktrace is the same data as the catch operator returns, for example:

    {'EXIT', {badarg, Stacktrace}} = catch abs(x)

    See also erlang:error/1 and erlang:error/2.

group_leader() -> GroupLeader

    Types
        GroupLeader = pid()

    Returns the pid of the group leader for the process which evaluates the function.

    Every process is a member of some process group and all groups have a group leader. All IO from the group is channeled to the group leader. When a new process is spawned, it gets the same group leader as the spawning process. Initially, at system start-up, init is both its own group leader and the group leader of all processes.

group_leader(GroupLeader, Pid) -> true

    Types
        GroupLeader = Pid = pid()

    Sets the group leader of Pid to GroupLeader. Typically, this is used when a processes started from a certain shell should have another group leader than init.

    See also group_leader/0.

halt()

    Halts the Erlang runtime system and indicates normal exit to the calling environment. Has no return value.
    > halt().
    os_prompt%

halt(Status)

    Types
        Status = int()>=0 | string()

    Status must be a non-negative integer, or a string. Halts the Erlang runtime system. Has no return value. If Status is an integer, it is returned as an exit status of Erlang to the calling environment. If Status is a string, produces an Erlang crash dump with String as slogan, and then exits with a non-zero status code.

    Note that on many platforms, only the status codes 0-255 are supported by the operating system.

erlang:hash(Term, Range) -> Hash

    Returns a hash value for Term within the range 1..Range. The allowed range is 1..2^27-1.

  Warning:

This BIF is deprecated as the hash value may differ on different architectures. Also the hash values for integer terms larger than 2^27 as well as large binaries are very poor. The BIF is retained for backward compatibility reasons (it may have been used to hash records into a file), but all new code should use one of the BIFs erlang:phash/2 or erlang:phash2/1, 2 instead.

hd(List) -> term()

    Types
        List = [term()]

    Returns the head of List, that is, the first element.
    > hd([1,2,3,4,5]).
    1

    Allowed in guard tests.

    Failure: badarg if List is the empty list [].

erlang:hibernate(Module, Function, Args)

    Types
        Module = Function = atom()
        Args = [term()]

    Puts the calling process into a wait state where its memory allocation has been reduced as much as possible, which is useful if the process does not expect to receive any messages in the near future.

    The process will be awaken when a message is sent to it, and control will resume in Module:Function with the arguments given by Args with the call stack emptied, meaning that the process will terminate when that function returns. Thus erlang:hibernate/3 will never return to its caller.

    If the process has any message in its message queue, the process will be awaken immediately in the same way as described above.

    In more technical terms, what erlang:hibernate/3 does is the following. It discards the call stack for the process. Then it garbage collects the process. After the garbage collection, all live data is in one continuous heap. The heap is then shrunken to the exact same size as the live data which it holds (even if that size is less than the minimum heap size for the process).

    If the size of the live data in the process is less than the minimum heap size, the first garbage collection occurring after the process has been awaken will ensure that the heap size is changed to a size not smaller than the minimum heap size.

    Note that emptying the call stack means that any surrounding catch is removed and has to be re-inserted after hibernation. One effect of this is that processes started using proc_lib (also indirectly, such as gen_server processes), should use proc_lib:hibernate/3 instead to ensure that the exception handler continues to work when the process wakes up.

integer_to_list(Integer) -> string()

    Types
        Integer = int()

    Returns a string which corresponds to the text representation of Integer.
    > integer_to_list(77).
    "77"

erlang:integer_to_list(Integer, Base) -> string()

    Types
        Integer = int()
        Base = 2..36

    Returns a string which corresponds to the text representation of Integer in base Base.
    > erlang:integer_to_list(1023, 16).
    "3FF"

iolist_to_binary(IoListOrBinary) -> binary()

    Types
        IoListOrBinary = iolist() | binary()

    Returns a binary which is made from the integers and binaries in IoListOrBinary.
    > Bin1 = <<1,2,3>>.
    <<1,2,3>>
    > Bin2 = <<4,5>>.
    <<4,5>>
    > Bin3 = <<6>>.
    <<6>>
    > iolist_to_binary([Bin1,1,[2,3,Bin2],4|Bin3]).
    <<1,2,3,1,2,3,4,5,4,6>>

iolist_size(Item) -> int()

    Types
        Item = iolist() | binary()

    Returns an integer which is the size in bytes of the binary that would be the result of iolist_to_binary(Item).
    > iolist_size([1,2|<<3,4>>]).
    4

is_alive() -> bool()

    Returns true if the local node is alive; that is, if the node can be part of a distributed system. Otherwise, it returns false.

is_atom(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is an atom; otherwise returns false.

    Allowed in guard tests.

is_binary(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is a binary; otherwise returns false.

    A binary always contains a complete number of bytes.

    Allowed in guard tests.

is_bitstring(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is a bitstring (including a binary); otherwise returns false.

    Allowed in guard tests.

is_boolean(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is either the atom true or the atom false (i.e. a boolean); otherwise returns false.

    Allowed in guard tests.

erlang:is_builtin(Module, Function, Arity) -> bool()

    Types
        Module = Function = atom()
        Arity = int()

    Returns true if Module:Function/Arity is a BIF implemented in C; otherwise returns false. This BIF is useful for builders of cross reference tools.

is_float(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is a floating point number; otherwise returns false.

    Allowed in guard tests.

is_function(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is a fun; otherwise returns false.

    Allowed in guard tests.

is_function(Term, Arity) -> bool()

    Types
        Term = term()
        Arity = int()

    Returns true if Term is a fun that can be applied with Arity number of arguments; otherwise returns false.

    Allowed in guard tests.

  Warning:

Currently, is_function/2 will also return true if the first argument is a tuple fun (a tuple containing two atoms). In a future release, tuple funs will no longer be supported and is_function/2 will return false if given a tuple fun.

is_integer(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is an integer; otherwise returns false.

    Allowed in guard tests.

is_list(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is a list with zero or more elements; otherwise returns false.

    Allowed in guard tests.

is_number(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is either an integer or a floating point number; otherwise returns false.

    Allowed in guard tests.

is_pid(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is a pid (process identifier); otherwise returns false.

    Allowed in guard tests.

is_port(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is a port identifier; otherwise returns false.

    Allowed in guard tests.

is_process_alive(Pid) -> bool()

    Types
        Pid = pid()

    Pid must refer to a process at the local node. Returns true if the process exists and is alive, that is, is not exiting and has not exited. Otherwise, returns false.

is_record(Term, RecordTag) -> bool()

    Types
        Term = term()
        RecordTag = atom()

    Returns true if Term is a tuple and its first element is RecordTag. Otherwise, returns false.

  Note:

Normally the compiler treats calls to is_record/2 specially. It emits code to verify that Term is a tuple, that its first element is RecordTag, and that the size is correct. However, if the RecordTag is not a literal atom, the is_record/2 BIF will be called instead and the size of the tuple will not be verified.

Allowed in guard tests, if RecordTag is a literal atom.

is_record(Term, RecordTag, Size) -> bool()

    Types
        Term = term()
        RecordTag = atom()
        Size = int()

    RecordTag must be an atom. Returns true if Term is a tuple, its first element is RecordTag, and its size is Size. Otherwise, returns false.

    Allowed in guard tests, provided that RecordTag is a literal atom and Size is a literal integer.

  Note:

This BIF is documented for completeness. In most cases is_record/2 should be used.

is_reference(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is a reference; otherwise returns false.

    Allowed in guard tests.

is_tuple(Term) -> bool()

    Types
        Term = term()

    Returns true if Term is a tuple; otherwise returns false.

    Allowed in guard tests.

length(List) -> int()

    Types
        List = [term()]

    Returns the length of List.
    > length([1,2,3,4,5,6,7,8,9]).
    9

    Allowed in guard tests.

link(Pid) -> true

    Types
        Pid = pid() | port()

    Creates a link between the calling process and another process (or port) Pid, if there is not such a link already. If a process attempts to create a link to itself, nothing is done. Returns true.

    If Pid does not exist, the behavior of the BIF depends on if the calling process is trapping exits or not (see process_flag/2):

        *   If the calling process is not trapping exits, and checking Pid is cheap -- that is, if Pid is local -- link/1 fails with reason noproc.

        *   Otherwise, if the calling process is trapping exits, and/or Pid is remote, link/1 returns true, but an exit signal with reason noproc is sent to the calling process.

list_to_atom(String) -> atom()

    Types
        String = string()

    Returns the atom whose text representation is String.
    > list_to_atom("Erlang").
    'Erlang'

list_to_binary(IoList) -> binary()

    Types
        IoList = iolist()

    Returns a binary which is made from the integers and binaries in IoList.
    > Bin1 = <<1,2,3>>.
    <<1,2,3>>
    > Bin2 = <<4,5>>.
    <<4,5>>
    > Bin3 = <<6>>.
    <<6>>
    > list_to_binary([Bin1,1,[2,3,Bin2],4|Bin3]).
    <<1,2,3,1,2,3,4,5,4,6>>

list_to_bitstring(BitstringList) -> bitstring()

    Types
        BitstringList = [BitstringList | bitstring() | char()]

    Returns a bitstring which is made from the integers and bitstrings in BitstringList. (The last tail in BitstringList is allowed to be a bitstring.)
    > Bin1 = <<1,2,3>>.
    <<1,2,3>>
    > Bin2 = <<4,5>>.
    <<4,5>>
    > Bin3 = <<6,7:4,>>.
    <<6>>
    > list_to_binary([Bin1,1,[2,3,Bin2],4|Bin3]).
    <<1,2,3,1,2,3,4,5,4,6,7:46>>

list_to_existing_atom(String) -> atom()

    Types
        String = string()

    Returns the atom whose text representation is String, but only if there already exists such atom.

    Failure: badarg if there does not already exist an atom whose text representation is String.

list_to_float(String) -> float()

    Types
        String = string()

    Returns the float whose text representation is String.
    > list_to_float("2.2017764e+0").
    2.2017764

    Failure: badarg if String contains a bad representation of a float.

list_to_integer(String) -> int()

    Types
        String = string()

    Returns an integer whose text representation is String.
    > list_to_integer("123").
    123

    Failure: badarg if String contains a bad representation of an integer.

erlang:list_to_integer(String, Base) -> int()

    Types
        String = string()
        Base = 2..36

    Returns an integer whose text representation in base Base is String.
    > erlang:list_to_integer("3FF", 16).
    1023

    Failure: badarg if String contains a bad representation of an integer.

list_to_pid(String) -> pid()

    Types
        String = string()

    Returns a pid whose text representation is String.

  Warning:

This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.
> list_to_pid("<0.4.1>").
<0.4.1>

Failure: badarg if String contains a bad representation of a pid.

list_to_tuple(List) -> tuple()

    Types
        List = [term()]

    Returns a tuple which corresponds to List. List can contain any Erlang terms.
    > list_to_tuple([share, ['Ericsson_B', 163]]).
    {share, ['Ericsson_B', 163]}

load_module(Module, Binary) -> {module, Module} | {error, Reason}

    Types
        Module = atom()
        Binary = binary()
        Reason = badfile | not_purged | badfile

    If Binary contains the object code for the module Module, this BIF loads that object code. Also, if the code for the module Module already exists, all export references are replaced so they point to the newly loaded code. The previously loaded code is kept in the system as old code, as there may still be processes which are executing that code. It returns either {module, Module}, or {error, Reason} if loading fails. Reason is one of the following:

        badfile:
            The object code in Binary has an incorrect format.

        not_purged:
            Binary contains a module which cannot be loaded because old code for this module already exists.

        badfile:
            The object code contains code for another module than Module

  Warning:

This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

erlang:loaded() -> [Module]

    Types
        Module = atom()

    Returns a list of all loaded Erlang modules (current and/or old code), including preloaded modules.

    See also code(3).

erlang:localtime() -> {Date, Time}

    Types
        Date = {Year, Month, Day}
        Time = {Hour, Minute, Second}
        Year = Month = Day = Hour = Minute = Second = int()

    Returns the current local date and time {{Year, Month, Day}, {Hour, Minute, Second}}.

    The time zone and daylight saving time correction depend on the underlying OS.
    > erlang:localtime().
    {{1996,11,6},{14,45,17}}

erlang:localtime_to_universaltime({Date1, Time1}) -> {Date2, Time2}

    Types
        Date1 = Date2 = {Year, Month, Day}
        Time1 = Time2 = {Hour, Minute, Second}
        Year = Month = Day = Hour = Minute = Second = int()

    Converts local date and time to Universal Time Coordinated (UTC), if this is supported by the underlying OS. Otherwise, no conversion is done and {Date1, Time1} is returned.
    > erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}).
    {{1996,11,6},{13,45,17}}

    Failure: badarg if Date1 or Time1 do not denote a valid date or time.

erlang:localtime_to_universaltime({Date1, Time1}, IsDst) -> {Date2, Time2}

    Types
        Date1 = Date2 = {Year, Month, Day}
        Time1 = Time2 = {Hour, Minute, Second}
        Year = Month = Day = Hour = Minute = Second = int()
        IsDst = true | false | undefined

    Converts local date and time to Universal Time Coordinated (UTC) just like erlang:localtime_to_universaltime/1, but the caller decides if daylight saving time is active or not.

    If IsDst == true the {Date1, Time1} is during daylight saving time, if IsDst == false it is not, and if IsDst == undefined the underlying OS may guess, which is the same as calling erlang:localtime_to_universaltime({Date1, Time1}).
    > erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, true).
    {{1996,11,6},{12,45,17}}
    > erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, false).
    {{1996,11,6},{13,45,17}}
    > erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, undefined).
    {{1996,11,6},{13,45,17}}

    Failure: badarg if Date1 or Time1 do not denote a valid date or time.

make_ref() -> ref()

    Returns an almost unique reference.

    The returned reference will re-occur after approximately 2^82 calls; therefore it is unique enough for practical purposes.
    > make_ref().
    #Ref<0.0.0.135>

erlang:make_tuple(Arity, InitialValue) -> tuple()

    Types
        Arity = int()
        InitialValue = term()

    Returns a new tuple of the given Arity, where all elements are InitialValue.
    > erlang:make_tuple(4, []).
    {[],[],[],[]}

erlang:md5(Data) -> Digest

    Types
        Data = iodata()
        Digest = binary()

    Computes an MD5 message digest from Data, where the length of the digest is 128 bits (16 bytes). Data is a binary or a list of small integers and binaries.

    See The MD5 Message Digest Algorithm (RFC 1321) for more information about MD5.

  Warning:

The MD5 Message Digest Algorithm is not considered safe for code-signing or software integrity purposes.

erlang:md5_final(Context) -> Digest

    Types
        Context = Digest = binary()

    Finishes the update of an MD5 Context and returns the computed MD5 message digest.

erlang:md5_init() -> Context

    Types
        Context = binary()

    Creates an MD5 context, to be used in subsequent calls to md5_update/2.

erlang:md5_update(Context, Data) -> NewContext

    Types
        Data = iodata()
        Context = NewContext = binary()

    Updates an MD5 Context with Data, and returns a NewContext.

erlang:memory() -> [{Type, Size}]

    Types
        Type, Size -- see below

    Returns a list containing information about memory dynamically allocated by the Erlang emulator. Each element of the list is a tuple {Type, Size}. The first element Typeis an atom describing memory type. The second element Sizeis memory size in bytes. A description of each memory type follows:

        total:
            The total amount of memory currently allocated, which is the same as the sum of memory size for processes and system.

        processes:
            The total amount of memory currently allocated by the Erlang processes.

        processes_used:
            The total amount of memory currently used by the Erlang processes.

            This memory is part of the memory presented as processes memory.

        system:
            The total amount of memory currently allocated by the emulator that is not directly related to any Erlang process.

            Memory presented as processes is not included in this memory.

        atom:
            The total amount of memory currently allocated for atoms.

            This memory is part of the memory presented as system memory.

        atom_used:
            The total amount of memory currently used for atoms.

            This memory is part of the memory presented as atom memory.

        binary:
            The total amount of memory currently allocated for binaries.

            This memory is part of the memory presented as system memory.

        code:
            The total amount of memory currently allocated for Erlang code.

            This memory is part of the memory presented as system memory.

        ets:
            The total amount of memory currently allocated for ets tables.

            This memory is part of the memory presented as system memory.

        maximum:
            The maximum total amount of memory allocated since the emulator was started.

            This tuple is only present when the emulator is run with instrumentation.

            For information on how to run the emulator with instrumentation see instrument(3) and/or erl(1).

  Note:

The system value is not complete. Some allocated memory that should be part of the system value are not.

When the emulator is run with instrumentation, the system value is more accurate, but memory directly allocated by malloc (and friends) are still not part of the system value. Direct calls to malloc are only done from OS specific runtime libraries and perhaps from user implemented Erlang drivers that do not use the memory allocation functions in the driver interface.

Since the total value is the sum of processes and system the error in system will propagate to the total value.

The different amounts of memory that are summed are not gathered atomically which also introduce an error in the result.

The different values has the following relation to each other. Values beginning with an uppercase letter is not part of the result.
        total = processes + system
        processes = processes_used + ProcessesNotUsed
        system = atom + binary + code + ets + OtherSystem
        atom = atom_used + AtomNotUsed
        RealTotal = processes + RealSystem
        RealSystem = system + MissedSystem

More tuples in the returned list may be added in the future.

  Note:

The total value is supposed to be the total amount of memory dynamically allocated by the emulator. Shared libraries, the code of the emulator itself, and the emulator stack(s) are not supposed to be included. That is, the total value is not supposed to be equal to the total size of all pages mapped to the emulator. Furthermore, due to fragmentation and pre-reservation of memory areas, the size of the memory segments which contain the dynamically allocated memory blocks can be substantially larger than the total size of the dynamically allocated memory blocks.

  Note:

Since erts version 5.6.4 erlang:memory/0 requires that all erts_alloc(3) allocators are enabled (default behaviour).

Failure:

    notsup:
        If an erts_alloc(3) allocator has been disabled.

erlang:memory(Type | [Type]) -> Size | [{Type, Size}]

    Types
        Type, Size -- see below

    Returns the memory size in bytes allocated for memory of type Type. The argument can also be given as a list of Type atoms, in which case a corresponding list of {Type, Size} tuples is returned.

  Note:

Since erts version 5.6.4 erlang:memory/1 requires that all erts_alloc(3) allocators are enabled (default behaviour).

Failures:

    badarg:
        If Type is not one of the memory types listed in the documentation of erlang:memory/0.

    badarg:
        If maximum is passed as Type and the emulator is not run in instrumented mode.

    notsup:
        If an erts_alloc(3) allocator has been disabled.

See also erlang:memory/0.

module_loaded(Module) -> bool()

    Types
        Module = atom()

    Returns true if the module Module is loaded, otherwise returns false. It does not attempt to load the module.

  Warning:

This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

erlang:monitor(Type, Item) -> MonitorRef

    Types
        Type = process
        Item = pid() | {RegName, Node} | RegName
        RegName = atom()
        Node = node()
        MonitorRef = reference()

    The calling process starts monitoring Item which is an object of type Type.

    Currently only processes can be monitored, i.e. the only allowed Type is process, but other types may be allowed in the future.

    Item can be:

        pid():
            The pid of the process to monitor.

        {RegName, Node}:
            A tuple consisting of a registered name of a process and a node name. The process residing on the node Node with the registered name RegName will be monitored.

        RegName:
            The process locally registered as RegName will be monitored.

  Note:

When a process is monitored by registered name, the process that has the registered name at the time when erlang:monitor/2 is called will be monitored. The monitor will not be effected, if the registered name is unregistered.

A 'DOWN' message will be sent to the monitoring process if Item dies, if Item does not exist, or if the connection is lost to the node which Item resides on. A 'DOWN' message has the following pattern:
{'DOWN', MonitorRef, Type, Object, Info}

where MonitorRef and Type are the same as described above, and:

    Object:
        A reference to the monitored object:

            *   the pid of the monitored process, if Item was specified as a pid.

            *   {RegName, Node}, if Item was specified as {RegName, Node}.

            *   {RegName, Node}, if Item was specified as RegName. Node will in this case be the name of the local node (node()).

    Info:
        Either the exit reason of the process, noproc (non-existing process), or noconnection (no connection to Node).

  Note:

If/when erlang:monitor/2 is extended (e.g. to handle other item types than process), other possible values for Object, and Info in the 'DOWN' message will be introduced.

The monitoring is turned off either when the 'DOWN' message is sent, or when erlang:demonitor/1 is called.

If an attempt is made to monitor a process on an older node (where remote process monitoring is not implemented or one where remote process monitoring by registered name is not implemented), the call fails with badarg.

Making several calls to erlang:monitor/2 for the same Item is not an error; it results in as many, completely independent, monitorings.

  Note:

The format of the 'DOWN' message changed in the 5.2 version of the emulator (OTP release R9B) for monitor by registered name. The Object element of the 'DOWN' message could in earlier versions sometimes be the pid of the monitored process and sometimes be the registered name. Now the Object element is always a tuple consisting of the registered name and the node name. Processes on new nodes (emulator version 5.2 or greater) will always get 'DOWN' messages on the new format even if they are monitoring processes on old nodes. Processes on old nodes will always get 'DOWN' messages on the old format.

monitor_node(Node, Flag) -> true

    Types
        Node = node()
        Flag = bool()

    Monitors the status of the node Node. If Flag is true, monitoring is turned on; if Flag is false, monitoring is turned off.

    Making several calls to monitor_node(Node, true) for the same Node is not an error; it results in as many, completely independent, monitorings.

    If Node fails or does not exist, the message {nodedown, Node} is delivered to the process. If a process has made two calls to monitor_node(Node, true) and Node terminates, two nodedown messages are delivered to the process. If there is no connection to Node, there will be an attempt to create one. If this fails, a nodedown message is delivered.

    Nodes connected through hidden connections can be monitored as any other node.

    Failure: badargif the local node is not alive.

erlang:monitor_node(Node, Flag, Options) -> true

    Types
        Node = node()
        Flag = bool()
        Options = [Option]
        Option = allow_passive_connect

    Behaves as monitor_node/2 except that it allows an extra option to be given, namely allow_passive_connect. The option allows the BIF to wait the normal net connection timeout for the monitored node to connect itself, even if it cannot be actively connected from this node (i.e. it is blocked). The state where this might be useful can only be achieved by using the kernel option dist_auto_connect once. If that kernel option is not used, the allow_passive_connect option has no effect.

  Note:

The allow_passive_connect option is used internally and is seldom needed in applications where the network topology and the kernel options in effect is known in advance.

Failure: badarg if the local node is not alive or the option list is malformed.

node() -> Node

    Types
        Node = node()

    Returns the name of the local node. If the node is not alive, nonode@nohost is returned instead.

    Allowed in guard tests.

node(Arg) -> Node

    Types
        Arg = pid() | port() | ref()
        Node = node()

    Returns the node where Arg is located. Arg can be a pid, a reference, or a port. If the local node is not alive, nonode@nohost is returned.

    Allowed in guard tests.

nodes() -> Nodes

    Types
        Nodes = [node()]

    Returns a list of all visible nodes in the system, excluding the local node. Same as nodes(visible).

nodes(Arg | [Arg]) -> Nodes

    Types
        Arg = visible | hidden | connected | this | known
        Nodes = [node()]

    Returns a list of nodes according to argument given. The result returned when the argument is a list, is the list of nodes satisfying the disjunction(s) of the list elements.

    Arg can be any of the following:

        visible:
            Nodes connected to this node through normal connections.

        hidden:
            Nodes connected to this node through hidden connections.

        connected:
            All nodes connected to this node.

        this:
            This node.

        known:
            Nodes which are known to this node, i.e., connected, previously connected, etc.

    Some equalities: [node()] = nodes(this), nodes(connected) = nodes([visible, hidden]), and nodes() = nodes(visible).

    If the local node is not alive, nodes(this) == nodes(known) == [nonode@nohost], for any other Arg the empty list [] is returned.

now() -> {MegaSecs, Secs, MicroSecs}

    Types
        MegaSecs = Secs = MicroSecs = int()

    Returns the tuple {MegaSecs, Secs, MicroSecs} which is the elapsed time since 00:00 GMT, January 1, 1970 (zero hour) on the assumption that the underlying OS supports this. Otherwise, some other point in time is chosen. It is also guaranteed that subsequent calls to this BIF returns continuously increasing values. Hence, the return value from now() can be used to generate unique time-stamps. It can only be used to check the local time of day if the time-zone info of the underlying operating system is properly configured.

open_port(PortName, PortSettings) -> port()

    Types
        PortName = {spawn, Command} | {fd, In, Out}
        Command = string()
        In = Out = int()
        PortSettings = [Opt]
        Opt = {packet, N} | stream | {line, L} | {cd, Dir} | {env, Env} | exit_status | use_stdio | nouse_stdio | stderr_to_stdout | in | out | binary | eof
        N = 1 | 2 | 4
        L = int()
        Dir = string()
        Env = [{Name, Val}]
        Name = string()
        Val = string() | false

    Returns a port identifier as the result of opening a new Erlang port. A port can be seen as an external Erlang process. PortName is one of the following:

        {spawn, Command}:
            Starts an external program. Command is the name of the external program which will be run. Command runs outside the Erlang work space unless an Erlang driver with the name Command is found. If found, that driver will be started. A driver runs in the Erlang workspace, which means that it is linked with the Erlang runtime system.

            When starting external programs on Solaris, the system call vfork is used in preference to fork for performance reasons, although it has a history of being less robust. If there are problems with using vfork, setting the environment variable ERL_NO_VFORK to any value will cause fork to be used instead.

        {fd, In, Out}:
            Allows an Erlang process to access any currently opened file descriptors used by Erlang. The file descriptor In can be used for standard input, and the file descriptor Out for standard output. It is only used for various servers in the Erlang operating system (shell and user). Hence, its use is very limited.

    PortSettings is a list of settings for the port. Valid settings are:

        {packet, N}:
            Messages are preceded by their length, sent in N bytes, with the most significant byte first. Valid values for N are 1, 2, or 4.

        stream:
            Output messages are sent without packet lengths. A user-defined protocol must be used between the Erlang process and the external object.

        {line, L}:
            Messages are delivered on a per line basis. Each line (delimited by the OS-dependent newline sequence) is delivered in one single message. The message data format is {Flag, Line}, where Flag is either eol or noeol and Line is the actual data delivered (without the newline sequence).

            L specifies the maximum line length in bytes. Lines longer than this will be delivered in more than one message, with the Flag set to noeol for all but the last message. If end of file is encountered anywhere else than immediately following a newline sequence, the last line will also be delivered with the Flag set to noeol. In all other cases, lines are delivered with Flag set to eol.

            The {packet, N} and {line, L} settings are mutually exclusive.

        {cd, Dir}:
            This is only valid for {spawn, Command}. The external program starts using Dir as its working directory. Dir must be a string. Not available on VxWorks.

        {env, Env}:
            This is only valid for {spawn, Command}. The environment of the started process is extended using the environment specifications in Env.

            Env should be a list of tuples {Name, Val}, where Name is the name of an environment variable, and Val is the value it is to have in the spawned port process. Both Name and Val must be strings. The one exception is Val being the atom false (in analogy with os:getenv/1), which removes the environment variable. Not available on VxWorks.

        exit_status:
            This is only valid for {spawn, Command} where Command refers to an external program.

            When the external process connected to the port exits, a message of the form {Port, {exit_status, Status}} is sent to the connected process, where Status is the exit status of the external process. If the program aborts, on Unix the same convention is used as the shells do (i.e., 128+signal).

            If the eof option has been given as well, the eof message and the exit_status message appear in an unspecified order.

            If the port program closes its stdout without exiting, the exit_status option will not work.

        use_stdio:
            This is only valid for {spawn, Command}. It allows the standard input and output (file descriptors 0 and 1) of the spawned (UNIX) process for communication with Erlang.

        nouse_stdio:
            The opposite of use_stdio. Uses file descriptors 3 and 4 for communication with Erlang.

        stderr_to_stdout:
            Affects ports to external programs. The executed program gets its standard error file redirected to its standard output file. stderr_to_stdout and nouse_stdio are mutually exclusive.

        in: The port can only be used for input.

        out:
            The port can only be used for output.

        binary:
            All IO from the port are binary data objects as opposed to lists of bytes.

        eof:
            The port will not be closed at the end of the file and produce an exit signal. Instead, it will remain open and a {Port, eof} message will be sent to the process holding the port.

        hide:
            When running on Windows, suppress creation of a new console window when spawning the port program. (This option has no effect on other platforms.)

    The default is stream for all types of port and use_stdio for spawned ports.

    Failure: If the port cannot be opened, the exit reason is badarg, system_limit, or the Posix error code which most closely describes the error, or einval if no Posix code is appropriate:

        badarg:
            Bad input arguments to open_port.

        system_limit:
            All available ports in the Erlang emulator are in use.

        enomem:
            There was not enough memory to create the port.

        eagain:
            There are no more available operating system processes.

        enametoolong:
            The external command given was too long.

        emfile:
            There are no more available file descriptors (for the operating system process that the Erlang emulator runs in).

        enfile:
            The file table is full (for the entire operating system).

    During use of a port opened using {spawn, Name}, errors arising when sending messages to it are reported to the owning process using signals of the form {'EXIT', Port, PosixCode}. See file(3) for possible values of PosixCode.

     The maximum number of ports that can be open at the same time is 1024 by default, but can be configured by the environment variable ERL_MAX_PORTS.

erlang:phash(Term, Range) -> Hash

    Types
        Term = term()
        Range = 1..2^32
        Hash = 1..Range

    Portable hash function that will give the same hash for the same Erlang term regardless of machine architecture and ERTS version (the BIF was introduced in ERTS 4.9.1.1). Range can be between 1 and 2^32, the function returns a hash value for Term within the range 1..Range.

    This BIF could be used instead of the old deprecated erlang:hash/2 BIF, as it calculates better hashes for all data-types, but consider using phash2/1, 2 instead.

erlang:phash2(Term [, Range]) -> Hash

    Types
        Term = term()
        Range = 1..2^32
        Hash = 0..Range-1

    Portable hash function that will give the same hash for the same Erlang term regardless of machine architecture and ERTS version (the BIF was introduced in ERTS 5.2). Range can be between 1 and 2^32, the function returns a hash value for Term within the range 0..Range-1. When called without the Range argument, a value in the range 0..2^27-1 is returned.

    This BIF should always be used for hashing terms. It distributes small integers better than phash/2, and it is faster for bignums and binaries.

    Note that the range 0..Range-1 is different from the range of phash/2 (1..Range).

pid_to_list(Pid) -> string()

    Types
        Pid = pid()

    Returns a string which corresponds to the text representation of Pid.

  Warning:

This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

port_close(Port) -> true

    Types
        Port = port() | atom()

    Closes an open port. Roughly the same as Port ! {self(), close} except for the error behaviour (see below), and that the port does not reply with {Port, closed}. Any process may close a port with port_close/1, not only the port owner (the connected process).

    For comparison: Port ! {self(), close} fails with badarg if Port cannot be sent to (i.e., Port refers neither to a port nor to a process). If Port is a closed port nothing happens. If Port is an open port and the calling process is the port owner, the port replies with {Port, closed} when all buffers have been flushed and the port really closes, but if the calling process is not the port owner the port owner fails with badsig.

    Note that any process can close a port using Port ! {PortOwner, close} just as if it itself was the port owner, but the reply always goes to the port owner.

    In short: port_close(Port) has a cleaner and more logical behaviour than Port ! {self(), close}.

    Failure: badarg if Port is not an open port or the registered name of an open port.

port_command(Port, Data) -> true

    Types
        Port = port() | atom()
        Data = iodata()

    Sends data to a port. Same as Port ! {self(), {command, Data}} except for the error behaviour (see below). Any process may send data to a port with port_command/2, not only the port owner (the connected process).

    For comparison: Port ! {self(), {command, Data}} fails with badarg if Port cannot be sent to (i.e., Port refers neither to a port nor to a process). If Port is a closed port the data message disappears without a sound. If Port is open and the calling process is not the port owner, the port owner fails with badsig. The port owner fails with badsig also if Data is not a valid IO list.

    Note that any process can send to a port using Port ! {PortOwner, {command, Data}} just as if it itself was the port owner.

    In short: port_command(Port, Data) has a cleaner and more logical behaviour than Port ! {self(), {command, Data}}.

    Failure: badarg if Port is not an open port or the registered name of an open port.

port_connect(Port, Pid) -> true

    Types
        Port = port() | atom()
        Pid = pid()

    Sets the port owner (the connected port) to Pid. Roughly the same as Port ! {self(), {connect, Pid}} except for the following:

        *   The error behavior differs, see below.

        *   The port does not reply with {Port, connected}.

        *   The new port owner gets linked to the port.

    The old port owner stays linked to the port and have to call unlink(Port) if this is not desired. Any process may set the port owner to be any process with port_connect/2.

    For comparison: Port ! {self(), {connect, Pid}} fails with badarg if Port cannot be sent to (i.e., Port refers neither to a port nor to a process). If Port is a closed port nothing happens. If Port is an open port and the calling process is the port owner, the port replies with {Port, connected} to the old port owner. Note that the old port owner is still linked to the port, and that the new is not. If Port is an open port and the calling process is not the port owner, the port owner fails with badsig. The port owner fails with badsig also if Pid is not an existing local pid.

    Note that any process can set the port owner using Port ! {PortOwner, {connect, Pid}} just as if it itself was the port owner, but the reply always goes to the port owner.

    In short: port_connect(Port, Pid) has a cleaner and more logical behaviour than Port ! {self(), {connect, Pid}}.

    Failure: badarg if Port is not an open port or the registered name of an open port, or if Pid is not an existing local pid.

port_control(Port, Operation, Data) -> Res

    Types
        Port = port() | atom()
        Operation = int()
        Data = Res = iodata()

    Performs a synchronous control operation on a port. The meaning of Operation and Data depends on the port, i.e., on the port driver. Not all port drivers support this control feature.

    Returns: a list of integers in the range 0 through 255, or a binary, depending on the port driver. The meaning of the returned data also depends on the port driver.

    Failure: badarg if Port is not an open port or the registered name of an open port, if Operation cannot fit in a 32-bit integer, if the port driver does not support synchronous control operations, or if the port driver so decides for any reason (probably something wrong with Operation or Data).

erlang:port_call(Port, Operation, Data) -> term()

    Types
        Port = port() | atom()
        Operation = int()
        Data = term()

    Performs a synchronous call to a port. The meaning of Operation and Data depends on the port, i.e., on the port driver. Not all port drivers support this feature.

    Port is a port identifier, referring to a driver.

    Operation is an integer, which is passed on to the driver.

    Data is any Erlang term. This data is converted to binary term format and sent to the port.

    Returns: a term from the driver. The meaning of the returned data also depends on the port driver.

    Failure: badarg if Port is not an open port or the registered name of an open port, if Operation cannot fit in a 32-bit integer, if the port driver does not support synchronous control operations, or if the port driver so decides for any reason (probably something wrong with Operation or Data).

erlang:port_info(Port) -> [{Item, Info}] | undefined

    Types
        Port = port() | atom()
        Item, Info -- see below

    Returns a list containing tuples with information about the Port, or undefined if the port is not open. The order of the tuples is not defined, nor are all the tuples mandatory.

        {registered_name, RegName}:
            RegName (an atom) is the registered name of the port. If the port has no registered name, this tuple is not present in the list.

        {id, Index}:
            Index (an integer) is the internal index of the port. This index may be used to separate ports.

        {connected, Pid}:
            Pid is the process connected to the port.

        {links, Pids}:
            Pids is a list of pids to which processes the port is linked.

        {name, String}:
            String is the command name set by open_port.

        {input, Bytes}:
            Bytes is the total number of bytes read from the port.

        {output, Bytes}:
            Bytes is the total number of bytes written to the port.

    Failure: badarg if Port is not a local port.

erlang:port_info(Port, Item) -> {Item, Info} | undefined | []

    Types
        Port = port() | atom()
        Item, Info -- see below

    Returns information about Port as specified by Item, or undefined if the port is not open. Also, if Item == registered_name and the port has no registered name, [] is returned.

    For valid values of Item, and corresponding values of Info, see erlang:port_info/1.

    Failure: badarg if Port is not a local port.

erlang:port_to_list(Port) -> string()

    Types
        Port = port()

    Returns a string which corresponds to the text representation of the port identifier Port.

  Warning:

This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

erlang:ports() -> [port()]

    Returns a list of all ports on the local node.

pre_loaded() -> [Module]

    Types
        Module = atom()

    Returns a list of Erlang modules which are pre-loaded in the system. As all loading of code is done through the file system, the file system must have been loaded previously. Hence, at least the module init must be pre-loaded.

erlang:process_display(Pid, Type) -> void()

    Types
        Pid = pid()
        Type = backtrace

    Writes information about the local process Pid on standard error. The currently allowed value for the atom Type is backtrace, which shows the contents of the call stack, including information about the call chain, with the current function printed first. The format of the output is not further defined.

process_flag(Flag, Value) -> OldValue

    Types
        Flag, Value, OldValue -- see below

    Sets certain flags for the process which calls this function. Returns the old value of the flag.

        process_flag(trap_exit, Boolean):
            When trap_exit is set to true, exit signals arriving to a process are converted to {'EXIT', From, Reason} messages, which can be received as ordinary messages. If trap_exit is set to false, the process exits if it receives an exit signal other than normal and the exit signal is propagated to its linked processes. Application processes should normally not trap exits.

            See also exit/2.

        process_flag(error_handler, Module):
            This is used by a process to redefine the error handler for undefined function calls and undefined registered processes. Inexperienced users should not use this flag since code auto-loading is dependent on the correct operation of the error handling module.

        process_flag(min_heap_size, MinHeapSize):
            This changes the minimum heap size for the calling process.

        process_flag(priority, Level):

            This sets the process priority. Level is an atom. There are currently four priority levels: low, normal, high, and max. The default priority level is normal. NOTE: The max priority level is reserved for internal use in the Erlang runtime system, and should not be used by others.

            Internally in each priority level processes are scheduled in a round robin fashion.

            Execution of processes on priority normal and priority low will be interleaved. Processes on priority low will be selected for execution less frequently than processes on priority normal.

            When there are runnable processes on priority high no processes on priority low, or normal will be selected for execution. Note, however, that this does not mean that no processes on priority low, or normal will be able to run when there are processes on priority high running. On the runtime system with SMP support there might be more processes running in parallel than processes on priority high, i.e., a low, and a high priority process might execute at the same time.

            When there are runnable processes on priority max no processes on priority low, normal, or high will be selected for execution. As with the high priority, processes on lower priorities might execute in parallel with processes on priority max.

            Scheduling is preemptive. Regardless of priority, a process is preempted when it has consumed more than a certain amount of reductions since the last time it was selected for execution.

            NOTE: You should not depend on the scheduling to remain exactly as it is today. Scheduling, at least on the runtime system with SMP support, is very likely to be modified in the future in order to better utilize available processor cores.

            There is currently no automatic mechanism for avoiding priority inversion, such as priority inheritance, or priority ceilings. When using priorities you have to take this into account and handle such scenarios by yourself.

            Making calls from a high priority process into code that you don't have control over may cause the high priority process to wait for a processes with lower priority, i.e., effectively decreasing the priority of the high priority process during the call. Even if this isn't the case with one version of the code that you don't have under your control, it might be the case in a future version of it. This might, for example, happen if a high priority process triggers code loading, since the code server runs on priority normal.

            Other priorities than normal are normally not needed. When other priorities are used, they need to be used with care, especially the high priority must be used with care. A process on high priority should only perform work for short periods of time. Busy looping for long periods of time in a high priority process will most likely cause problems, since there are important servers in OTP running on priority normal.

        process_flag(save_calls, N):
            When there are runnable processes on priority max no processes on priority low, normal, or high will be selected for execution. As with the high priority, processes on lower priorities might execute in parallel with processes on priority max.

            N must be an integer in the interval 0..10000. If N > 0, call saving is made active for the process, which means that information about the N most recent global function calls, BIF calls, sends and receives made by the process are saved in a list, which can be retrieved with process_info(Pid, last_calls). A global function call is one in which the module of the function is explicitly mentioned. Only a fixed amount of information is saved: a tuple {Module, Function, Arity} for function calls, and the mere atoms send, 'receive' and timeout for sends and receives ('receive' when a message is received and timeout when a receive times out). If N = 0, call saving is disabled for the process, which is the default. Whenever the size of the call saving list is set, its contents are reset.

        process_flag(sensitive, Boolean):
            Set or clear the sensitive flag for the current process. When a process has been marked as sensitive by calling process_flag(sensitive, true), features in the run-time system that can be used for examining the data and/or inner working of the process are silently disabled.

            Features that are disabled include (but are not limited to) the following:

            Tracing: Trace flags can still be set for the process, but no trace messages of any kind will be generated. (If the sensitive flag is turned off, trace messages will again be generated if there are any trace flags set.)

            Sequential tracing: The sequential trace token will be propagated as usual, but no sequential trace messages will be generated.

            process_info/1, 2 cannot be used to read out the message queue or the process dictionary (both will be returned as empty lists).

            Stack back-traces cannot be displayed for the process.

            In crash dumps, the stack, messages, and the process dictionary will be omitted.

            If {save_calls, N} has been set for the process, no function calls will be saved to the call saving list. (The call saving list will not be cleared; furthermore, send, receive, and timeout events will still be added to the list.)

process_flag(Pid, Flag, Value) -> OldValue

    Types
        Pid = pid()
        Flag, Value, OldValue -- see below

    Sets certain flags for the process Pid, in the same manner as process_flag/2. Returns the old value of the flag. The allowed values for Flag are only a subset of those allowed in process_flag/2, namely: save_calls.

    Failure: badarg if Pid is not a local process.

process_info(Pid) -> InfoResult

    Types
        Pid = pid()
        Item = atom()
        Info = term()
        InfoTuple = {Item, Info}
        InfoTupleList = [InfoTuple]
        InfoResult = InfoTupleList | undefined

    Returns a list containing InfoTuples with miscellaneous information about the process identified by Pid, or undefined if the process is not alive.

    The order of the InfoTuples is not defined, nor are all the InfoTuples mandatory. The InfoTuples part of the result may be changed without prior notice. Currently InfoTuples with the following Items are part of the result: current_function, initial_call, status, message_queue_len, messages, links, dictionary, trap_exit, error_handler, priority, group_leader, total_heap_size, heap_size, stack_size, reductions, and garbage_collection. If the process identified by Pid has a registered name also an InfoTuple with Item == registered_name will appear.

    See process_info/2 for information about specific InfoTuples.

  Warning:

This BIF is intended for debugging only, use process_info/2 for all other purposes.

Failure: badarg if Pid is not a local process.

process_info(Pid, ItemSpec) -> InfoResult

    Types
        Pid = pid()
        Item = atom()
        Info = term()
        ItemList = [Item]
        ItemSpec = Item | ItemList
        InfoTuple = {Item, Info}
        InfoTupleList = [InfoTuple]
        InfoResult = InfoTuple | InfoTupleList | undefined | []

    Returns information about the process identified by Pid as specified by the ItemSpec, or undefined if the process is not alive.

    If the process is alive and ItemSpec is a single Item, the returned value is the corresponding InfoTuple unless ItemSpec == registered_name and the process has no registered name. In this case [] is returned. This strange behavior is due to historical reasons, and is kept for backward compatibility.

    If ItemSpec is an ItemList, the result is an InfoTupleList. The InfoTuples in the InfoTupleList will appear with the corresponding Items in the same order as the Items appeared in the ItemList. Valid Items may appear multiple times in the ItemList.

  Note:

If registered_name is part of an ItemList and the process has no name registered a {registered_name, []} InfoTuple will appear in the resulting InfoTupleList. This behavior is different than when ItemSpec == registered_name, and than when process_info/1 is used.

Currently the following InfoTuples with corresponding Items are valid:

    {backtrace, Bin}:
        The binary Bin contains the same information as the output from erlang:process_display(Pid, backtrace). Use binary_to_list/1 to obtain the string of characters from the binary.

    {binary, BinInfo}:
        BinInfo is a list containing miscellaneous information about binaries currently being referred to by this process. This InfoTuple may be changed or removed without prior notice.

    {catchlevel, CatchLevel}:
        CatchLevel is the number of currently active catches in this process. This InfoTuple may be changed or removed without prior notice.

    {current_function, {Module, Function, Args}}:
        Module, Function, Args is the current function call of the process.

    {dictionary, Dictionary}:
        Dictionary is the dictionary of the process.

    {error_handler, Module}:
        Module is the error handler module used by the process (for undefined function calls, for example).

    {garbage_collection, GCInfo}:
        GCInfo is a list which contains miscellaneous information about garbage collection for this process. The content of GCInfo may be changed without prior notice.

    {group_leader, GroupLeader}:
        GroupLeader is group leader for the IO of the process.

    {heap_size, Size}:
        Size is the size in words of youngest heap generation of the process. This generation currently include the stack of the process. This information is highly implementation dependent, and may change if the implementation change.

    {initial_call, {Module, Function, Arity}}:
        Module, Function, Arity is the initial function call with which the process was spawned.

    {links, Pids}:
        Pids is a list of pids, with processes to which the process has a link.

    {last_calls, false|Calls}:
        The value is false if call saving is not active for the process (see process_flag/3). If call saving is active, a list is returned, in which the last element is the most recent called.

    {memory, Size}:
        Size is the size in bytes of the process. This includes call stack, heap and internal structures.

    {message_binary, BinInfo}:
        BinInfo is a list containing miscellaneous information about binaries currently being referred to by the message area. This InfoTuple is only valid on an emulator using the hybrid heap type. This InfoTuple may be changed or removed without prior notice.

    {message_queue_len, MessageQueueLen}:
        MessageQueueLen is the number of messages currently in the message queue of the process. This is the length of the list MessageQueue returned as the info item messages (see below).

    {messages, MessageQueue}:
        MessageQueue is a list of the messages to the process, which have not yet been processed.

    {monitored_by, Pids}:
        A list of pids that are monitoring the process (with erlang:monitor/2).

    {monitors, Monitors}:
        A list of monitors (started by erlang:monitor/2) that are active for the process. For a local process monitor or a remote process monitor by pid, the list item is {process, Pid}, and for a remote process monitor by name, the list item is {process, {RegName, Node}}.

    {priority, Level}:
        Level is the current priority level for the process. For more information on priorities see process_flag(priority, Level).

    {reductions, Number}:
        Number is the number of reductions executed by the process.

    {registered_name, Atom}:
        Atom is the registered name of the process. If the process has no registered name, this tuple is not present in the list.

    {sequential_trace_token, [] | SequentialTraceToken}:
        SequentialTraceToken the sequential trace token for the process. This InfoTuple may be changed or removed without prior notice.

    {stack_size, Size}:
        Size is the stack size of the process in words.

    {status, Status}:
        Status is the status of the process. Status is waiting (waiting for a message), running, runnable (ready to run, but another process is running), or suspended (suspended on a "busy" port or by the erlang:suspend_process/[1, 2] BIF).

    {suspending, SuspendeeList}:
        SuspendeeList is a list of {Suspendee, ActiveSuspendCount, OutstandingSuspendCount} tuples. Suspendee is the pid of a process that have been or is to be suspended by the process identified by Pid via the erlang:suspend_process/2 BIF, or the erlang:suspend_process/1 BIF. ActiveSuspendCount is the number of times the Suspendee has been suspended by Pid. OutstandingSuspendCount is the number of not yet completed suspend requests sent by Pid. That is, if ActiveSuspendCount /= 0, Suspendee is currently in the suspended state, and if OutstandingSuspendCount /= 0 the asynchronous option of erlang:suspend_process/2 has been used and the suspendee has not yet been suspended by Pid. Note that the ActiveSuspendCount and OutstandingSuspendCount are not the total suspend count on Suspendee, only the parts contributed by Pid.

    {total_heap_size, Size}:
        Size is the total size in words of all heap fragments of the process. This currently include the stack of the process.

    {trace, InternalTraceFlags}:
        InternalTraceFlags is an integer representing internal trace flag for this process. This InfoTuple may be changed or removed without prior notice.

    {trap_exit, Boolean}:
        Boolean is true if the process is trapping exits, otherwise it is false.

Note however, that not all implementations support every one of the above Items.

Failure: badarg if Pid is not a local process, or if Item is not a valid Item.

processes() -> [pid()]

    Returns a list of process identifiers corresponding to all the processes currently existing on the local node.

    Note that a process that is exiting, exists but is not alive, i.e., is_process_alive/1 will return false for a process that is exiting, but its process identifier will be part of the result returned from processes/0.
    > processes().
    [<0.0.0>,<0.2.0>,<0.4.0>,<0.5.0>,<0.7.0>,<0.8.0>]

purge_module(Module) -> void()

    Types
        Module = atom()

    Removes old code for Module. Before this BIF is used, erlang:check_process_code/2 should be called to check that no processes are executing old code in the module.

  Warning:

This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

Failure: badarg if there is no old code for Module.

put(Key, Val) -> OldVal | undefined

    Types
        Key = Val = OldVal = term()

    Adds a new Key to the process dictionary, associated with the value Val, and returns undefined. If Key already exists, the old value is deleted and replaced by Val and the function returns the old value.

  Note:

The values stored when put is evaluated within the scope of a catch will not be retracted if a throw is evaluated, or if an error occurs.
> X = put(name, walrus), Y = put(name, carpenter),
Z = get(name),
{X, Y, Z}.
{undefined,walrus,carpenter}

erlang:raise(Class, Reason, Stacktrace)

    Types
        Class = error | exit | throw
        Reason = term()
        Stacktrace = [{Module, Function, Arity | Args} | {Fun, Args}]
        Module = Function = atom()
        Arity = int()
        Args = [term()]
        Fun = [fun()]

    Stops the execution of the calling process with an exception of given class, reason and call stack backtrace (stacktrace).

  Warning:

This BIF is intended for debugging and for use in the Erlang operating system. In general, it should be avoided in applications, unless you know very well what you are doing.

Class is one of error, exit or throw, so if it were not for the stacktrace erlang:raise(Class, Reason, Stacktrace) is equivalent to erlang:Class(Reason). Reason is any term and Stacktrace is a list as returned from get_stacktrace(), that is a list of 3-tuples {Module, Function, Arity | Args} where Module and Function are atoms and the third element is an integer arity or an argument list. The stacktrace may also contain {Fun, Args} tuples where Fun is a local fun and Args is an argument list.

The stacktrace is used as the exception stacktrace for the calling process; it will be truncated to the current maximum stacktrace depth.

Because evaluating this function causes the process to terminate, it has no return value - unless the arguments are invalid, in which case the function returns the error reason, that is badarg. If you want to be really sure not to return you can call erlang:error(erlang:raise(Class, Reason, Stacktrace)) and hope to distinguish exceptions later.

erlang:read_timer(TimerRef) -> int() | false

    Types
        TimerRef = ref()

    TimerRef is a timer reference returned by erlang:send_after/3 or erlang:start_timer/3. If the timer is active, the function returns the time in milliseconds left until the timer will expire, otherwise false (which means that TimerRef was never a timer, that it has been cancelled, or that it has already delivered its message).

    See also erlang:send_after/3, erlang:start_timer/3, and erlang:cancel_timer/1.

erlang:ref_to_list(Ref) -> string()

    Types
        Ref = ref()

    Returns a string which corresponds to the text representation of Ref.

  Warning:

This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

register(RegName, Pid | Port) -> true

    Types
        RegName = atom()
        Pid = pid()
        Port = port()

    Associates the name RegName with a pid or a port identifier. RegName, which must be an atom, can be used instead of the pid / port identifier in the send operator (RegName ! Message).
    > register(db, Pid).
    true

    Failure: badarg if Pid is not an existing, local process or port, if RegName is already in use, if the process or port is already registered (already has a name), or if RegName is the atom undefined.

registered() -> [RegName]

    Types
        RegName = atom()

    Returns a list of names which have been registered using register/2.
    > registered().
    [code_server, file_server, init, user, my_db]

erlang:resume_process(Suspendee) -> true

    Types
        Suspendee = pid()

    Decreases the suspend count on the process identified by Suspendee. Suspendee should previously have been suspended via erlang:suspend_process/2, or erlang:suspend_process/1 by the process calling erlang:resume_process(Suspendee). When the suspend count on Suspendee reach zero, Suspendee will be resumed, i.e., the state of the Suspendee is changed from suspended into the state Suspendee was in before it was suspended.

  Warning:

This BIF is intended for debugging only.

Failures:

    badarg:
        If Suspendee isn't a process identifier.

    badarg:
        If the process calling erlang:resume_process/1 had not previously increased the suspend count on the process identified by Suspendee.

    badarg:
        If the process identified by Suspendee is not alive.

round(Number) -> int()

    Types
        Number = number()

    Returns an integer by rounding Number.
    > round(5.5).
    6

    Allowed in guard tests.

self() -> pid()

    Returns the pid (process identifier) of the calling process.
    > self().
    <0.26.0>

    Allowed in guard tests.

erlang:send(Dest, Msg) -> Msg

    Types
        Dest = pid() | port() | RegName | {RegName, Node}
        Msg = term()
        RegName = atom()
        Node = node()

    Sends a message and returns Msg. This is the same as Dest ! Msg.

    Dest may be a remote or local pid, a (local) port, a locally registered name, or a tuple {RegName, Node} for a registered name at another node.

erlang:send(Dest, Msg, [Option]) -> Res

    Types
        Dest = pid() | port() | RegName | {RegName, Node}
        RegName = atom()
        Node = node()
        Msg = term()
        Option = nosuspend | noconnect
        Res = ok | nosuspend | noconnect

    Sends a message and returns ok, or does not send the message but returns something else (see below). Otherwise the same as erlang:send/2. See also erlang:send_nosuspend/2,3. for more detailed explanation and warnings.

    The possible options are:

        nosuspend:
            If the sender would have to be suspended to do the send, nosuspend is returned instead.

        noconnect:
            If the destination node would have to be auto-connected before doing the send, noconnect is returned instead.

  Warning:

As with erlang:send_nosuspend/2, 3: Use with extreme care!

erlang:send_after(Time, Dest, Msg) -> TimerRef

    Types
        Time = int()
        0 <= Time <= 4294967295
        Dest = pid() | RegName
        LocalPid = pid() (of a process, alive or dead, on the local node)
        Msg = term()
        TimerRef = ref()

    Starts a timer which will send the message Msg to Dest after Time milliseconds.

    If Dest is an atom, it is supposed to be the name of a registered process. The process referred to by the name is looked up at the time of delivery. No error is given if the name does not refer to a process.

    If Dest is a pid, the timer will be automatically canceled if the process referred to by the pid is not alive, or when the process exits. This feature was introduced in erts version 5.4.11. Note that timers will not be automatically canceled when Dest is an atom.

    See also erlang:start_timer/3, erlang:cancel_timer/1, and erlang:read_timer/1.

    Failure: badarg if the arguments does not satisfy the requirements specified above.

erlang:send_nosuspend(Dest, Msg) -> bool()

    Types
        Dest = pid() | port() | RegName | {RegName, Node}
        RegName = atom()
        Node = node()
        Msg = term()

    The same as erlang:send(Dest, Msg, [nosuspend]), but returns true if the message was sent and false if the message was not sent because the sender would have had to be suspended.

    This function is intended for send operations towards an unreliable remote node without ever blocking the sending (Erlang) process. If the connection to the remote node (usually not a real Erlang node, but a node written in C or Java) is overloaded, this function will not send the message but return false instead.

    The same happens, if Dest refers to a local port that is busy. For all other destinations (allowed for the ordinary send operator '!') this function sends the message and returns true.

    This function is only to be used in very rare circumstances where a process communicates with Erlang nodes that can disappear without any trace causing the TCP buffers and the drivers queue to be over-full before the node will actually be shut down (due to tick timeouts) by net_kernel. The normal reaction to take when this happens is some kind of premature shutdown of the other node.

    Note that ignoring the return value from this function would result in unreliable message passing, which is contradictory to the Erlang programming model. The message is not sent if this function returns false.

    Note also that in many systems, transient states of overloaded queues are normal. The fact that this function returns false does not in any way mean that the other node is guaranteed to be non-responsive, it could be a temporary overload. Also a return value of true does only mean that the message could be sent on the (TCP) channel without blocking, the message is not guaranteed to have arrived at the remote node. Also in the case of a disconnected non-responsive node, the return value is true (mimics the behaviour of the ! operator). The expected behaviour as well as the actions to take when the function returns false are application and hardware specific.

  Warning:

Use with extreme care!

erlang:send_nosuspend(Dest, Msg, Options) -> bool()

    Types
        Dest = pid() | port() | RegName | {RegName, Node}
        RegName = atom()
        Node = node()
        Msg = term()
        Option = noconnect

    The same as erlang:send(Dest, Msg, [nosuspend | Options]), but with boolean return value.

    This function behaves like erlang:send_nosuspend/2), but takes a third parameter, a list of options. The only currently implemented option is noconnect. The option noconnect makes the function return false if the remote node is not currently reachable by the local node. The normal behaviour is to try to connect to the node, which may stall the process for a shorter period. The use of the noconnect option makes it possible to be absolutely sure not to get even the slightest delay when sending to a remote process. This is especially useful when communicating with nodes who expect to always be the connecting part (i.e. nodes written in C or Java).

    Whenever the function returns false (either when a suspend would occur or when noconnect was specified and the node was not already connected), the message is guaranteed not to have been sent.

  Warning:

Use with extreme care!

erlang:set_cookie(Node, Cookie) -> true

    Types
        Node = node()
        Cookie = atom()

    Sets the magic cookie of Node to the atom Cookie. If Node is the local node, the function also sets the cookie of all other unknown nodes to Cookie (see Distributed Erlang in the Erlang Reference Manual).

    Failure: function_clause if the local node is not alive.

setelement(Index, Tuple1, Value) -> Tuple2

    Types
        Index = 1..tuple_size(Tuple1)
        Tuple1 = Tuple2 = tuple()
        Value = term()

    Returns a tuple which is a copy of the argument Tuple1 with the element given by the integer argument Index (the first element is the element with index 1) replaced by the argument Value.
    > setelement(2, {10, green, bottles}, red).
    {10,red,bottles}

size(Item) -> int()

    Types
        Item = tuple() | binary()

    Returns an integer which is the size of the argument Item, which must be either a tuple or a binary.
    > size({morni, mulle, bwange}).
    3

    Allowed in guard tests.

spawn(Fun) -> pid()

    Types
        Fun = fun()

    Returns the pid of a new process started by the application of Fun to the empty list []. Otherwise works like spawn/3.

spawn(Node, Fun) -> pid()

    Types
        Node = node()
        Fun = fun()

    Returns the pid of a new process started by the application of Fun to the empty list [] on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn/3.

spawn(Module, Function, Args) -> pid()

    Types
        Module = Function = atom()
        Args = [term()]

    Returns the pid of a new process started by the application of Module:Function to Args. The new process created will be placed in the system scheduler queue and be run some time later.

    error_handler:undefined_function(Module, Function, Args) is evaluated by the new process if Module:Function/Arity does not exist (where Arity is the length of Args). The error handler can be redefined (see process_flag/2). If error_handler is undefined, or the user has redefined the default error_handler its replacement is undefined, a failure with the reason undef will occur.
    > spawn(speed, regulator, [high_speed, thin_cut]).
    <0.13.1>

spawn(Node, Module, Function, ArgumentList) -> pid()

    Types
        Node = node()
        Module = Function = atom()
        Args = [term()]

    Returns the pid of a new process started by the application of Module:Function to Args on Node. If Node does not exists, a useless pid is returned. Otherwise works like spawn/3.

spawn_link(Fun) -> pid()

    Types
        Fun = fun()

    Returns the pid of a new process started by the application of Fun to the empty list []. A link is created between the calling process and and the new process, atomically. Otherwise works like spawn/3.

spawn_link(Node, Fun) ->

    Types
        Node = node()
        Fun = fun()

    Returns the pid of a new process started by the application of Fun to the empty list [] on Node. A link is created between the calling process and and the new process, atomically. If Node does not exist, a useless pid is returned (and due to the link, an exit signal with exit reason noconnection will be received). Otherwise works like spawn/3.

spawn_link(Module, Function, Args) -> pid()

    Types
        Module = Function = atom()
        Args = [term()]

    Returns the pid of a new process started by the application of Module:Function to Args. A link is created between the calling process and the new process, atomically. Otherwise works like spawn/3.

spawn_link(Node, Module, Function, Args) -> pid()

    Types
        Node = node()
        Module = Function = atom()
        Args = [term()]

    Returns the pid of a new process started by the application of Module:Function to Args on Node. A link is created between the calling process and the new process, atomically. If Node does not exist, a useless pid is returned (and due to the link, an exit signal with exit reason noconnection will be received). Otherwise works like spawn/3.

spawn_monitor(Fun) -> {pid(),reference()}

    Types
        Fun = fun()

    Returns the pid of a new process started by the application of Fun to the empty list [] and reference for a monitor created to the new process. Otherwise works like spawn/3.

spawn_monitor(Module, Function, Args) -> {pid(),reference()}

    Types
        Module = Function = atom()
        Args = [term()]

    A new process is started by the application of Module:Function to Args, and the process is monitored at the same time. Returns the pid and a reference for the monitor. Otherwise works like spawn/3.

spawn_opt(Fun, [Option]) -> pid() | {pid(),reference()}

    Types
        Fun = fun()
        Option = link | monitor | {priority, Level} | {fullsweep_after, Number} | {min_heap_size, Size}
        Level = low | normal | high
        Number = int()
        Size = int()

    Returns the pid of a new process started by the application of Fun to the empty list []. Otherwise works like spawn_opt/4.

    If the option monitor is given, the newly created process will be monitored and both the pid and reference for the monitor will be returned.

spawn_opt(Node, Fun, [Option]) -> pid()

    Types
        Node = node()
        Fun = fun()
        Option = link | {priority, Level} | {fullsweep_after, Number} | {min_heap_size, Size}
        Level = low | normal | high
        Number = int()
        Size = int()

    Returns the pid of a new process started by the application of Fun to the empty list [] on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn_opt/4.

spawn_opt(Module, Function, Args, [Option]) -> pid() | {pid(),reference()}

    Types
        Module = Function = atom()
        Args = [term()]
        Option = link | monitor | {priority, Level} | {fullsweep_after, Number} | {min_heap_size, Size}
        Level = low | normal | high
        Number = int()
        Size = int()

    Works exactly like spawn/3, except that an extra option list is given when creating the process.

    If the option monitor is given, the newly created process will be monitored and both the pid and reference for the monitor will be returned.

        link:
            Sets a link to the parent process (like spawn_link/3 does).

        monitor:
            Monitor the new process (just like erlang:monitor/2 does).

        {priority, Level}:
            Sets the priority of the new process. Equivalent to executing process_flag(priority, Level) in the start function of the new process, except that the priority will be set before the process is selected for execution for the first time. For more information on priorities see process_flag(priority, Level).

        {fullsweep_after, Number}:
            This option is only useful for performance tuning. In general, you should not use this option unless you know that there is problem with execution times and/or memory consumption, and you should measure to make sure that the option improved matters.

            The Erlang runtime system uses a generational garbage collection scheme, using an "old heap" for data that has survived at least one garbage collection. When there is no more room on the old heap, a fullsweep garbage collection will be done.

            The fullsweep_after option makes it possible to specify the maximum number of generational collections before forcing a fullsweep even if there is still room on the old heap. Setting the number to zero effectively disables the general collection algorithm, meaning that all live data is copied at every garbage collection.

            Here are a few cases when it could be useful to change fullsweep_after. Firstly, if binaries that are no longer used should be thrown away as soon as possible. (Set Number to zero.) Secondly, a process that mostly have short-lived data will be fullsweeped seldom or never, meaning that the old heap will contain mostly garbage. To ensure a fullsweep once in a while, set Number to a suitable value such as 10 or 20. Thirdly, in embedded systems with limited amount of RAM and no virtual memory, one might want to preserve memory by setting Number to zero. (The value may be set globally, see erlang:system_flag/2.)

        {min_heap_size, Size}:
            This option is only useful for performance tuning. In general, you should not use this option unless you know that there is problem with execution times and/or memory consumption, and you should measure to make sure that the option improved matters.

            Gives a minimum heap size in words. Setting this value higher than the system default might speed up some processes because less garbage collection is done. Setting too high value, however, might waste memory and slow down the system due to worse data locality. Therefore, it is recommended to use this option only for fine-tuning an application and to measure the execution time with various Size values.

spawn_opt(Node, Module, Function, Args, [Option]) -> pid()

    Types
        Node = node()
        Module = Function = atom()
        Args = [term()]
        Option = link | {priority, Level} | {fullsweep_after, Number} | {min_heap_size, Size}
        Level = low | normal | high
        Number = int()
        Size = int()

    Returns the pid of a new process started by the application of Module:Function to Args on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn_opt/4.

split_binary(Bin, Pos) -> {Bin1, Bin2}

    Types
        Bin = Bin1 = Bin2 = binary()
        Pos = 1..byte_size(Bin)

    Returns a tuple containing the binaries which are the result of splitting Bin into two parts at position Pos. This is not a destructive operation. After the operation, there will be three binaries altogether.
    > B = list_to_binary("0123456789").
    <<"0123456789">>
    > byte_size(B).
    10
    > {B1, B2} = split_binary(B,3).
    {<<"012">>,<<"3456789">>}
    > byte_size(B1).
    3
    > byte_size(B2).
    7

erlang:start_timer(Time, Dest, Msg) -> TimerRef

    Types
        Time = int()
        0 <= Time <= 4294967295
        Dest = LocalPid | RegName
        LocalPid = pid() (of a process, alive or dead, on the local node)
        RegName = atom()
        Msg = term()
        TimerRef = ref()

    Starts a timer which will send the message {timeout, TimerRef, Msg} to Dest after Time milliseconds.

    If Dest is an atom, it is supposed to be the name of a registered process. The process referred to by the name is looked up at the time of delivery. No error is given if the name does not refer to a process.

    If Dest is a pid, the timer will be automatically canceled if the process referred to by the pid is not alive, or when the process exits. This feature was introduced in erts version 5.4.11. Note that timers will not be automatically canceled when Dest is an atom.

    See also erlang:send_after/3, erlang:cancel_timer/1, and erlang:read_timer/1.

    Failure: badarg if the arguments does not satisfy the requirements specified above.

statistics(Type) -> Res

    Types
        Type, Res -- see below

    Returns information about the system as specified by Type:

        context_switches:
            Returns {ContextSwitches, 0}, where ContextSwitches is the total number of context switches since the system started.

        exact_reductions:

            Returns {Total_Exact_Reductions, Exact_Reductions_Since_Last_Call}.

            NOTE:statistics(exact_reductions) is a more expensive operation than statistics(reductions) especially on an Erlang machine with SMP support.

        garbage_collection:
            Returns {Number_of_GCs, Words_Reclaimed, 0}. This information may not be valid for all implementations.

        io: Returns {{input, Input}, {output, Output}}, where Input is the total number of bytes received through ports, and Output is the total number of bytes output to ports.

        reductions:

            Returns {Total_Reductions, Reductions_Since_Last_Call}.

            NOTE: From erts version 5.5 (OTP release R11B) this value does not include reductions performed in current time slices of currently scheduled processes. If an exact value is wanted, use statistics(exact_reductions).

        run_queue:
            Returns the length of the run queue, that is, the number of processes that are ready to run.

        runtime:
            Returns {Total_Run_Time, Time_Since_Last_Call}.

        wall_clock:
            Returns {Total_Wallclock_Time, Wallclock_Time_Since_Last_Call}. wall_clock can be used in the same manner as runtime, except that real time is measured as opposed to runtime or CPU time.

    All times are in milliseconds.
    > statistics(runtime).
    {1690,1620}
    > statistics(reductions).
    {2046,11}
    > statistics(garbage_collection).
    {85,23961,0}

erlang:suspend_process(Suspendee, OptList) -> true | false

    Types
        Suspendee = pid()
        OptList = [Opt]
        Opt = atom()

    Increases the suspend count on the process identified by Suspendee and puts it in the suspended state if it isn't already in the suspended state. A suspended process will not be scheduled for execution until the process has been resumed.

    A process can be suspended by multiple processes and can be suspended multiple times by a single process. A suspended process will not leave the suspended state until its suspend count reach zero. The suspend count of Suspendee is decreased when erlang:resume_process(Suspendee) is called by the same process that called erlang:suspend_process(Suspendee). All increased suspend counts on other processes acquired by a process will automatically be decreased when the process terminates.

    Currently the following options (Opts) are available:

        asynchronous:
            A suspend request is sent to the process identified by Suspendee. Suspendee will eventually suspend unless it is resumed before it was able to suspend. The caller of erlang:suspend_process/2 will return immediately, regardless of whether the Suspendee has suspended yet or not. Note that the point in time when the Suspendee will actually suspend cannot be deduced from other events in the system. The only guarantee given is that the Suspendee will eventually suspend (unless it is resumed). If the asynchronous option has not been passed, the caller of erlang:suspend_process/2 will be blocked until the Suspendee has actually suspended.

        unless_suspending:
            The process identified by Suspendee will be suspended unless the calling process already is suspending the Suspendee. If unless_suspending is combined with the asynchronous option, a suspend request will be sent unless the calling process already is suspending the Suspendee or if a suspend request already has been sent and is in transit. If the calling process already is suspending the Suspendee, or if combined with the asynchronous option and a send request already is in transit, false is returned and the suspend count on Suspendee will remain unchanged.

    If the suspend count on the process identified by Suspendee was increased, true is returned; otherwise, false is returned.

  Warning:

This BIF is intended for debugging only.

Failures:

    badarg:
        If Suspendee isn't a process identifier.

    badarg:
        If the process identified by Suspendee is same the process as the process calling erlang:suspend_process/2.

    badarg:
        If the process identified by Suspendee is not alive.

    badarg:
        If the process identified by Suspendee resides on another node.

    badarg:
        If OptList isn't a proper list of valid Opts.

    system_limit:
        If the process identified by Suspendee has been suspended more times by the calling process than can be represented by the currently used internal data structures. The current system limit is larger than 2 000 000 000 suspends, and it will never be less than that.

erlang:suspend_process(Suspendee) -> true

    Types
        Suspendee = pid()

    Suspends the process identified by Suspendee. The same as calling erlang:suspend_process(Suspendee, []). For more information see the documentation of erlang:suspend_process/2.

  Warning:

This BIF is intended for debugging only.

erlang:system_flag(Flag, Value) -> OldValue

    Types
        Flag, Value, OldValue -- see below

    Sets various system properties of the Erlang node. Returns the old value of the flag.

        erlang:system_flag(backtrace_depth, Depth):
            Sets the maximum depth of call stack back-traces in the exit reason element of 'EXIT' tuples.

        erlang:system_flag(fullsweep_after, Number):
            Number is a non-negative integer which indicates how many times generational garbages collections can be done without forcing a fullsweep collection. The value applies to new processes; processes already running are not affected.

            In low-memory systems (especially without virtual memory), setting the value to 0 can help to conserve memory.

            An alternative way to set this value is through the (operating system) environment variable ERL_FULLSWEEP_AFTER.

        erlang:system_flag(min_heap_size, MinHeapSize):
            Sets the default minimum heap size for processes. The size is given in words. The new min_heap_size only effects processes spawned after the change of min_heap_size has been made. The min_heap_size can be set for individual processes by use of spawn_opt/N or process_flag/2.

        erlang:system_flag(multi_scheduling, BlockState):

            BlockState = block | unblock

            If multi-scheduling is enabled, more than one scheduler thread is used by the emulator. Multi-scheduling can be blocked. When multi-scheduling has been blocked, only one scheduler thread will schedule Erlang processes.

            If BlockState =:= block, multi-scheduling will be blocked. If BlockState =:= unblock and no-one else is blocking multi-scheduling and this process has only blocked one time, multi-scheduling will be unblocked. One process can block multi-scheduling multiple times. If a process has blocked multiple times, it has to unblock exactly as many times as it has blocked before it has released its multi-scheduling block. If a process that has blocked multi-scheduling exits, it will release its blocking of multi-scheduling.

            The return values are disabled, blocked, or enabled. The returned value describes the state just after the call to erlang:system_flag(multi_scheduling, BlockState) has been made. The return values are described in the documentation of erlang:system_info(multi_scheduling).

            NOTE: Blocking of multi-scheduling should normally not be needed. If you feel that you need to block multi-scheduling, think through the problem at least a couple of times again. Blocking multi-scheduling should only be used as a last resort since it will most likely be a very inefficient way to solve the problem.

            See also erlang:system_info(multi_scheduling), erlang:system_info(multi_scheduling_blockers), and erlang:system_info(schedulers).

        erlang:system_flag(trace_control_word, TCW):
            Sets the value of the node's trace control word to TCW. TCW should be an unsigned integer. For more information see documentation of the set_tcw function in the match specification documentation in the ERTS User's Guide.

  Note:

The schedulers option has been removed as of erts version 5.5.3. The number of scheduler threads is determined at emulator boot time, and cannot be changed after that.

erlang:system_info(Type) -> Res

    Types
        Type, Res -- see below

    Returns various information about the current system (emulator) as specified by Type:

        allocated_areas:

            Returns a list of tuples with information about miscellaneous allocated memory areas.

            Each tuple contains an atom describing type of memory as first element and amount of allocated memory in bytes as second element. In those cases when there is information present about allocated and used memory, a third element is present. This third element contains the amount of used memory in bytes.

            erlang:system_info(allocated_areas) is intended for debugging, and the content is highly implementation dependent. The content of the results will therefore change when needed without prior notice.

            Note: The sum of these values is not the total amount of memory allocated by the emulator. Some values are part of other values, and some memory areas are not part of the result. If you are interested in the total amount of memory allocated by the emulator see erlang:memory/0,1.

        allocator:

            Returns {Allocator, Version, Features, Settings}.

            Types:

                *   Allocator = undefined | elib_malloc | glibc

                *   Version = [int()]

                *   Features = [atom()]

                *   Settings = [{Subsystem, [{Parameter, Value}]}]

                *   Subsystem = atom()

                *   Parameter = atom()

                *   Value = term()

            Explanation:

                *   Allocator corresponds to the malloc() implementation used. If Allocator equals undefined, the malloc() implementation used could not be identified. Currently elib_malloc and glibc can be identified.

                *   Version is a list of integers (but not a string) representing the version of the malloc() implementation used.

                *   Features is a list of atoms representing allocation features used.

                *   Settings is a list of subsystems, their configurable parameters, and used values. Settings may differ between different combinations of platforms, allocators, and allocation features. Memory sizes are given in bytes.

            See also "System Flags Effecting erts_alloc" in erts_alloc(3).

        alloc_util_allocators:

            Returns a list of the names of all allocators using the ERTS internal alloc_util framework as atoms. For more information see the "the alloc_util framework" section in the erts_alloc(3) documentation.

        {allocator, Alloc}:

            Returns information about the specified allocator. As of erts version 5.6.1 the return value is a list of {instance, InstanceNo, InstanceInfo} tuples where InstanceInfo contains information about a specific instance of the allocator. If Alloc is not a recognized allocator, undefined is returned. If Alloc is disabled, false is returned.

            Note: The information returned is highly implementation dependent and may be changed, or removed at any time without prior notice. It was initially intended as a tool when developing new allocators, but since it might be of interest for others it has been briefly documented.

            The recognized allocators are listed in erts_alloc(3). After reading the erts_alloc(3) documentation, the returned information should more or less speak for itself. But it can be worth explaining some things. Call counts are presented by two values. The first value is giga calls, and the second value is calls. mbcs, and sbcs are abbreviations for, respectively, multi-block carriers, and single-block carriers. Sizes are presented in bytes. When it is not a size that is presented, it is the amount of something. Sizes and amounts are often presented by three values, the first is current value, the second is maximum value since the last call to erlang:system_info({allocator, Alloc}), and the third is maximum value since the emulator was started. If only one value is present, it is the current value. fix_alloc memory block types are presented by two values. The first value is memory pool size and the second value used memory size.

        {allocator_sizes, Alloc}:

            Returns various size information for the specified allocator. The information returned is a subset of the information returned by erlang:system_info({allocator, Alloc}).

        c_compiler_used:
            Returns a two-tuple describing the C compiler used when compiling the runtime system. The first element is an atom describing the name of the compiler, or undefined if unknown. The second element is a term describing the version of the compiler, or undefined if unknown.

        check_io:
            Returns a list containing miscellaneous information regarding the emulators internal I/O checking. Note, the content of the returned list may vary between platforms and over time. The only thing guaranteed is that a list is returned.

        compat_rel:
            Returns the compatibility mode of the local node as an integer. The integer returned represents the Erlang/OTP release which the current emulator has been set to be backward compatible with. The compatibility mode can be configured at startup by using the command line flag +R, see erl(1).

        creation:
            Returns the creation of the local node as an integer. The creation is changed when a node is restarted. The creation of a node is stored in process identifiers, port identifiers, and references. This makes it (to some extent) possible to distinguish between identifiers from different incarnations of a node. Currently valid creations are integers in the range 1..3, but this may (probably will) change in the future. If the node is not alive, 0 is returned.

        debug_compiled:
            Returns true if the emulator has been debug compiled; otherwise, false.

        dist:
            Returns a binary containing a string of distribution information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.

        dist_ctrl:
            Returns a list of tuples {Node, ControllingEntity}, one entry for each connected remote node. The Node is the name of the node and the ControllingEntity is the port or pid responsible for the communication to that node. More specifically, the ControllingEntity for nodes connected via TCP/IP (the normal case) is the socket actually used in communication with the specific node.

        driver_version:
            Returns a string containing the erlang driver version used by the runtime system. It will be on the form "<major ver>.<minor ver>".

        elib_malloc:
            If the emulator uses the elib_malloc memory allocator, a list of two-element tuples containing status information is returned; otherwise, false is returned. The list currently contains the following two-element tuples (all sizes are presented in bytes):

                {heap_size, Size}:
                    Where Size is the current heap size.

                {max_alloced_size, Size}:
                    Where Size is the maximum amount of memory allocated on the heap since the emulator started.

                {alloced_size, Size}:
                    Where Size is the current amount of memory allocated on the heap.

                {free_size, Size}:
                    Where Size is the current amount of free memory on the heap.

                {no_alloced_blocks, No}:
                    Where No is the current number of allocated blocks on the heap.

                {no_free_blocks, No}:
                    Where No is the current number of free blocks on the heap.

                {smallest_alloced_block, Size}:
                    Where Size is the size of the smallest allocated block on the heap.

                {largest_free_block, Size}:
                    Where Size is the size of the largest free block on the heap.

        fullsweep_after:
            Returns {fullsweep_after, int()} which is the fullsweep_after garbage collection setting used by default. For more information see garbage_collection described below.

        garbage_collection:
            Returns a list describing the default garbage collection settings. A process spawned on the local node by a spawn or spawn_link will use these garbage collection settings. The default settings can be changed by use of system_flag/2. spawn_opt/4 can spawn a process that does not use the default settings.

        global_heaps_size:
            Returns the current size of the shared (global) heap.

        heap_sizes:
            Returns a list of integers representing valid heap sizes in words. All Erlang heaps are sized from sizes in this list.

        heap_type:
            Returns the heap type used by the current emulator. Currently the following heap types exist:

                private:
                    Each process has a heap reserved for its use and no references between heaps of different processes are allowed. Messages passed between processes are copied between heaps.

                shared:
                    One heap for use by all processes. Messages passed between processes are passed by reference.

                hybrid:
                    A hybrid of the private and shared heap types. A shared heap as well as private heaps are used.

        info:
            Returns a binary containing a string of miscellaneous system information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.

        kernel_poll:
            Returns true if the emulator uses some kind of kernel-poll implementation; otherwise, false.

        loaded:
            Returns a binary containing a string of loaded module information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.

        logical_processors:
            Returns the number of logical processors detected on the system as an integer or the atom unknown if the emulator wasn't able to detect any.

        machine:
            Returns a string containing the Erlang machine name.

        modified_timing_level:
            Returns the modified timing level (an integer) if modified timing has been enabled; otherwise, undefined. See the +T command line flag in the documentation of the erl(1) command for more information on modified timing.

        multi_scheduling:

            Returns disabled, blocked, or enabled. A description of the return values:

                disabled:
                    The emulator has only one scheduler thread. The emulator does not have SMP support, or have been started with only one scheduler thread.

                blocked:
                    The emulator has more than one scheduler thread, but all scheduler threads but one have been blocked, i.e., only one scheduler thread will schedule Erlang processes and execute Erlang code.

                enabled:
                    The emulator has more than one scheduler thread, and no scheduler threads have been blocked, i.e., all available scheduler threads will schedule Erlang processes and execute Erlang code.

            See also erlang:system_flag(multi_scheduling, BlockState), erlang:system_info(multi_scheduling_blockers), and erlang:system_info(schedulers).

        multi_scheduling_blockers:

            Returns a list of PIDs when multi-scheduling is blocked; otherwise, the empty list. The PIDs in the list is PIDs of the processes currently blocking multi-scheduling. A PID will only be present once in the list, even if the corresponding process has blocked multiple times.

            See also erlang:system_flag(multi_scheduling, BlockState), erlang:system_info(multi_scheduling), and erlang:system_info(schedulers).

        otp_release:

            Returns a string containing the OTP release number.

        process_count:
            Returns the number of processes currently existing at the local node as an integer. The same value as length(processes()) returns.

        process_limit:
            Returns the maximum number of concurrently existing processes at the local node as an integer. This limit can be configured at startup by using the command line flag +P, see erl(1).

        procs:
            Returns a binary containing a string of process and port information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.

        scheduler_id:

            Returns the scheduler id (SchedulerId) of the scheduler thread that the calling process is executing on. SchedulerId is a positive integer; where 1 <= SchedulerId <= erlang:system_info(schedulers). See also erlang:system_info(schedulers).

        schedulers:

            Returns the number of scheduler threads used by the emulator. A scheduler thread schedules Erlang processes and Erlang ports, and execute Erlang code and Erlang linked in driver code.

            The number of scheduler threads is determined at emulator boot time and cannot be changed after that.

            See also erlang:system_info(scheduler_id), erlang:system_flag(multi_scheduling, BlockState), erlang:system_info(multi_scheduling), and and erlang:system_info(multi_scheduling_blockers).

        smp_support:
            Returns true if the emulator has been compiled with smp support; otherwise, false.

        system_version:
            Returns a string containing the emulator type and version as well as some important properties such as the size of the thread pool, etc.

        system_architecture:
            Returns a string containing the processor and OS architecture the emulator is built for.

        threads:
            Returns true if the emulator has been compiled with thread support; otherwise, false is returned.

        thread_pool_size:

            Returns the number of async threads in the async thread pool used for asynchronous driver calls (driver_async()) as an integer.

        trace_control_word:
            Returns the value of the node's trace control word. For more information see documentation of the function get_tcw in "Match Specifications in Erlang", ERTS User's Guide.

        version:

            Returns a string containing the version number of the emulator.

        wordsize:
            Returns the word size in bytes as an integer, i.e. on a 32-bit architecture 4 is returned, and on a 64-bit architecture 8 is returned.

  Note:

The scheduler argument has changed name to scheduler_id. This in order to avoid mixup with the schedulers argument. The scheduler argument was introduced in ERTS version 5.5 and renamed in ERTS version 5.5.1.

erlang:system_monitor() -> MonSettings

    Types
        MonSettings -> {MonitorPid, Options} | undefined
        MonitorPid = pid()
        Options = [Option]
        Option = {long_gc, Time} | {large_heap, Size} | busy_port | busy_dist_port
        Time = Size = int()

    Returns the current system monitoring settings set by erlang:system_monitor/2 as {MonitorPid, Options}, or undefined if there are no settings. The order of the options may be different from the one that was set.

erlang:system_monitor(undefined | {MonitorPid, Options}) -> MonSettings

    Types
        MonitorPid, Options, MonSettings -- see below

    When called with the argument undefined, all system performance monitoring settings are cleared.

    Calling the function with {MonitorPid, Options} as argument, is the same as calling erlang:system_monitor(MonitorPid, Options).

    Returns the previous system monitor settings just like erlang:system_monitor/0.

erlang:system_monitor(MonitorPid, [Option]) -> MonSettings

    Types
        MonitorPid = pid()
        Option = {long_gc, Time} | {large_heap, Size} | busy_port | busy_dist_port
        Time = Size = int()
        MonSettings = {OldMonitorPid, [Option]}
        OldMonitorPid = pid()

    Sets system performance monitoring options. MonitorPid is a local pid that will receive system monitor messages, and the second argument is a list of monitoring options:

        {long_gc, Time}:
            If a garbage collection in the system takes at least Time wallclock milliseconds, a message {monitor, GcPid, long_gc, Info} is sent to MonitorPid. GcPid is the pid that was garbage collected and Info is a list of two-element tuples describing the result of the garbage collection. One of the tuples is {timeout, GcTime} where GcTime is the actual time for the garbage collection in milliseconds. The other tuples are tagged with heap_size, heap_block_size, stack_size, mbuf_size, old_heap_size, and old_heap_block_size. These tuples are explained in the documentation of the gc_start trace message (see erlang:trace/3). New tuples may be added, and the order of the tuples in the Info list may be changed at any time without prior notice.

        {large_heap, Size}:
            If a garbage collection in the system results in the allocated size of a heap being at least Size words, a message {monitor, GcPid, large_heap, Info} is sent to MonitorPid. GcPid and Info are the same as for long_gc above, except that the tuple tagged with timeout is not present. Note: As of erts version 5.6 the monitor message is sent if the sum of the sizes of all memory blocks allocated for all heap generations is equal to or larger than Size. Previously the monitor message was sent if the memory block allocated for the youngest generation was equal to or larger than Size.

        busy_port:
            If a process in the system gets suspended because it sends to a busy port, a message {monitor, SusPid, busy_port, Port} is sent to MonitorPid. SusPid is the pid that got suspended when sending to Port.

        busy_dist_port:
            If a process in the system gets suspended because it sends to a process on a remote node whose inter-node communication was handled by a busy port, a message {monitor, SusPid, busy_dist_port, Port} is sent to MonitorPid. SusPid is the pid that got suspended when sending through the inter-node communication port Port.

    Returns the previous system monitor settings just like erlang:system_monitor/0.

  Note:

If a monitoring process gets so large that it itself starts to cause system monitor messages when garbage collecting, the messages will enlarge the process's message queue and probably make the problem worse.

Keep the monitoring process neat and do not set the system monitor limits too tight.

Failure: badarg if MonitorPid does not exist.

erlang:system_profile() -> ProfilerSettings

    Types
        ProfilerSettings -> {ProfilerPid, Options} | undefined
        ProfilerPid = pid() | port()
        Options = [Option]
        Option = runnable_procs | runnable_ports | scheduler | exclusive

    Returns the current system profiling settings set by erlang:system_profile/2 as {ProfilerPid, Options}, or undefined if there are no settings. The order of the options may be different from the one that was set.

erlang:system_profile(ProfilerPid, Options) -> ProfilerSettings

    Types
        ProfilerSettings -> {ProfilerPid, Options} | undefined
        ProfilerPid = pid() | port()
        Options = [Option]
        Option = runnable_procs | runnable_ports | scheduler | exclusive

    Sets system profiler options. ProfilerPid is a local pid or port that will receive profiling messages. The receiver is excluded from all profiling. The second argument is a list of profiling options:

        runnable_procs:
            If a process is put into or removed from the runqueue a message, {profile, Pid, State, Mfa, Ts}, is sent to ProfilerPid. Running processes that is reinsertet into the runqueue after completing its reductions does not trigger this message.

        runnable_ports:
            If a port is put into or removed from the runqueue a message, {profile, Port, State, 0, Ts}, is sent to ProfilerPid.

        scheduler:
            If a scheduler is put to sleep or awoken a message, {profile, scheduler, Id, State, NoScheds, Ts}, is sent to ProfilerPid.

        exclusive:
            If a synchronous call to a port from a process is done, the calling process is considered not runnable during the call runtime to the port. The calling process is notified as inactive and subsequently active when the port callback returns.

  Note:

erlang:system_profile is considered experimental and its behaviour may change in the future.

term_to_binary(Term) -> ext_binary()

    Types
        Term = term()

    Returns a binary data object which is the result of encoding Term according to the Erlang external term format.

    This can be used for a variety of purposes, for example writing a term to a file in an efficient way, or sending an Erlang term to some type of communications channel not supported by distributed Erlang.

    See also binary_to_term/1.

term_to_binary(Term, [Option]) -> ext_binary()

    Types
        Term = term()
        Option = compressed | {compressed, Level} | {minor_version, Version}

    Returns a binary data object which is the result of encoding Term according to the Erlang external term format.

    If the option compressed is provided, the external term format will be compressed. The compressed format is automatically recognized by binary_to_term/1 in R7B and later.

    It is also possible to specify a compression level by giving the option {compressed, Level}, where Level is an integer from 0 through 9. 0 means that no compression will be done (it is the same as not giving any compressed option); 1 will take the least time but may not compress as well as the higher levels; 9 will take the most time and may produce a smaller result. Note the "mays" in the preceding sentence; depending on the input term, level 9 compression may or may not produce a smaller result than level 1 compression.

    Currently, compressed gives the same result as {compressed, 6}.

    The option {minor_version, Version} can be use to control some details of the encoding. This option was introduced in R11B-4. Currently, the allowed values for Version are 0 and 1.

    {minor_version, 1} forces any floats in the term to be encoded in a more space-efficient and exact way (namely in the 64-bit IEEE format, rather than converted to a textual representation). binary_to_term/1 in R11B-4 and later is able decode the new representation.

    {minor_version, 0} is currently the default, meaning that floats will be encoded using a textual representation; this option is useful if you want to ensure that releases prior to R11B-4 can decode resulting binary.

    See also binary_to_term/1.

throw(Any)

    Types
        Any = term()

    A non-local return from a function. If evaluated within a catch, catch will return the value Any.
    > catch throw({hello, there}).
    {hello,there}

    Failure: nocatch if not evaluated within a catch.

time() -> {Hour, Minute, Second}

    Types
        Hour = Minute = Second = int()

    Returns the current time as {Hour, Minute, Second}.

    The time zone and daylight saving time correction depend on the underlying OS.
    > time().
    {9,42,44}

tl(List1) -> List2

    Types
        List1 = List2 = [term()]

    Returns the tail of List1, that is, the list minus the first element.
    > tl([geesties, guilies, beasties]).
    [guilies, beasties]

    Allowed in guard tests.

    Failure: badarg if List is the empty list [].

erlang:trace(PidSpec, How, FlagList) -> int()

    Types
        PidSpec = pid() | existing | new | all
        How = bool()
        FlagList = [Flag]
        Flag -- see below

    Turns on (if How == true) or off (if How == false) the trace flags in FlagList for the process or processes represented by PidSpec.

    PidSpec is either a pid for a local process, or one of the following atoms:

        existing:
            All processes currently existing.

        new:
            All processes that will be created in the future.

        all:
            All currently existing processes and all processes that will be created in the future.

    FlagList can contain any number of the following flags (the "message tags" refers to the list of messages following below):

        all:
            Set all trace flags except {tracer, Tracer} and cpu_timestamp that are in their nature different than the others.

        send:
            Trace sending of messages.

            Message tags: send, send_to_non_existing_process.

        'receive':
            Trace receiving of messages.

            Message tags: 'receive'.

        procs:
            Trace process related events.

            Message tags: spawn, exit, register, unregister, link, unlink, getting_linked, getting_unlinked.

        call:
            Trace certain function calls. Specify which function calls to trace by calling erlang:trace_pattern/3.

            Message tags: call, return_from.

        silent:
            Used in conjunction with the call trace flag. The call, return_from and return_to trace messages are inhibited if this flag is set, but if there are match specs they are executed as normal.

            Silent mode is inhibited by executing erlang:trace(_, false, [silent|_]), or by a match spec executing the {silent, false} function.

            The silent trace flag facilitates setting up a trace on many or even all processes in the system. Then the interesting trace can be activated and deactivated using the {silent, Bool} match spec function, giving a high degree of control of which functions with which arguments that triggers the trace.

            Message tags: call, return_from, return_to. Or rather, the absence of.

        return_to:
            Used in conjunction with the call trace flag. Trace the actual return from a traced function back to its caller. Only works for functions traced with the local option to erlang:trace_pattern/3.

            The semantics is that a trace message is sent when a call traced function actually returns, that is, when a chain of tail recursive calls is ended. There will be only one trace message sent per chain of tail recursive calls, why the properties of tail recursiveness for function calls are kept while tracing with this flag. Using call and return_to trace together makes it possible to know exactly in which function a process executes at any time.

            To get trace messages containing return values from functions, use the {return_trace} match_spec action instead.

            Message tags: return_to.

        running:
            Trace scheduling of processes.

            Message tags: in, and out.

        exiting:
            Trace scheduling of an exiting processes.

            Message tags: in_exiting, out_exiting, and out_exited.

        garbage_collection:
            Trace garbage collections of processes.

            Message tags: gc_start, gc_end.

        timestamp:
            Include a time stamp in all trace messages. The time stamp (Ts) is of the same form as returned by erlang:now().

        cpu_timestamp:
            A global trace flag for the Erlang node that makes all trace timestamps be in CPU time, not wallclock. It is only allowed with PidSpec==all. If the host machine operating system does not support high resolution CPU time measurements, trace/3 exits with badarg.

        arity:
            Used in conjunction with the call trace flag. {M, F, Arity} will be specified instead of {M, F, Args} in call trace messages.

        set_on_spawn:
            Makes any process created by a traced process inherit its trace flags, including the set_on_spawn flag.

        set_on_first_spawn:
            Makes the first process created by a traced process inherit its trace flags, excluding the set_on_first_spawn flag.

        set_on_link:
            Makes any process linked by a traced process inherit its trace flags, including the set_on_link flag.

        set_on_first_link:
            Makes the first process linked to by a traced process inherit its trace flags, excluding the set_on_first_link flag.

        {tracer, Tracer}:
            Specify where to send the trace messages. Tracer must be the pid of a local process or the port identifier of a local port. If this flag is not given, trace messages will be sent to the process that called erlang:trace/3.

    The effect of combining set_on_first_link with set_on_link is the same as having set_on_first_link alone. Likewise for set_on_spawn and set_on_first_spawn.

    If the timestamp flag is not given, the tracing process will receive the trace messages described below. Pid is the pid of the traced process in which the traced event has occurred. The third element of the tuple is the message tag.

    If the timestamp flag is given, the first element of the tuple will be trace_ts instead and the timestamp is added last in the tuple.

        {trace, Pid, 'receive', Msg}:
            When Pid receives the message Msg.

        {trace, Pid, send, Msg, To}:
            When Pid sends the message Msg to the process To.

        {trace, Pid, send_to_non_existing_process, Msg, To}:
            When Pid sends the message Msg to the non-existing process To.

        {trace, Pid, call, {M, F, Args}}:
            When Pid calls a traced function. The return values of calls are never supplied, only the call and its arguments.

            Note that the trace flag arity can be used to change the contents of this message, so that Arity is specified instead of Args.

        {trace, Pid, return_to, {M, F, Arity}}:
            When Pid returns to the specified function. This trace message is sent if both the call and the return_to flags are set, and the function is set to be traced on local function calls. The message is only sent when returning from a chain of tail recursive function calls where at least one call generated a call trace message (that is, the functions match specification matched and {message, false} was not an action).

        {trace, Pid, return_from, {M, F, Arity}, ReturnValue}:
            When Pid returns from the specified function. This trace message is sent if the call flag is set, and the function has a match specification with a return_trace or exception_trace action.

        {trace, Pid, exception_from, {M, F, Arity}, {Class, Value}}:
            When Pid exits from the specified function due to an exception. This trace message is sent if the call flag is set, and the function has a match specification with an exception_trace action.

        {trace, Pid, spawn, Pid2, {M, F, Args}}:
            When Pid spawns a new process Pid2 with the specified function call as entry point.

            Note that Args is supposed to be the argument list, but may be any term in the case of an erroneous spawn.

        {trace, Pid, exit, Reason}:
            When Pid exits with reason Reason.

        {trace, Pid, link, Pid2}:
            When Pid links to a process Pid2.

        {trace, Pid, unlink, Pid2}:
            When Pid removes the link from a process Pid2.

        {trace, Pid, getting_linked, Pid2}:
            When Pid gets linked to a process Pid2.

        {trace, Pid, getting_unlinked, Pid2}:
            When Pid gets unlinked from a process Pid2.

        {trace, Pid, register, RegName}:
            When Pid gets the name RegName registered.

        {trace, Pid, unregister, RegName}:
            When Pid gets the name RegName unregistered. Note that this is done automatically when a registered process exits.

        {trace, Pid, in, {M, F, Arity} | 0}:
            When Pid is scheduled to run. The process will run in function {M, F, Arity}. On some rare occasions the current function cannot be determined, then the last element Arity is 0.

        {trace, Pid, out, {M, F, Arity} | 0}:
            When Pid is scheduled out. The process was running in function {M, F, Arity}. On some rare occasions the current function cannot be determined, then the last element Arity is 0.

        {trace, Pid, gc_start, Info}:

            Sent when garbage collection is about to be started. Info is a list of two-element tuples, where the first element is a key, and the second is the value. You should not depend on the tuples have any defined order. Currently, the following keys are defined:

                heap_size:
                    The size of the used part of the heap.

                heap_block_size:
                    The size of the memory block used for storing the heap and the stack.

                old_heap_size:
                    The size of the used part of the old heap.

                old_heap_block_size:
                    The size of the memory block used for storing the old heap.

                stack_size:
                    The actual size of the stack.

                recent_size:
                    The size of the data that survived the previous garbage collection.

                mbuf_size:
                    The combined size of message buffers associated with the process.

            All sizes are in words.

        {trace, Pid, gc_end, Info}:
            Sent when garbage collection is finished. Info contains the same kind of list as in the gc_start message, but the sizes reflect the new sizes after garbage collection.

    If the tracing process dies, the flags will be silently removed.

    Only one process can trace a particular process. For this reason, attempts to trace an already traced process will fail.

    Returns: A number indicating the number of processes that matched PidSpec. If PidSpec is a pid, the return value will be 1. If PidSpec is all or existing the return value will be the number of processes running, excluding tracer processes. If PidSpec is new, the return value will be 0.

    Failure: If specified arguments are not supported. For example cpu_timestamp is not supported on all platforms.

erlang:trace_delivered(Tracee) -> Ref

    Types
        Tracee = pid() | all
        Ref = reference()

    The delivery of trace messages is dislocated on the time-line compared to other events in the system. If you know that the Tracee has passed some specific point in its execution, and you want to know when at least all trace messages corresponding to events up to this point have reached the tracer you can use erlang:trace_delivered(Tracee). A {trace_delivered, Tracee, Ref} message is sent to the caller of erlang:trace_delivered(Tracee) when it is guaranteed that all trace messages have been delivered to the tracer up to the point that the Tracee had reached at the time of the call to erlang:trace_delivered(Tracee).

    Note that the trace_delivered message does not imply that trace messages have been delivered; instead, it implies that all trace messages that should be delivered have been delivered. It is not an error if Tracee isn't, and hasn't been traced by someone, but if this is the case, no trace messages will have been delivered when the trace_delivered message arrives.

    Note that Tracee has to refer to a process currently, or previously existing on the same node as the caller of erlang:trace_delivered(Tracee) resides on. The special Tracee atom all denotes all processes that currently are traced in the node.

    An example: Process A is tracee, port B is tracer, and process C is the port owner of B. C wants to close B when A exits. C can ensure that the trace isn't truncated by calling erlang:trace_delivered(A) when A exits and wait for the {trace_delivered, A, Ref} message before closing B.

    Failure: badarg if Tracee does not refer to a process (dead or alive) on the same node as the caller of erlang:trace_delivered(Tracee) resides on.

erlang:trace_info(PidOrFunc, Item) -> Res

    Types
        PidOrFunc = pid() | new | {Module, Function, Arity} | on_load
        Module = Function = atom()
        Arity = int()
        Item, Res -- see below

    Returns trace information about a process or function.

    To get information about a process, PidOrFunc should be a pid or the atom new. The atom new means that the default trace state for processes to be created will be returned. Item must have one of the following values:

        flags:
            Return a list of atoms indicating what kind of traces is enabled for the process. The list will be empty if no traces are enabled, and one or more of the followings atoms if traces are enabled: send, 'receive', set_on_spawn, call, return_to, procs, set_on_first_spawn, set_on_link, running, garbage_collection, timestamp, and arity. The order is arbitrary.

        tracer:
            Return the identifier for process or port tracing this process. If this process is not being traced, the return value will be [].

    To get information about a function, PidOrFunc should be a three-element tuple: {Module, Function, Arity} or the atom on_load. No wildcards are allowed. Returns undefined if the function does not exist or false if the function is not traced at all. Item must have one of the following values:

        traced:
            Return global if this function is traced on global function calls, local if this function is traced on local function calls (i.e local and global function calls), and false if neither local nor global function calls are traced.

        match_spec:
            Return the match specification for this function, if it has one. If the function is locally or globally traced but has no match specification defined, the returned value is [].

        meta:
            Return the meta trace tracer process or port for this function, if it has one. If the function is not meta traced the returned value is false, and if the function is meta traced but has once detected that the tracer proc is invalid, the returned value is [].

        meta_match_spec:
            Return the meta trace match specification for this function, if it has one. If the function is meta traced but has no match specification defined, the returned value is [].

        call_count:
            Return the call count value for this function or true for the pseudo function on_load if call count tracing is active. Return false otherwise. See also erlang:trace_pattern/3.

        all:
            Return a list containing the {Item, Value} tuples for all other items, or return false if no tracing is active for this function.

    The actual return value will be {Item, Value}, where Value is the requested information as described above. If a pid for a dead process was given, or the name of a non-existing function, Value will be undefined.

    If PidOrFunc is the on_load, the information returned refers to the default value for code that will be loaded.

erlang:trace_pattern(MFA, MatchSpec) -> int()

    The same as erlang:trace_pattern(MFA, MatchSpec, []), retained for backward compatibility.

erlang:trace_pattern(MFA, MatchSpec, FlagList) -> int()

    Types
        MFA, MatchSpec, FlagList -- see below

    This BIF is used to enable or disable call tracing for exported functions. It must be combined with erlang:trace/3 to set the call trace flag for one or more processes.

    Conceptually, call tracing works like this: Inside the Erlang virtual machine there is a set of processes to be traced and a set of functions to be traced. Tracing will be enabled on the intersection of the set. That is, if a process included in the traced process set calls a function included in the traced function set, the trace action will be taken. Otherwise, nothing will happen.

    Use erlang:trace/3 to add or remove one or more processes to the set of traced processes. Use erlang:trace_pattern/2 to add or remove exported functions to the set of traced functions.

    The erlang:trace_pattern/3 BIF can also add match specifications to an exported function. A match specification comprises a pattern that the arguments to the function must match, a guard expression which must evaluate to true and an action to be performed. The default action is to send a trace message. If the pattern does not match or the guard fails, the action will not be executed.

    The MFA argument should be a tuple like {Module, Function, Arity} or the atom on_load (described below). It can be the module, function, and arity for an exported function (or a BIF in any module). The '_' atom can be used to mean any of that kind. Wildcards can be used in any of the following ways:

        {Module, Function, '_'}:
            All exported functions of any arity named Function in module Module.

        {Module, '_', '_'}:
            All exported functions in module Module.

        {'_', '_', '_'}:
            All exported functions in all loaded modules.

    Other combinations, such as {Module, '_', Arity}, are not allowed. Local functions will match wildcards only if the local option is in the FlagList.

    If the MFA argument is the atom on_load, the match specification and flag list will be used on all modules that are newly loaded.

    The MatchSpec argument can take any of the following forms:

        false:
            Disable tracing for the matching function(s). Any match specification will be removed.

        true:
            Enable tracing for the matching function(s).

        MatchSpecList:
            A list of match specifications. An empty list is equivalent to true. See the ERTS User's Guide for a description of match specifications.

        restart:
            For the FlagList option call_count: restart the existing counters. The behaviour is undefined for other FlagList options.

        pause:
            For the FlagList option call_count: pause the existing counters. The behaviour is undefined for other FlagList options.

    The FlagList parameter is a list of options. The following options are allowed:

        global:
            Turn on or off call tracing for global function calls (that is, calls specifying the module explicitly). Only exported functions will match and only global calls will generate trace messages. This is the default.

        local:
            Turn on or off call tracing for all types of function calls. Trace messages will be sent whenever any of the specified functions are called, regardless of how they are called. If the return_to flag is set for the process, a return_to message will also be sent when this function returns to its caller.

        meta | {meta, Pid}:
            Turn on or off meta tracing for all types of function calls. Trace messages will be sent to the tracer process or port Pid whenever any of the specified functions are called, regardless of how they are called. If no Pid is specified, self() is used as a default tracer process.

            Meta tracing traces all processes and does not care about the process trace flags set by trace/3, the trace flags are instead fixed to [call, timestamp].

            The match spec function {return_trace} works with meta trace and send its trace message to the same tracer process.

        call_count:
            Starts (MatchSpec == true) or stops (MatchSpec == false) call count tracing for all types of function calls. For every function a counter is incremented when the function is called, in any process. No process trace flags need to be activated.

            If call count tracing is started while already running, the count is restarted from zero. Running counters can be paused with MatchSpec == pause. Paused and running counters can be restarted from zero with MatchSpec == restart.

            The counter value can be read with erlang:trace_info/2.

    The global and local options are mutually exclusive and global is the default (if no options are specified). The call_count and meta options perform a kind of local tracing, and can also not be combined with global. A function can be either globally or locally traced. If global tracing is specified for a specified set of functions; local, meta and call count tracing for the matching set of local functions will be disabled, and vice versa.

    When disabling trace, the option must match the type of trace that is set on the function, so that local tracing must be disabled with the local option and global tracing with the global option (or no option at all), and so forth.

    There is no way to directly change part of a match specification list. If a function has a match specification, you can replace it with a completely new one. If you need to change an existing match specification, use the erlang:trace_info/2 BIF to retrieve the existing match specification.

    Returns the number of exported functions that matched the MFA argument. This will be zero if none matched at all.

trunc(Number) -> int()

    Types
        Number = number()

    Returns an integer by the truncating Number.
    > trunc(5.5).
    5

    Allowed in guard tests.

tuple_size(Tuple) -> int()

    Types
        Tuple = tuple()

    Returns an integer which is the number of elements in Tuple.
    > tuple_size({morni, mulle, bwange}).
    3

    Allowed in guard tests.

tuple_to_list(Tuple) -> [term()]

    Types
        Tuple = tuple()

    Returns a list which corresponds to Tuple. Tuple may contain any Erlang terms.
    > tuple_to_list({share, {'Ericsson_B', 163}}).
    [share,{'Ericsson_B',163}]

erlang:universaltime() -> {Date, Time}

    Types
        Date = {Year, Month, Day}
        Time = {Hour, Minute, Second}
        Year = Month = Day = Hour = Minute = Second = int()

    Returns the current date and time according to Universal Time Coordinated (UTC), also called GMT, in the form {{Year, Month, Day}, {Hour, Minute, Second}} if supported by the underlying operating system. If not, erlang:universaltime() is equivalent to erlang:localtime().
    > erlang:universaltime().
    {{1996,11,6},{14,18,43}}

erlang:universaltime_to_localtime({Date1, Time1}) -> {Date2, Time2}

    Types
        Date1 = Date2 = {Year, Month, Day}
        Time1 = Time2 = {Hour, Minute, Second}
        Year = Month = Day = Hour = Minute = Second = int()

    Converts Universal Time Coordinated (UTC) date and time to local date and time, if this is supported by the underlying OS. Otherwise, no conversion is done, and {Date1, Time1} is returned.
    > erlang:universaltime_to_localtime({{1996,11,6},{14,18,43}}).
    {{1996,11,7},{15,18,43}}

    Failure: badarg if Date1 or Time1 do not denote a valid date or time.

unlink(Id) -> true

    Types
        Id = pid() | port()

    Removes the link, if there is one, between the calling process and the process or port referred to by Id.

    Returns true and does not fail, even if there is no link to Id, or if Id does not exist.

    Once unlink(Id) has returned it is guaranteed that the link between the caller and the entity referred to by Id has no effect on the caller in the future (unless the link is setup again). If caller is trapping exits, an {'EXIT', Id, _} message due to the link might have been placed in the callers message queue prior to the call, though. Note, the {'EXIT', Id, _} message can be the result of the link, but can also be the result of Id calling exit/2. Therefore, it may be appropriate to cleanup the message queue when trapping exits after the call to unlink(Id), as follow:
        unlink(Id),
        receive
            {'EXIT', Id, _} ->
                true
        after 0 ->
                true
        end

  Note:

Prior to OTP release R11B (erts version 5.5) unlink/1 behaved completely asynchronous, i.e., the link was active until the "unlink signal" reached the linked entity. This had one undesirable effect, though. You could never know when you were guaranteed not to be effected by the link.

Current behavior can be viewed as two combined operations: asynchronously send an "unlink signal" to the linked entity and ignore any future results of the link.

unregister(RegName) -> true

    Types
        RegName = atom()

    Removes the registered name RegName, associated with a pid or a port identifier.
    > unregister(db).
    true

    Users are advised not to unregister system processes.

    Failure: badarg if RegName is not a registered name.

whereis(RegName) -> pid() | port() | undefined

    Returns the pid or port identifier with the registered name RegName. Returns undefined if the name is not registered.
    > whereis(db).
    <0.43.0>

erlang:yield() -> true

    Voluntarily let other processes (if any) get a chance to execute. Using erlang:yield() is similar to receive after 1 -> ok end, except that yield() is faster.