dc(1) -- arbitrary-precision reverse-Polish notation calculator
===============================================================

SYNOPSIS
--------

`dc` [`-hiPvVx`] [`--version`] [`--help`] [`--interactive`] [`--no-prompt`]
[`--extended-register`] [`-e` *expr*] [`--expression=`*expr*...]
[`-f` *file*...] [`-file=`*file*...] [*file*...]

DESCRIPTION
-----------

dc(1) is an arbitrary-precision calculator. It uses a stack (reverse Polish
notation) to store numbers and results of computations. Arithmetic operations
pop arguments off of the stack and push the results.

If no files are given on the command-line as extra arguments (i.e., not as `-f`
or `--file` arguments), then dc(1) reads from `stdin`. Otherwise, those files
are processed, and dc(1) will then exit.

This is different from the dc(1) on OpenBSD and possibly other dc(1)
implementations, where `-e` (`--expression`) and `-f` (`--file`) arguments cause
dc(1) to execute them and exit. The reason for this is that this dc(1) allows
users to set arguments in the environment variable `DC_ENV_ARGS` (see the
ENVIRONMENT VARIABLES section). Any expressions given on the command-line should
be used to set up a standard environment. For example, if a user wants the
`scale` always set to `10`, they can set `DC_ENV_ARGS` to "-e 10k", and this
dc(1) will always start with a `scale` of `10`.

If users want to have dc(1) exit after processing all input from `-e` and `-f`
arguments (and their equivalents), then they can just simply add "-e q" as the
last command-line argument.

OPTIONS
-------

The following are the options that dc(1) accepts.

  * `-h`, `--help`:
    Prints a usage message and quits.

  * `-v`, `-V`, `--version`:
    Print the version information (copyright header) and exit.

  * `-i`, `--interactive`:
    Forces interactive mode.

    dc(1) has an interactive mode and a non-interactive mode. The interactive
    mode is turned on automatically when both `stdin` and `stdout` are hooked to
    a terminal, but this flag can turn it on in other cases. In interactive
    mode, dc(1) attempts to recover from errors (see the RESET section), and in
    normal execution, flushes `stdout` as soon as execution is done for the
    current input.

    This is a **non-portable extension**.

  * `-P`, `--no-prompt`:
    Disables the prompt in interactive mode. This is mostly for those users that
    do not want a prompt or are not used to having them in `dc`. Most of those
    users would want to put this option in `DC_ENV_ARGS`.

    If the prompt has been disabled while building dc(1), this option is a
    no-op.

    This is a **non-portable extension**.

  * `-x` `--extended-register`:
    Enables extended register mode. See the REGISTERS section for more
    information.

    This is a **non-portable extension**.

  * `-e` *expr*, `--expression`=*expr*:
    Evaluates `expr`. If multiple expressions are given, they are evaluated in
    order. If files are given as well (see below), the expressions and files are
    evaluated in the order given. This means that if a file is given before an
    expression, the file is read in and evaluated first.

    In other dc(1) implementations, this option causes the program to execute
    the expressions and then exit. This dc(1) does not, unless the
    `DC_EXPR_EXIT` is defined (see the ENVIRONMENT VARIABLES section).

    This is a **non-portable extension**.

  * `-f` *file*, `--file`=*file*:
    Reads in `file` and evaluates it. If expressions are also given (see above),
    the expressions are evaluated in the order given.

    In other dc(1) implementations, this option causes the program to execute
    the files and then exit. This dc(1) does not, unless the
    `DC_EXPR_EXIT` is defined (see the ENVIRONMENT VARIABLES section).

    This is a **non-portable extension**.

STDOUT
------

Any non-error output is written to `stdout`.

**Note**: Unlike other dc(1) implementations, this dc(1) will issue a fatal
error (see the EXIT STATUS section) if it cannot write to `stdout`, so if
`stdout` is closed, as in `dc <file> >&-`, it will quit with an error. This is
done so that dc(1) can report problems when `stdout` is redirected to a file.

If there are scripts that depend on the behavior of other dc(1) implementations,
it is recommended that those scripts be changed to redirect `stdout` to
`/dev/null`.

STDERR
------

Any error output is written to `stderr`.

**Note**: Unlike other dc(1) implementations, this dc(1) will issue a fatal
error (see the EXIT STATUS section) if it cannot write to `stderr`, so if
`stderr` is closed, as in `dc <file> 2>&-`, it will quit with an error. This is
done so that dc(1) can report problems when `stderr` is redirected to a file.

If there are scripts that depend on the behavior of other dc(1) implementations,
it is recommended that those scripts be changed to redirect `stderr` to
`/dev/null`.

SYNTAX
------

`ibase` is a register (see the REGISTERS section) determining how to interpret
constant numbers. It is the "input" base, or the number base used for
interpreting input numbers. `ibase` is initially `10`. The max allowable value
for `ibase` is `16`. The min allowable value for `ibase` is `2`. The max
allowable value for `ibase` can be queried in dc(1) programs with the `T`
command.

`obase` is a register (see the REGISTERS section) determining how to output
results. It is the "output" base, or the number base used for outputting
numbers. `obase` is initially `10`. The max allowable value for `obase` is
`DC_BASE_MAX`. The min allowable value for `obase` is `2` unless dc(1) was built
with the extra math option. If it was, then the min allowable value is `0`. In
this case, if `obase` is `0`, values are output in scientific notation, and if
`obase` is `1`, values are output in engineering notation. (Outputting in
scientific or engineering notation are **non-portable extensions**.) The max
allowable value for `obase` can be queried in dc(1) programs with the `U`
command.

The **scale** of an expression is the number of digits in the result of the
expression right of the decimal point, and `scale` is a register (see the
REGISTERS section) that sets the precision of any operations (with exceptions).
`scale` is initially `0`. `scale` cannot be negative. The max allowable value
for `scale` can be queried in dc(1) programs with the `V` command.

Each item in the input source code, either a number (see the NUMBERS section) or
a command (see the COMMANDS section), is processed and executed, in order. Input
is processed immediately when entered.

### Comments

Comments go from `#` until, and not including, the next newline. This is a
**non-portable extension**.

NUMBERS
-------

Numbers are strings made up of digits, uppercase letters up to `F`, and at most
`1` period for a radix. Numbers can have up to `DC_NUM_MAX` digits. Uppercase
letters equal `9` + their position in the alphabet (i.e., `A` equals `10`, or
`9 + 1`). If a digit or letter makes no sense with the current value of `ibase`,
they are set to the value of the highest valid digit in `ibase`.

Single-character numbers (i.e., `A`) take the value that they would have if they
were valid digits, regardless of the value of `ibase`. This means that `A`
always equals decimal `10` and `F` always equals decimal `15`.

In addition, if dc(1) was built with the extra math option, it accepts numbers
in scientific notation. For dc(1), an example is `1.89237e9`, which is equal to
`1892370000`. Negative exponents are also allowed, so `4.2890e_3` is equal to
`0.0042890`.

**WARNING**: Both the number and the exponent in scientific notation are
interpreted according to the current `ibase`, but the number is still multiplied
by `10^exponent` regardless of the current `ibase`. For example, if `ibase` is
`16` and dc(1) is given the number string `"FFeA"`, the resulting decimal number
will be `2550000000000`, and if dc(1) is given the number string `"10e_4"`, the
resulting decimal number will be `0.0016`.

Accepting input as scientific notation is a **non-portable extension**.

COMMANDS
--------

The valid commands are listed below.

### Printing

These commands are used for printing.

Note that if dc(1) has been built with the extra math option enabled, both
scientific notation and engineering notation are available for printing numbers.
Scientific notation is activated by assigning `0` to `obase` using `0o` (in any
other context, an `obase` of `0` is invalid), and engineering notation is
activated by assigning `1` to `obase` using `1o` (which is also invalid in any
other context). To deactivate them, just assign a different value to `obase`.

Printing numbers in scientific notation and/or engineering notation is a
**non-portable extension**.

  * `p`:
    Prints the value on top of the stack, whether number or string, and prints a
    newline after.

    This does not alter the stack.

  * `n`:
    Prints the value on top of the stack, whether number or string, and pops it
    off of the stack.

  * `P`:
    Pops a value off the stack.

    If the value is a number, it is truncated and the absolute value of the
    result is printed as though `obase` is `UCHAR_MAX + 1` and each digit is
    interpreted as an ASCII character, making it a byte stream.

    If the value is a string, it is printed without a trailing newline.

    This is a **non-portable extension**.

  * `f`:
    Prints the entire contents of the stack, in order from newest to oldest,
    without altering anything.

    Users should use this command when they get lost.

### Arithmetic

These are the commands used for arithmetic.

  * `+`:
    The top two values are popped off the stack, added, and the result is pushed
    onto the stack. The **scale** of the result is equal to the max **scale** of
    both operands.

  * `-`:
    The top two values are popped off the stack, subtracted, and the result is
    pushed onto the stack. The **scale** of the result is equal to the max
    **scale** of both operands.

  * `*`:
    The top two values are popped off the stack, multiplied, and the result is
    pushed onto the stack. If `a` is the **scale** of the first expression and
    `b` is the **scale** of the second expression, the **scale** of the result
    is equal to `min(a+b,max(scale,a,b))` where `min` and `max` return the
    obvious values.

  * `/`:
    The top two values are popped off the stack, divided, and the result is
    pushed onto the stack. The **scale** of the result is equal to `scale`.

    The first value popped off of the stack must be non-zero.

  * `%`:
    The top two values are popped off the stack, remaindered, and the result is
    pushed onto the stack.

    Remaindering is equivalent to 1) Computing `a/b` to current `scale`, and 2)
    Using the result of step 1 to calculate `a-(a/b)*b` to **scale**
    `max(scale + scale(b), scale(a))`.

    The first value popped off of the stack must be non-zero.

  * `~`:
    The top two values are popped off the stack, divided and remaindered, and
    the results (divided first, remainder second) are pushed onto the stack.
    This is equivalent to `x y / x y %` except that `x` and `y` are only
    evaluated once.

    The first value popped off of the stack must be non-zero.

    This is a **non-portable extension**.

  * `^`:
    The top two values are popped off the stack, the second is raised to the
    power of the first, and the result is pushed onto the stack.

    The first value popped off of the stack must be an integer, and if that
    value is negative, the second value popped off of the stack must be
    non-zero.

  * `v`:
    The top value is popped off the stack, its square root is computed, and the
    result is pushed onto the stack. The **scale** of the result is equal to
    `scale`.

    The value popped off of the stack must be non-negative.

  * `_`:
    If this command *immediately* precedes a number (i.e., no spaces or other
    commands), then that number is input as a negative number.

    Otherwise, the top value on the stack is popped and copied, and the copy is
    negated and pushed onto the stack. This behavior without a number is a
    **non-portable extension**.

  * `b`:
    The top value is popped off the stack, and if it is zero, it is pushed back
    onto the stack. Otherwise, its absolute value is pushed onto the stack.

    This is a **non-portable extension**.

  * `|`:
    The top three values are popped off the stack, a modular exponentiation is
    computed, and the result is pushed onto the stack.

    The first value popped is used as the reduction modulus and must be an
    integer and non-zero. The second value popped is used as the exponent and
    must be an integer and non-negative. The third value popped is the base and
    must be an integer.

    This is a **non-portable extension**.

  * `$`:
    The top value is popped off the stack and copied, and the copy is truncated
    and pushed onto the stack.

    This is a **non-portable extension**.

  * `@`:
    The top two values are popped off the stack, and the precision of the second
    is set to the value of the first, whether by truncation or extension.

    The first value popped off of the stack must be an integer and non-negative.

    This is a **non-portable extension**.

  * `H`:
    The top two values are popped off the stack, and the second is shifted left
    (radix shifted right) to the value of the first.

    The first value popped off of the stack must be an integer and non-negative.

    This is a **non-portable extension**.

  * `h`:
    The top two values are popped off the stack, and the second is shifted right
    (radix shifted left) to the value of the first.

    The first value popped off of the stack must be an integer and non-negative.

    This is a **non-portable extension**.

  * `G`:
    The top two values are popped off of the stack, they are compared, and a `1`
    is pushed if they are equal, or `0` otherwise.

    This is a **non-portable extension**.

  * `N`:
    The top value is popped off of the stack, and if it a `0`, a `1` is pushed;
    otherwise, a `0` is pushed.

    This is a **non-portable extension**.

  * `(`:
    The top two values are popped off of the stack, they are compared, and a `1`
    is pushed if the first is less than the second, or `0` otherwise.

    This is a **non-portable extension**.

  * `{`:
    The top two values are popped off of the stack, they are compared, and a `1`
    is pushed if the first is less than or equal to the second, or `0`
    otherwise.

    This is a **non-portable extension**.

  * `)`:
    The top two values are popped off of the stack, they are compared, and a `1`
    is pushed if the first is greater than the second, or `0` otherwise.

    This is a **non-portable extension**.

  * `}`:
    The top two values are popped off of the stack, they are compared, and a `1`
    is pushed if the first is greater than or equal to the second, or `0`
    otherwise.

    This is a **non-portable extension**.

  * `M`:
    The top two values are popped off of the stack. If they are both non-zero, a
    `1` is pushed onto the stack. If either of them is zero, or both of them
    are, then a `0` is pushed onto the stack.

    This is like the `&&` operator in bc(1), and it is not a short-circuit
    operator.

    This is a **non-portable extension**.

  * `m`:
    The top two values are popped off of the stack. If at least one of them is
    non-zero, a `1` is pushed onto the stack. If both of them are zero, then a
    `0` is pushed onto the stack.

    This is like the `||` operator in bc(1), and it is not a short-circuit
    operator.

    This is a **non-portable extension**.

### Stack Control

These commands control the stack.

  * `c`:
    Removes all items from ("clears") the stack.

  * `d`:
    Copies the item on top of the stack ("duplicates") and pushes the copy onto
    the stack.

  * `r`:
    Swaps ("reverses") the two top items on the stack.

  * `R`:
    Pops ("removes") the top value from the stack.

### Register Control

These commands control registers (see the REGISTERS section).

  * `s`*r*:
    Pops the value off the top of the stack and stores it into register `r`.

  * `l`*r*:
    Copies the value in register `r` and pushes it onto the stack. This does not
    alter the contents of `r`.

  * `S`*r*:
    Pops the value off the top of the (main) stack and pushes it onto the stack
    of register `r`. The previous value of the register becomes inaccessible.

  * `L`*r*:
    Pops the value off the top of the stack for register `r` and push it onto
    the main stack. The previous value in the stack for register `r`, if any, is
    now accessible via the `l`*r* command.

### Parameters

These commands control the values of `ibase`, `obase`, and `scale` (see the
SYNTAX section).

  * `i`:
    Pops the value off of the top of the stack and uses it to set `ibase`, which
    must be between `2` and `16`, inclusive.

    If the value on top of the stack has any **scale**, the **scale** is
    ignored.

  * `o`:
    Pops the value off of the top of the stack and uses it to set `obase`, which
    must be between `2` and `DC_BASE_MAX`, inclusive (see bc(1)). The value can
    be either `0` or `1` if dc(1) was built with the extra math option.

    If the value on top of the stack has any **scale**, the **scale** is
    ignored.

  * `k`:
    Pops the value off of the top of the stack and uses it to set `scale`, which
    must be non-negative.

    If the value on top of the stack has any **scale**, the **scale** is
    ignored.

  * `I`:
    Pushes the current value of `ibase` onto the main stack.

  * `O`:
    Pushes the current value of `obase` onto the main stack.

  * `K`:
    Pushes the current value of `scale` onto the main stack.

  * `T`:
    Pushes the maximum allowable value of `ibase` onto the main stack.

    This is a **non-portable extension**.

  * `U`:
    Pushes the maximum allowable value of `obase` onto the main stack.

    This is a **non-portable extension**.

  * `V`:
    Pushes the maximum allowable value of `scale` onto the main stack.

    This is a **non-portable extension**.

### Strings

The following commands control strings.

dc(1) can work with both numbers and strings, and registers (see the REGISTERS
section) can hold both strings and numbers. dc(1) always knows whether the
contents of a register are a string or a number.

While arithmetic operations have to have numbers, and will print an error if
given a string, other commands accept strings.

Strings can also be executed as macros. For example, if the string `[1pR]` is
executed as a macro, then the code `1pR` is executed, meaning that the `1` will
be printed with a newline after and then popped from the stack.

  * `[`*characters*`]`:
    Makes a string containing *characters* and pushes it onto the stack.

    If there are brackets (`[` and `]`) in the string, then they must be
    balanced. Unbalanced brackets can be escaped using a backslash (`\`)
    character.

    If there is a backslash character in the string, the character after it
    (even another backslash) is put into the string verbatim, but the (first)
    backslash is not.

  * `a`:
    The value on top of the stack is popped.

    If it is a number, it is truncated and its absolute value is taken. The
    result mod `UCHAR_MAX + 1` is calculated. If that result is `0`, push an
    empty string; otherwise, push a one-character string where the character is
    the result of the mod interpreted as an ASCII character.

    If it is a string, then a new string is made. If the original string is
    empty, the new string is empty. If it is not, then the first character of
    the original string is used to create the new string as a one-character
    string. The new string is then pushed onto the stack.

    This is a **non-portable extension**.

  * `x`:
    Pops a value off of the top of the stack.

    If it is a number, it is pushed onto the stack.

    If it is a string, it is executed as a macro.

    This behavior is the norm whenever a macro is executed, whether by this
    command or by the conditional execution commands below.

  * `>`*r*:
    Pops two values off of the stack that must be numbers and compares them. If
    the first value is greater than the second, then the contents of register
    `r` are executed.

    For example, `0 1>a` will execute the contents of register `a`, and `1 0>a`
    will not.

  * `>`*r*`e`*s*:
    Like the above, but will execute register `s` if the comparison fails.

    This is a **non-portable extension**.

  * `!>`*r*:
    Pops two values off of the stack that must be numbers and compares them. If
    the first value is not greater than the second (less than or equal to), then
    the contents of register `r` are executed.

  * `!>`*r*`e`*s*:
    Like the above, but will execute register `s` if the comparison fails.

    This is a **non-portable extension**.

  * `<`*r*:
    Pops two values off of the stack that must be numbers and compares them. If
    the first value is less than the second, then the contents of register `r`
    are executed.

  * `<`*r*`e`*s*:
    Like the above, but will execute register `s` if the comparison fails.

    This is a **non-portable extension**.

  * `!<`*r*:
    Pops two values off of the stack that must be numbers and compares them. If
    the first value is not less than the second (greater than or equal to), then
    the contents of register `r` are executed.

  * `!<`*r*`e`*s*:
    Like the above, but will execute register `s` if the comparison fails.

    This is a **non-portable extension**.

  * `=`*r*:
    Pops two values off of the stack that must be numbers and compares them. If
    the first value is equal to the second, then the contents of register `r`
    are executed.

  * `=`*r*`e`*s*:
    Like the above, but will execute register `s` if the comparison fails.

    This is a **non-portable extension**.

  * `!=`*r*:
    Pops two values off of the stack that must be numbers and compares them. If
    the first value is not equal to the second, then the contents of register
    `r` are executed.

  * `!=`*r*`e`*s*:
    Like the above, but will execute register `s` if the comparison fails.

    This is a **non-portable extension**.

  * `?`:
    Reads a line from the `stdin` and executes it. This is to allow macros to
    request input from users.

  * `q`:
    During execution of a macro, this exits the execution of that macro and the
    execution of the macro that executed it. If there are no macros, or only one
    macro executing, dc(1) exits.

  * `Q`:
    Pops a value from the stack which must be non-negative and is used the
    number of macro executions to pop off of the execution stack. If the number
    of levels to pop is greater than the number of executing macros, dc(1)
    exits.

### Status

These commands query status of the stack or its top value.

  * `Z`:
    Pops a value off of the stack.

    If it is a number, calculates the number of significant decimal digits it
    has and pushes the result.

    If it is a string, pushes the number of characters the string has.

  * `X`:
    Pops a value off of the stack.

    If it is a number, pushes the **scale** of the value onto the stack.

    If it is a string, pushes `0`.

  * `z`:
    Pushes the current stack depth (before execution of this command).

### Arrays

These commands manipulate arrays.

  * `:`*r*:
    Pops the top two values off of the stack. The second value will be stored in
    the array `r` (see the REGISTERS section), indexed by the first value.

  * `;`*r*:
    Pops the value on top of the stack and uses it as an index into the array
    `r`. The selected value is then pushed onto the stack.

REGISTERS
---------

Registers are names that can store strings, numbers, and arrays. (Number/string
registers do not interfere with array registers.)

Each register is also its own stack, so the current register value is the top of
the stack for the register. All registers, when first referenced, have one value
(`0`) in their stack.

In non-extended register mode, a register name is just the single character that
follows any command that needs a register name. The only exception is a newline
(`'\n'`); it is a parse error for a newline to be used as a register name.

### Extended Register Mode

Unlike most other dc(1) implentations, this dc(1) provides nearly unlimited
amounts of registers, if extended register mode is enabled.

If extended register mode is enabled (`-x` or `--extended-register` command-line
arguments are given), then normal single character registers are used
***unless*** the character immediately following a command that needs a register
name is a space (according to `isspace()`) and not a newline (`'\n'`).

In that case, the register name is found according to the regex
`[a-z][a-z0-9_]*` (like bc(1)), and it is a parse error if the next
non-space characters do not match that regex.

RESET
-----

When dc(1) encounters an error or a signal that it has a non-default handler
for, it resets. This means that several things happen.

First, any macros that are executing are stopped and popped off the stack.
The behavior is not unlike that of exceptions in programming languages. Then
the execution point is set so that any code waiting to execute (after all
functions returned) is skipped.

Thus, when dc(1) resets, it skips any remaining code waiting to be executed.
Then, if it is interactive mode, and the error was not a fatal error (see the
EXIT STATUS section), it asks for more input; otherwise, it exits with the
appropriate return code.

PERFORMANCE
-----------

Most dc(1) implementations use `char` types to calculate the value of `1`
decimal digit at a time, but that can be slow. This dc(1) does something
different.

It uses large integers to calculate more than `1` decimal digit at a time. If
built in a environment where `DC_LONG_BIT` (see the LIMITS section) is `64`,
then each integer has `9` decimal digits. If built in an environment where
`DC_LONG_BIT` is `32` then each integer has `4` decimal digits. This value (the
number of decimal digits per large integer) is called `DC_BASE_DIGS`.

In addition, this dc(1) uses an even larger integer for overflow checking. This
integer type depends on the value of `DC_LONG_BIT`, but is always at least twice
as large as the integer type used to store digits.

LIMITS
------

The following are the limits on dc(1):

  * `DC_LONG_BIT`:
    The number of bits in the `long` type in the environment where dc(1) was
    built. This determines how many decimal digits can be stored in a single
    large integer (see the PERFORMANCE section).

  * `DC_BASE_DIGS`:
    The number of decimal digits per large integer (see the PERFORMANCE
    section). Depends on `DC_LONG_BIT`.

  * `DC_BASE_POW`:
    The max decimal number that each large integer can store (see
    `DC_BASE_DIGS`) plus `1`. Depends on `DC_BASE_DIGS`.

  * `DC_OVERFLOW_MAX`:
    The max number that the overflow type (see the PERFORMANCE section) can
    hold. Depends on `DC_LONG_BIT`.

  * `DC_BASE_DIGS`:
    The number of decimal digits per large integer (see the PERFORMANCE
    section).

  * `DC_BASE_MAX`:
    The maximum output base. Set at `DC_BASE_POW`.

  * `DC_DIM_MAX`:
    The maximum size of arrays. Set at `SIZE_MAX-1`.

  * `DC_SCALE_MAX`:
    The maximum `scale`. Set at `DC_OVERFLOW_MAX-1`.

  * `DC_STRING_MAX`:
    The maximum length of strings. Set at `DC_OVERFLOW_MAX-1`.

  * `DC_NAME_MAX`:
    The maximum length of identifiers. Set at `DC_OVERFLOW_MAX-1`.

  * `DC_NUM_MAX`:
    The maximum length of a number (in decimal digits), which includes digits
    after the decimal point. Set at `DC_OVERFLOW_MAX-1`.

  * Exponent:
    The maximum allowable exponent (positive or negative). Set at
    `DC_OVERFLOW_MAX`.

  * Number of vars:
    The maximum number of vars/arrays. Set at `SIZE_MAX-1`.

These limits are meant to be effectively non-existent; the limits are so large
(at least on 64-bit machines) that there should not be any point at which they
become a problem. In fact, memory should be exhausted before these limits should
be hit.

ENVIRONMENT VARIABLES
---------------------

dc(1) recognizes the following environment variables:

  * `DC_ENV_ARGS`:
    This is another way to give command-line arguments to dc(1). They should be
    in the same format as all other command-line arguments. These are always
    processed first, so any files given in `DC_ENV_ARGS` will be processed
    before files given on the command-line. This gives the user the ability to
    set up "standard" options and files to be used at every invocation. The most
    useful thing for such files to contain would be useful functions that the
    user might want every time dc(1) runs. Another use would be to use the `-e`
    option to set `scale` to a value other than `0`.

  * `DC_LINE_LENGTH`:
    If this environment variable exists and contains an integer that is greater
    than `1` and is less than `UINT16_MAX` (`2^16-1`), dc(1) will output lines
    to that length, including the backslash newline combo. The default line
    length is `70`.

  * `DC_EXPR_EXIT`:
    If this variable exists (no matter the contents), dc(1) will exit
    immediately after executing expressions and files given by the `-e` and/or
    `-f` command-line options (and any equivalents).

EXIT STATUS
-----------

dc(1) returns the following exit statuses:

  * `0`:
    No error.

  * `1`:
    A math error occurred. This follows standard practice of using `1` for
    expected errors, since math errors will happen in the process of normal
    execution.

    Math errors include divide by `0`, taking the square root of a negative
    number, attempting to convert a negative number to a hardware integer,
    overflow when converting a number to a hardware integer, and attempting to
    use a non-integer where an integer is required.

    Converting to a hardware integer happens for the second operand of the power
    (`^`), places (`@`), left shift (`H`), and right shift (`h`) operators.

  * `2`:
    A parse error occurred.

    Parse errors include unexpected `EOF`, using an invalid character, failing
    to find the end of a string or comment, and using a token where it is
    invalid.

  * `3`:
    A runtime error occurred.

    Runtime errors include assigning an invalid number to `ibase`, `obase`, or
    `scale`; give a bad expression to a `read()` call, calling `read()` inside
    of a `read()` call, type errors, and attempting an operation when the stack
    has too few elements.

  * `4`:
    A fatal error occurred.

    Fatal errors include memory allocation errors, I/O errors, failing to open
    files, attempting to use files that do not have only ASCII characters (dc(1)
    only accepts ASCII characters), attempting to open a directory as a file,
    and giving invalid command-line options.

The exit status `4` is special; when a fatal error occurs, dc(1) always exits
and returns `4`, no matter what mode dc(1) is in.

The other statuses will only be returned when dc(1) is not in interactive mode,
since dc(1) resets its state (see the RESET section) and accepts more input when
one of those errors occurs in interactive mode. This is also the case when
interactive mode is forced by the `-i` option.

These exit statuses allow dc(1) to be used in shell scripting with error
checking, and its normal behavior can be forced by using `-i`.

SIGNAL HANDLING
---------------

If dc(1) has been compiled with the signal handling, sending a `SIGINT` will
cause dc(1) to stop execution of the current input and reset (see the RESET
section), asking for more input.

Otherwise, `SIGTERM` and `SIGQUIT` cause dc(1) to clean up and exit, and it uses
the default handler for all other signals.

If dc(1) has not been compiled with signal handling, it uses the default signal
handlers for all signals.

COMMAND LINE HISTORY
--------------------

dc(1) supports interactive command-line editing, if compiled with the history
option enabled. If `stdin` is hooked to a terminal, it is enabled. Previous
lines can be recalled and edited with the arrow keys.

**Note**: when dc(1) is built with history support, tabs are converted to 8
spaces.

LOCALES
-------

This dc(1) ships with support for adding error messages for different locales.

SEE ALSO
--------

bc(1)

STANDARDS
---------

The dc(1) utility operators are compliant with the operators in the bc(1)
[IEEE Std 1003.1-2017 (“POSIX.1-2017”)][1] specification.

AUTHOR
------

This dc(1) was made from scratch by Gavin D. Howard.

BUGS
----

None are known. Report bugs at https://github.com/gavinhoward/bc.

[1]: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/bc.html
