THREAD(3)

NAME
alt, chancreate, chanfree, chanprint, chansetname, mainstacksize, proccreate, procdata, recv, recvp, recvul, send, sendp, sendul, nbrecv, nbrecvp, nbrecvul, nbsend, nbsendp, nbsendul, threadcreate, threaddata, threadexec, threadexecl, threadexits, threadexitsall, threadgetgrp, threadgetname, threadint, threadintgrp, threadkill, threadkillgrp, threadmain, threadmaybackground, threadnotify, threadid, threadpid, threadpin, threadunpin, threadsetgrp, threadsetname, threadsetstate, threadspawn, threadspawnd, threadspawnl, threadwaitchan, yield - thread and proc management

SYNOPSIS

    #include <u.h>
    #include <libc.h>
    #include <thread.h>

    #define	CHANEND		0
    #define	CHANSND		1
    #define	CHANRCV		2
    #define	CHANNOP		3
    #define	CHANNOBLK	4

    typedef struct Alt Alt;
    struct Alt {
    	Channel	*c;
    	void	*v;
    	int	op;
    	Channel	**tag;
    	int	entryno;
    	char	*name;
    };

void	threadmain(int argc, char *argv[])
int	threadmaybackground(void)
int	mainstacksize
int	proccreate(void (*fn)(void*), void *arg, uint stacksize)
int	threadcreate(void (*fn)(void*), void *arg, uint stacksize)
void	threadexits(char *status)
void	threadexitsall(char *status)
void	yield(void)
int	threadpin(void)
int	threadunpin(void)
int	threadid(void)
int	threadgrp(void)
int	threadsetgrp(int group)
int	threadpid(int id)
int	threadint(int id)
int	threadintgrp(int group)
int	threadkill(int id)
int	threadkillgrp(int group)
void	threadsetname(char *name)
char*	threadgetname(void)
void**	threaddata(void)
void**	procdata(void)
Channel*	chancreate(int elsize, int nel)
void	chanfree(Channel *c)
int	alt(Alt *alts)
int	recv(Channel *c, void *v)
void*	recvp(Channel *c)
ulong	recvul(Channel *c)
int	nbrecv(Channel *c, void *v)
void*	nbrecvp(Channel *c)
ulong	nbrecvul(Channel *c)
int	send(Channel *c, void *v)
int	sendp(Channel *c, void *v)
int	sendul(Channel *c, ulong v)
int	nbsend(Channel *c, void *v)
int	nbsendp(Channel *c, void *v)
int	nbsendul(Channel *c, ulong v)
int	chanprint(Channel *c, char *fmt, ...)
int	threadspawnl(int fd[3], char *file, ...)
int	threadspawn(int fd[3], char *file, char *args[])
int	threadspawnd(int fd[3], char *file, char *args[], char *dir)
int	threadexecl(Channel *cpid, int fd[3], char *file, ...)
int	threadexec(Channel *cpid, int fd[3], char *file, char *args[])
Channel*	threadwaitchan(void)
int	threadnotify(int (*f)(void*, char*), int in)

DESCRIPTION

The thread library provides parallel programming support similar to that of the languages Alef and Newsqueak. Threads and procs occupy a shared address space, communicating and synchronizing through channels and shared variables.

A proc is a Plan 9 process that contains one or more cooperatively scheduled threads. Programs using threads must replace main by threadmain. The thread library provides a main function that sets up a proc with a single thread executing threadmain.

Every thread is backed by an operating system-provided pthread and runs on its system-provided stack; mainstacksize and the the stack size arguments to proccreate and threadcreate are ignored. Although each thread is backed by a separate pthread, the threads in a proc are still scheduled non-preemptively as on Plan 9 and as described below.

Threadcreate creates a new thread in the calling proc, returning a unique integer identifying the thread; the thread executes fn(arg) on a stack of size stacksize. Thread stacks are allocated in shared memory, making it valid to pass pointers to stack variables between threads and procs. Proccreate creates a new proc, and inside that proc creates a single thread as threadcreate would, returning the id of the created thread. Be aware that the calling thread may continue execution before the newly created proc and thread are scheduled. Because of this, arg should not point to data on the stack of a function that could return before the new process is scheduled.

Threadexits terminates the calling thread. If the thread is the last in its proc, threadexits also terminates the proc, using status as the exit status. Threadexitsall terminates all procs in the program, using status as the exit status.

When the last thread in threadmain's proc exits, the program will appear to its parent to have exited. The remaining procs will still run together, but as a background program. This functionality can only be relied upon if the program defines a function threadmaybackground returning a non-zero result. Programs that do not define such a threadmaybackground will crash instead should the last thread in threadmain's proc exit leaving behind other running procs.

The threads in a proc are coroutines, scheduled nonpreemptively in a round-robin fashion. A thread must explicitly relinquish control of the processor before another thread in the same proc is run. Calls that do this are yield, proccreate, threadexec, threadexecl, threadexits, threadspawn, threadspawnd, threadspawnl, alt, send, and recv (and the calls related to send and recv—see their descriptions further on). Procs are scheduled by the operating system. Therefore, threads in different procs can preempt one another in arbitrary ways and should synchronize their actions using qlocks (see lock(3)) or channel communication. System calls such as read(3) block the entire proc; all threads in a proc block until the system call finishes.

Threadpin disables scheduling inside a proc, `pinning' the current thread as the only runnable one in the current proc. Threadunpin reenables scheduling, allowing other procs to run once the current thread relinquishes the processor. Threadpin and threadunpin can lead to deadlock. Used carefully, they can make library routines that use qlocks appear atomic relative to the current proc, like a system call.

As mentioned above, each thread has a unique integer thread id. Thread ids are not reused; they are unique across the life of the program. Threadid returns the id for the current thread. Each thread also has a thread group id. The initial thread has a group id of zero. Each new thread inherits the group id of the thread that created it. Threadgrp returns the group id for the current thread; threadsetgrp sets it. Threadpid returns the pid of the Plan 9 process containing the thread identified by id, or -1 if no such thread is found.

Threadint interrupts a thread that is blocked in a channel operation or system call. Threadintgrp interrupts all threads with the given group id. Threadkill marks a thread to die when it next relinquishes the processor (via one of the calls listed above). If the thread is blocked in a channel operation or system call, it is also interrupted. Threadkillgrp kills all threads with the given group id. Note that threadkill and threadkillgrp will not terminate a thread that never relinquishes the processor.

Primarily for debugging, threads can have string names associated with them. Threadgetname returns the current thread's name; threadsetname sets it. The pointer returned by threadgetname is only valid until the next call to threadsetname.

Also for debugging, threads have a string state associated with them. Threadsetstate sets the state string. There is no threadgetstate; since the thread scheduler resets the state to Running every time it runs the thread, it is only useful for debuggers to inspect the state.

Threaddata returns a pointer to a per-thread pointer that may be modified by threaded programs for per-thread storage. Similarly, procdata returns a pointer to a per-proc pointer.

Threadexecl and threadexec are threaded analogues of exec and execl (see exec(3)); on success, they replace the calling thread and invoke the external program, never returning. (Unlike on Plan 9, the calling thread need not be the only thread in its proc—the other threads will continue executing.) On error, they return -1. If cpid is not null, the pid of the invoked program will be sent along cpid (using sendul) once the program has been started, or -1 will be sent if an error occurs. Threadexec and threadexecl will not access their arguments after sending a result along cpid. Thus, programs that malloc the argv passed to threadexec can safely free it once they have received the cpid response.

Threadexecl and threadexec will duplicate (see dup(3)) the three file descriptors in fd onto standard input, output, and error for the external program and then close them in the calling thread. Beware of code that sets

        fd[0] = 0;
        fd[1] = 1;
        fd[2] = 2;

to use the current standard files. The correct code is

        fd[0] = dup(0, -1);
        fd[1] = dup(1, -1);
        fd[2] = dup(2, -1);

Threadspawnl and threadspawn are like threadexecl and threadexec but do not replace the current thread. They return the pid of the invoked program on success, or -1 on error. Threadspawnd is like threadspawn but takes as its final argument the directory in which to run the invoked program. The child will attempt to change into that directory before running the program, but it is only best effort: failure to change into the directory does not stop the running of the program.

Threadwaitchan returns a channel of pointers to Waitmsg structures (see wait(3)). When an exec'ed process exits, a pointer to a Waitmsg is sent to this channel. These Waitmsg structures have been allocated with malloc(3) and should be freed after use.

A Channel is a buffered or unbuffered queue for fixed-size messages. Procs and threads send messages into the channel and recv messages from the channel. If the channel is unbuffered, a send operation blocks until the corresponding recv operation occurs and vice versa. Chancreate allocates a new channel for messages of size elsize and with a buffer holding nel messages. If nel is zero, the channel is unbuffered. Chanfree frees a channel that is no longer used. Chanfree can be called by either sender or receiver after the last item has been sent or received. Freeing the channel will be delayed if there is a thread blocked on it until that thread unblocks (but chanfree returns immediately).

The name element in the Channel structure is a description intended for use in debugging. Chansetname sets the name.

Send sends the element pointed at by v to the channel c. If v is null, zeros are sent. Recv receives an element from c and stores it in v. If v is null, the received value is discarded. Send and recv return 1 on success, -1 if interrupted. Nbsend and nbrecv behave similarly, but return 0 rather than blocking.

Sendp, nbsendp, sendul, and nbsendul send a pointer or an unsigned long; the channel must have been initialized with the appropriate elsize. Recvp, nbrecvp, recvul, and nbrecvul receive a pointer or an unsigned long; they return zero when a zero is received, when interrupted, or (for nbrecvp and nbrecvul) when the operation would have blocked. To distinguish between these three cases, use recv or nbrecv.

Alt can be used to recv from or send to one of a number of channels, as directed by an array of Alt structures, each of which describes a potential send or receive operation. In an Alt structure, c is the channel; v the value pointer (which may be null); and op the operation: CHANSND for a send operation, CHANRECV for a recv operation; CHANNOP for no operation (useful when alt is called with a varying set of operations). The array of Alt structures is terminated by an entry with op CHANEND or CHANNOBLK. If at least one Alt structure can proceed, one of them is chosen at random to be executed. Alt returns the index of the chosen structure. If no operations can proceed and the list is terminated with CHANNOBLK, alt returns the index of the terminating CHANNOBLK structure. Otherwise, alt blocks until one of the operations can proceed, eventually returning the index of the structure executes. Alt returns -1 when interrupted. The tag and entryno fields in the Alt structure are used internally by alt and need not be initialized. They are not used between alt calls.

Chanprint formats its arguments in the manner of print(3) and sends the result to the channel c. The string delivered by chanprint is allocated with malloc(3) and should be freed upon receipt.

Thread library functions do not return on failure; if errors occur, the entire program is aborted.

Threaded programs should use threadnotify in place of atnotify (see notify(3)).

It is safe to use sysfatal(3) in threaded programs. Sysfatal will print the error string and call threadexitsall.

It is not safe to call rfork in a threaded program, except to call rfork(RFNOTEG) from the main proc before any other procs have been created. To create new processes, use proccreate.

FILES
/acid/thread contains useful acid(1) functions for debugging threaded programs.

/src/libthread/test contains some example programs.

SOURCE
/src/libthread

SEE ALSO
intro(3), ioproc(3)

BUGS
To avoid name conflicts, alt, nbrecv, nbrecvp, nbrecvul, nbsend, nbsendp, nbsendul, recv, recvp, recvul, send, sendp, and sendul are defined as macros that expand to chanalt, channbrecv, and so on. Yield is defined as a macro that expands to threadyield. See intro(3).

Threadint, threadintgrp, threadkill, threadkillgrp and threadpid are unimplemented.

The implementation of threadnotify may not be correct.

There appears to be a race in the Linux NPTL implementation of pthread_exit . Call threadexitsall rather than coordinating a simultaneous threadexits among many threads.