mirror of https://github.com/mkerrisk/man-pages
269 lines
7.1 KiB
Groff
269 lines
7.1 KiB
Groff
'\" t
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.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.TH SEM_WAIT 3 2007-07-26 "Linux" "Linux Programmer's Manual"
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.SH NAME
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sem_wait, sem_timedwait, sem_trywait \- lock a semaphore
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.SH SYNOPSIS
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.nf
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.B #include <semaphore.h>
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.sp
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.BI "int sem_wait(sem_t *" sem );
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.sp
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.BI "int sem_trywait(sem_t *" sem );
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.sp
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.BI "int sem_timedwait(sem_t *" sem ", const struct timespec *" abs_timeout );
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.fi
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.sp
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Link with \fI\-lrt\fP or \fI\-pthread\fP.
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.sp
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.in -4n
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.in
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.sp
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.BR sem_timedwait ():
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_POSIX_C_SOURCE\ >=\ 200112L || _XOPEN_SOURCE\ >=\ 600
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.SH DESCRIPTION
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.BR sem_wait ()
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decrements (locks) the semaphore pointed to by
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.IR sem .
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If the semaphore's value is greater than zero,
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then the decrement proceeds, and the function returns, immediately.
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If the semaphore currently has the value zero,
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then the call blocks until either it becomes possible to perform
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the decrement (i.e., the semaphore value rises above zero),
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or a signal handler interrupts the call.
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.BR sem_trywait ()
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is the same as
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.BR sem_wait (),
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except that if the decrement cannot be immediately performed,
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then call returns an error
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.RI ( errno
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set to
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.BR EAGAIN )
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instead of blocking.
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.BR sem_timedwait ()
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is the same as
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.BR sem_wait (),
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except that
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.I abs_timeout
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specifies a limit on the amount of time that the call
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should block if the decrement cannot be immediately performed.
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The
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.I abs_timeout
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argument points to a structure that specifies an absolute timeout
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in seconds and nanoseconds since the Epoch (00:00:00, 1 January 1970).
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This structure is defined as follows:
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.nf
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.in +4n
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struct timespec {
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time_t tv_sec; /* Seconds */
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long tv_nsec; /* Nanoseconds [0 .. 999999999] */
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};
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.in
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.fi
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.PP
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If the timeout has already expired by the time of the call,
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and the semaphore could not be locked immediately,
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then
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.BR sem_timedwait ()
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fails with a timeout error
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.RI ( errno
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set to
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.BR ETIMEDOUT ).
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If the operation can be performed immediately, then
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.BR sem_timedwait ()
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never fails with a timeout error, regardless of the value of
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.IR abs_timeout .
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Furthermore, the validity of
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.I abs_timeout
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is not checked in this case.
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.SH RETURN VALUE
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All of these functions return 0 on success;
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on error, the value of the semaphore is left unchanged,
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\-1 is returned, and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EINTR
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The call was interrupted by a signal handler.
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.TP
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.B EINVAL
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.I sem
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is not a valid semaphore.
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.PP
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The following additional error can occur for
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.BR sem_trywait ():
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.TP
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.B EAGAIN
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The operation could not be performed without blocking (i.e., the
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semaphore currently has the value zero).
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.PP
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The following additional errors can occur for
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.BR sem_timedwait ():
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.TP
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.B EINVAL
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The value of
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.I abs_timeout.tv_nsecs
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is less than 0, or greater than or equal to 1000 million.
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.TP
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.B ETIMEDOUT
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The call timed out before the semaphore could be locked.
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.\" POSIX.1-2001 also allows EDEADLK -- "A deadlock condition
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.\" was detected", but this does not occur on Linux(?).
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.SH CONFORMING TO
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POSIX.1-2001.
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.SH NOTES
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A signal handler always interrupts a blocked call to
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one of these functions, regardless of the use of the
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.BR sigaction (2)
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.B SA_RESTART
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flag.
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.\" sem_wait() is always interrupted on most other implementations,
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.\" but on FreeBSD 5.4 SA_RESTART does cause restarting.
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.SH EXAMPLE
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.PP
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The (somewhat trivial) program shown below operates on an
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unnamed semaphore.
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The program expects two command-line arguments.
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The first argument specifies a seconds value that is used to
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set an alarm timer to generate a
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.B SIGALRM
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signal.
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This handler performs a
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.BR sem_post (3)
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to increment the semaphore that is being waited on in
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.I main()
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using
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.BR sem_timedwait ().
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The second command-line argument specifies the length
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of the timeout, in seconds, for
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.BR sem_timedwait ().
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The following shows what happens on two different runs of the program:
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.in +4n
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.nf
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$ ./a.out 2 3
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About to call sem_timedwait()
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sem_post() from handler
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sem_getvalue() from handler; value = 1
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sem_timedwait() succeeded
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$ ./a.out 2 1
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About to call sem_timedwait()
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sem_timedwait() timed out
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.fi
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.in
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.PP
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The source code of the program is as follows:
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.nf
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#include <unistd.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <semaphore.h>
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#include <time.h>
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#include <assert.h>
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#include <errno.h>
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#include <signal.h>
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sem_t sem;
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#define handle_error(msg) \\
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do { perror(msg); exit(EXIT_FAILURE); } while (0)
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static void
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handler(int sig)
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{
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write(STDOUT_FILENO, "sem_post() from handler\\n", 24);
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if (sem_post(&sem) == \-1) {
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write(STDERR_FILENO, "sem_post() failed\\n", 18);
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_exit(EXIT_FAILURE);
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}
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}
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int
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main(int argc, char *argv[])
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{
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struct sigaction sa;
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struct timespec ts;
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int s;
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if (argc != 3) {
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fprintf(stderr, "Usage: %s <alarm\-secs> <wait\-secs>\\n",
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argv[0]);
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exit(EXIT_FAILURE);
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}
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if (sem_init(&sem, 0, 0) == \-1)
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handle_error("sem_init");
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/* Establish SIGALRM handler; set alarm timer using argv[1] */
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sa.sa_handler = handler;
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sigemptyset(&sa.sa_mask);
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sa.sa_flags = 0;
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if (sigaction(SIGALRM, &sa, NULL) == \-1)
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handle_error("sigaction");
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alarm(atoi(argv[1]));
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/* Calculate relative interval as current time plus
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number of seconds given argv[2] */
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if (clock_gettime(CLOCK_REALTIME, &ts) == \-1)
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handle_error("clock_gettime");
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ts.tv_sec += atoi(argv[2]);
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printf("main() about to call sem_timedwait()\\n");
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while ((s = sem_timedwait(&sem, &ts)) == \-1 && errno == EINTR)
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continue; /* Restart if interrupted by handler */
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/* Check what happened */
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if (s == \-1) {
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if (errno == ETIMEDOUT)
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printf("sem_timedwait() timed out\\n");
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else
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perror("sem_timedwait");
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} else
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printf("sem_timedwait() succeeded\\n");
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exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
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}
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.fi
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.SH "SEE ALSO"
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.\" FIXME . when this page has been added to the man-pages set:
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.\" .BR clock_gettime (3),
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.BR sem_getvalue (3),
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.BR sem_post (3),
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.BR sem_overview (7)
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