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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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sem_wait(3) Library Functions Manual sem_wait(3)
sem_wait, sem_timedwait, sem_trywait - lock a semaphore
POSIX threads library (libpthread, -lpthread)
#include <semaphore.h>
int sem_wait(sem_t *sem);
int sem_trywait(sem_t *sem);
int sem_timedwait(sem_t *restrict sem,
const struct timespec *restrict abs_timeout);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
sem_timedwait():
_POSIX_C_SOURCE >= 200112L
sem_wait() decrements (locks) the semaphore pointed to by sem.
If the semaphore's value is greater than zero, then the decrement
proceeds, and the function returns, immediately. If the
semaphore currently has the value zero, then the call blocks
until either it becomes possible to perform the decrement (i.e.,
the semaphore value rises above zero), or a signal handler
interrupts the call.
sem_trywait() is the same as sem_wait(), except that if the
decrement cannot be immediately performed, then call returns an
error (errno set to EAGAIN) instead of blocking.
sem_timedwait() is the same as sem_wait(), except that
abs_timeout specifies a limit on the amount of time that the call
should block if the decrement cannot be immediately performed.
The abs_timeout argument points to a timespec(3) structure that
specifies an absolute timeout in seconds and nanoseconds since
the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
If the timeout has already expired by the time of the call, and
the semaphore could not be locked immediately, then
sem_timedwait() fails with a timeout error (errno set to
ETIMEDOUT).
If the operation can be performed immediately, then
sem_timedwait() never fails with a timeout error, regardless of
the value of abs_timeout. Furthermore, the validity of
abs_timeout is not checked in this case.
All of these functions return 0 on success; on error, the value
of the semaphore is left unchanged, -1 is returned, and errno is
set to indicate the error.
EAGAIN (sem_trywait()) The operation could not be performed
without blocking (i.e., the semaphore currently has the
value zero).
EINTR The call was interrupted by a signal handler; see
signal(7).
EINVAL sem is not a valid semaphore.
EINVAL (sem_timedwait()) The value of abs_timeout.tv_nsecs is
less than 0, or greater than or equal to 1000 million.
ETIMEDOUT
(sem_timedwait()) The call timed out before the semaphore
could be locked.
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼─────────┤
│ sem_wait(), sem_trywait(), │ Thread safety │ MT-Safe │
│ sem_timedwait() │ │ │
└─────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2008.
POSIX.1-2001.
The (somewhat trivial) program shown below operates on an unnamed
semaphore. The program expects two command-line arguments. The
first argument specifies a seconds value that is used to set an
alarm timer to generate a SIGALRM signal. This handler performs
a sem_post(3) to increment the semaphore that is being waited on
in main() using sem_timedwait(). The second command-line
argument specifies the length of the timeout, in seconds, for
sem_timedwait(). The following shows what happens on two
different runs of the program:
$ ./a.out 2 3
About to call sem_timedwait()
sem_post() from handler
sem_timedwait() succeeded
$ ./a.out 2 1
About to call sem_timedwait()
sem_timedwait() timed out
Program source
#include <errno.h>
#include <semaphore.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#include <assert.h>
sem_t sem;
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
static void
handler(int sig)
{
write(STDOUT_FILENO, "sem_post() from handler\n", 24);
if (sem_post(&sem) == -1) {
write(STDERR_FILENO, "sem_post() failed\n", 18);
_exit(EXIT_FAILURE);
}
}
int
main(int argc, char *argv[])
{
struct sigaction sa;
struct timespec ts;
int s;
if (argc != 3) {
fprintf(stderr, "Usage: %s <alarm-secs> <wait-secs>\n",
argv[0]);
exit(EXIT_FAILURE);
}
if (sem_init(&sem, 0, 0) == -1)
handle_error("sem_init");
/* Establish SIGALRM handler; set alarm timer using argv[1]. */
sa.sa_handler = handler;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
if (sigaction(SIGALRM, &sa, NULL) == -1)
handle_error("sigaction");
alarm(atoi(argv[1]));
/* Calculate relative interval as current time plus
number of seconds given argv[2]. */
if (clock_gettime(CLOCK_REALTIME, &ts) == -1)
handle_error("clock_gettime");
ts.tv_sec += atoi(argv[2]);
printf("%s() about to call sem_timedwait()\n", __func__);
while ((s = sem_timedwait(&sem, &ts)) == -1 && errno == EINTR)
continue; /* Restart if interrupted by handler. */
/* Check what happened. */
if (s == -1) {
if (errno == ETIMEDOUT)
printf("sem_timedwait() timed out\n");
else
perror("sem_timedwait");
} else
printf("sem_timedwait() succeeded\n");
exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
}
clock_gettime(2), sem_getvalue(3), sem_post(3), timespec(3),
sem_overview(7), time(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 sem_wait(3)
Pages that refer to this page: PR_SET_TIMERSLACK(2const), sem_close(3), sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_post(3), sem_unlink(3), sem_overview(7), signal(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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