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sem_wait(3) — Linux manual page

sem_wait(3) Library Functions Manual sem_wait(3)

NAME top

 sem_wait, sem_timedwait, sem_trywait - lock a semaphore

LIBRARY top

 POSIX threads library (libpthread, -lpthread)

SYNOPSIS top

 #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

DESCRIPTION top

 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年01月01日 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.

RETURN VALUE top

 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.

ERRORS top

 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.

ATTRIBUTES top

 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() │ │ │
 └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

 POSIX.1-2008.

HISTORY top

 POSIX.1-2001.

EXAMPLES top

 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);
 }

SEE ALSO top

 clock_gettime(2), sem_getvalue(3), sem_post(3), timespec(3),
 sem_overview(7), time(7)

COLOPHON top

 This page is part of the man-pages (Linux kernel and C library
 user-space interface documentation) project. Information about
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 ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
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 ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
 This page was obtained from the tarball man-pages-6.10.tar.gz
 fetched from
 ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
 2025年02月02日. If you discover any rendering problems in this HTML
 version of the page, or you believe there is a better or more up-
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 part of the original manual page), send a mail to
 man-pages@man7.org
Linux man-pages 6.10 2024年07月23日 sem_wait(3)

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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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