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sem_wait(3) — Linux manual page

NAME \|LIBRARY \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|ERRORS \|ATTRIBUTES \|STANDARDS \|HISTORY \|EXAMPLES \|SEE ALSO \|COLOPHON

sem_wait(3)              Library Functions Manualsem_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 *restrictsem,const struct timespec *restrictabs_timeout);   Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):sem_timedwait():           _POSIX_C_SOURCE >= 200112L

DESCRIPTION top

sem_wait() decrements (locks) the semaphore pointed to bysem.  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 assem_wait(), except that if the       decrement cannot be immediately performed, then call returns an       error (errno set toEAGAIN) instead of blocking.sem_timedwait() is the same assem_wait(), except thatabs_timeout       specifies a limit on the amount of time that the call should block       if the decrement cannot be immediately performed.  Theabs_timeout       argument points to atimespec(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, thensem_timedwait() fails with a timeout error (errno set toETIMEDOUT).       If the operation can be performed immediately, thensem_timedwait() never fails with a timeout error, regardless of       the value ofabs_timeout.  Furthermore, the validity ofabs_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, anderrno 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).EINTRThe call was interrupted by a signal handler; seesignal(7).EINVALsem is not a valid semaphore.EINVAL(sem_timedwait()) The value ofabs_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, seeattributes(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 aSIGALRMsignal.  This handler performs asem_post(3) to increment the semaphore that is being waited on inmain() usingsem_timedwait().  The second command-line argument       specifies the length of the timeout, in seconds, forsem_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 outProgram 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);       }

COLOPHON top

       This page is part of theman-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-6.15.tar.gz       fetched from       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on       2025-08-11.  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 isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-05-17sem_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)