sem_wait, sem_timedwait, sem_trywait — lock a semaphore


#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);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_POSIX_C_SOURCE >= 200112L
[Note] Note

Link with −pthread.


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 structure that specifies an absolute timeout in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). This structure is defined as follows:

struct timespec {
  time_t   tv_sec;
/* Seconds */
  long   tv_nsec;
/* Nanoseconds [0 .. 999999999] */

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.



(sem_trywait()) The operation could not be performed without blocking (i.e., the semaphore currently has the value zero).


The call was interrupted by a signal handler; see signal(7).


sem is not a valid semaphore.


(sem_timedwait()) The value of abs_timeout.tv_nsecs is less than 0, or greater than or equal to 1000 million.


(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(), sem_timedwait() Thread safety MT-Safe


POSIX.1-2001, POSIX.1-2008.


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 <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <semaphore.h>
#include <time.h>
#include <assert.h>
#include <errno.h>
#include <signal.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);

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",

    if (sem_init(&sem, 0, 0) == −1)

    /* Establish SIGALRM handler; set alarm timer using argv[1]. */

    sa.sa_handler = handler;
    sa.sa_flags = 0;
    if (sigaction(SIGALRM, &sa, NULL) == −1)


    /* Calculate relative interval as current time plus
       number of seconds given argv[2]. */

    if (clock_gettime(CLOCK_REALTIME, &ts) == −1)

    ts.tv_sec += atoi(argv[2]);

    printf("main() about to call sem_timedwait()\n");
    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
        printf("sem_timedwait() succeeded\n");

    exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);


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


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  Copyright (C) 2006 Michael Kerrisk <>

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