NAME

sem_clockwait, sem_timedwait — lock a semaphore

SYNOPSIS

#include <semaphore.h>

int sem_clockwait(sem_t *restrict sem, clockid_t clock_id,
       const struct timespec *restrict abstime);
int sem_timedwait(sem_t *restrict sem,
       const struct timespec *restrict abstime);


DESCRIPTION

The sem_clockwait() and sem_timedwait() functions shall lock the semaphore referenced by sem as in the sem_wait() function. However, if the semaphore cannot be locked without waiting for another process or thread to unlock the semaphore by performing a sem_post() function, this wait shall be terminated when the specified timeout expires.

The timeout shall expire when the absolute time specified by abstime passes, as measured by the clock on which timeouts are based (that is, when the value of that clock equals or exceeds abstime), or if the absolute time specified by abstime has already been passed at the time of the call.

For sem_timedwait(), the timeout shall be based on the CLOCK_REALTIME clock. For sem_clockwait(), the timeout shall be based on the clock specified by the clock_id argument. The resolution of the timeout shall be the resolution of the clock on which it is based. Implementations shall support passing CLOCK_REALTIME and CLOCK_MONOTONIC to sem_clockwait() as the clock_id argument.

Under no circumstance shall the function fail with a timeout if the semaphore can be locked immediately. The validity of the abstime need not be checked if the semaphore can be locked immediately.

RETURN VALUE

The sem_clockwait() and sem_timedwait() functions shall return zero if the calling process successfully performed the semaphore lock operation on the semaphore designated by sem. If the call was unsuccessful, the state of the semaphore shall be unchanged, and the functions shall return a value of -1 and set errno to indicate the error.

ERRORS

The sem_clockwait() and sem_timedwait() functions shall fail if:

[EINVAL]

The process or thread would have blocked, and either the abstime parameter specified a nanoseconds field value less than zero or greater than or equal to 1000 million, or the sem_clockwait() function was passed an invalid or unsupported clock_id value.

[ETIMEDOUT]

The semaphore could not be locked before the specified timeout expired.

The sem_clockwait() and sem_timedwait() functions may fail if:

[EDEADLK]

A deadlock condition was detected.

[EINTR]

A signal interrupted the function.

[EINVAL]

The sem argument does not refer to a valid semaphore.

EXAMPLES

The 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() to increment the semaphore that is being waited on in main() using sem_clockwait(). The second command-line argument specifies the length of the timeout, in seconds, for sem_clockwait().

#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;


static void
handler(int sig)
{
    int sav_errno = errno;
    static const char info_msg[] = "sem_post() from handler\n";
    write(STDOUT_FILENO, info_msg, sizeof info_msg - 1);
    if (sem_post(&sem) == -1) {
        static const char err_msg[] = "sem_post() failed\n";
        write(STDERR_FILENO, err_msg, sizeof err_msg - 1);
        _exit(EXIT_FAILURE);
    }
    errno = sav_errno;
}


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) {
        perror("sem_init");
        exit(EXIT_FAILURE);
    }


    /* 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) {
        perror("sigaction");
        exit(EXIT_FAILURE);
    }


    alarm(atoi(argv[1]));


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


    if (clock_gettime(CLOCK_MONOTONIC, &ts) == -1) {
        perror("clock_gettime");
        exit(EXIT_FAILURE);
    }
    ts.tv_sec += atoi(argv[2]);


    printf("main() about to call sem_clockwait()\n");
    while ((s = sem_clockwait(&sem, CLOCK_MONOTONIC, &ts)) == -1 &&
           errno == EINTR)
        continue;       /* Restart if interrupted by handler */


    /* Check what happened */


    if (s == -1) {
        if (errno == ETIMEDOUT)
            printf("sem_clockwait() timed out\n");
        else
            perror("sem_clockwait");
    } else
        printf("sem_clockwait() succeeded\n");


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

APPLICATION USAGE

Applications using these functions may be subject to priority inversion, as discussed in XBD 3.275 Priority Inversion.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

sem_post, sem_trywait, semctl, semget, semop, time

XBD 3.275 Priority Inversion, <semaphore.h>, <time.h>

CHANGE HISTORY

First released in Issue 6. Derived from IEEE Std 1003.1d-1999.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/120 is applied, updating the ERRORS section so that the [EINVAL] error becomes optional.

Issue 7

The sem_timedwait() function is moved from the Semaphores option to the Base.

Functionality relating to the Timers option is moved to the Base.

An example is added.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0529 [138] is applied.

Issue 8

Austin Group Defect 592 is applied, removing text relating to <time.h> from the SYNOPSIS and DESCRIPTION sections.

Austin Group Defect 1216 is applied, adding sem_clockwait().