'\" t
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.\" Copyright (C) 2006 Michael Kerrisk <mtk-manpages@gmx.net>
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.TH SEM_WAIT 3 2006-03-25 "Linux" "Linux Programmer's Manual"
.SH NAME
sem_wait, sem_timedwait, sem_trywait \- lock a semaphore
.SH SYNOPSIS
.nf
.B #include <semaphore.h>
.sp
.BI "int sem_wait(sem_t *" sem );
.BI "int sem_trywait(sem_t *" sem );
.sp
.B #define _XOPEN_SOURCE 600
.B #include <semaphore.h>
.sp
.BI "int sem_timedwait(sem_t *" sem ", const struct timespec *" abs_timeout );
.fi
.SH DESCRIPTION
.BR sem_wait ()
decrements (locks) the semaphore pointed to by
.IR 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.

.BR sem_trywait ()
is the same as
.BR sem_wait (),
except that if the decrement cannot be immediately performed,
then call returns an error
.RI ( errno
set to
.BR EAGAIN )
instead of blocking.

.BR sem_timedwait ()
is the same as
.BR sem_wait (),
except that
.I abs_timeout
specifies a limit on the amount of time that the call
should block if the decrement cannot be immediately performed.
The
.I abs_timeout
argument points to a structure that specifies an absolute timeout
in seconds and nanoseconds since the Epoch (00:00:00, 1 January 1970).
This structure is defined as follows:

.nf
.in +0.5i
struct timespec {
    time_t tv_sec;      /* Seconds */
    long   tv_nsec;     /* Nanoseconds [0 .. 999999999] */
};
.in -0.5i
.fi
.PP
If the timeout has already expired by the time of the call,
and the semaphore could not be locked immediately,
then
.BR sem_timedwait ()
fails with a timeout error
.RI ( errno
set to
.BR ETIMEDOUT ).

If the operation can be performed immediately, then
.BR sem_timedwait ()
never fails with a timeout error, regardless of the value of
.IR abs_timeout .
Furthermore, the validity of
.I abs_timeout
is not checked in this case.
.SH RETURN VALUE
All of these functions return 0 on success;
on error, the value of the semaphore is left unchanged,
\-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINTR
The call was interrupted by a signal handler.
.TP
.B EINVAL
.I sem
is not a valid semaphore.
.PP
The following additional error can occur for
.BR sem_trywait ():
.TP
.B EAGAIN
The operation could not be performed without blocking (i.e., the
semaphore currently has the value zero).
.PP
The following additional errors can occur for
.BR sem_timedwait ():
.TP
.B EINVAL
The value of
.I abs_timeout.tv_nsecs
is less than 0, or greater than or equal to 1000 million.
.TP
.B ETIMEDOUT
The call timed out before the semaphore could be locked.
.\" POSIX.1-2001 also allows EDEADLK -- "A deadlock condition
.\" was detected", but this does not occur on Linux(?).
.SH CONFORMING TO
POSIX.1-2001.
.SH NOTES
A signal handler always interrupts a blocked call to
one of these functions, regardless of the use of the
.BR sigaction (2)
.B SA_RESTART
flag.
.\" sem_wait() is always interrupted on most other implementations,
.\" but on FreeBSD 5.4 SA_RESTART does cause restarting.
.SH EXAMPLE
.PP
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
.B SIGALRM
signal.
This handler performs a
.BR sem_post (3)
to increment the semaphore that is being waited on in
.I main()
using
.BR sem_timedwait ().
The second command-line argument specifies the length
of the timeout, in seconds, for
.BR sem_timedwait ().
The following shows what happens on two different runs of the program:

.in +0.5i
.nf
$ ./a.out 2 3
About to call sem_timedwait()
sem_post() from handler
sem_getvalue() from handler; value = 1
sem_timedwait() succeeded
$ ./a.out 2 1
About to call sem_timedwait()
sem_timedwait() timed out
.fi
.in -0.5i
.PP
The source code of the program is as follows:
.nf

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

#define die(msg) { perror(msg); exit(EXIT_FAILURE); }

sem_t sem;

static void
handler(int sig)
{
    int sval;

    printf("sem_post() from handler\\n");
    if (sem_post(&sem) == -1)
        die("sem_post");

    if (sem_getvalue(&sem, &sval) == -1)
        die("sem_getvalue");
    printf("sem_getvalue() from handler; value = %d\\n", sval);
} /* handler */

int
main(int argc, char *argv[])
{
    struct sigaction sa;
    struct timespec ts;
    int s;

    assert(argc == 3);  /* Usage: ./a.out alarm-secs wait-secs */

    if (sem_init(&sem, 0, 0) == -1)
        die("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)
        die("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)
        die("clock_gettime");
    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 when interrupted by handler */

    /* Check what happened */

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

    exit(EXIT_SUCCESS);
}
.fi
.SH "SEE ALSO"
.\" FIXME . when this page has been added to the man-pages set:
.\" .BR clock_gettime (3),
.BR sem_getvalue (3),
.BR sem_post (3),
.BR feature_test_macros (7),
.BR sem_overview (7)