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.TH SEM_WAIT 3 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

sem_wait, sem_timedwait, sem_trywait \- lock a semaphore

.SH SYNOPSIS

.nf

.B #include <semaphore.h>

.PP

.BI "int sem_wait(sem_t *" sem );

.PP

.BI "int sem_trywait(sem_t *" sem );

.PP

.BI "int sem_timedwait(sem_t *" sem ", const struct timespec *" abs_timeout );

.fi

.PP

Link with \fI\-pthread\fP.

.PP

.in -4n

Feature Test Macro Requirements for glibc (see

.BR feature_test_macros (7)):

.in

.PP

.BR sem_timedwait ():

_POSIX_C_SOURCE\ >=\ 200112L

.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.

.PP

.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.

.PP

.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, 1970-01-01 00:00:00 +0000 (UTC).

This structure is defined as follows:

.PP

.in +4n

.EX

struct timespec {

    time_t tv_sec;      /* Seconds */

    long   tv_nsec;     /* Nanoseconds [0 .. 999999999] */

};

.EE

.in

.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 ).

.PP

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; see

.BR signal (7).

.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 ATTRIBUTES

For an explanation of the terms used in this section, see

.BR attributes (7).

.TS

allbox;

lbw26 lb lb

l l l.

Interface	Attribute	Value

T{

.BR sem_wait (),

.BR sem_trywait (),

.BR sem_timedwait ()

T}	Thread safety	MT-Safe

.TE

.SH CONFORMING TO

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

.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:

.PP

.in +4n

.EX

.RB "$" " ./a.out 2 3"

About to call sem_timedwait()

sem_post() from handler

sem_timedwait() succeeded

.RB "$" " ./a.out 2 1"

About to call sem_timedwait()

sem_timedwait() timed out

.EE

.in

.SS Program source

\&

.EX

#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);

        _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("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

            perror("sem_timedwait");

    } else

        printf("sem_timedwait() succeeded\\n");

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

}

.EE

.SH SEE ALSO

.BR clock_gettime (2),

.BR sem_getvalue (3),

.BR sem_post (3),

.BR sem_overview (7),

.BR time (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.