sem_wait(3) - Linux manual page (original) (raw)
semwait(3) Library Functions Manual semwait(3)
NAME top
sem_wait, sem_timedwait, sem_trywait - lock a semaphoreLIBRARY 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 *restrict** _sem_**,**
**const struct timespec *restrict** _abstimeout_**);**Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
**sem_timedwait**():
_POSIX_C_SOURCE >= 200112LDESCRIPTION top
**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](../man3/errno.3.html)_ set to **EAGAIN**) instead of blocking.
**sem_timedwait**() is the same as **sem_wait**(), except that _abstimeout_
specifies a limit on the amount of time that the call should block
if the decrement cannot be immediately performed. The _abstimeout_
argument points to a **timespec**(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, then
**sem_timedwait**() fails with a timeout error (_[errno](../man3/errno.3.html)_ set to
**ETIMEDOUT**).
If the operation can be performed immediately, then
**sem_timedwait**() never fails with a timeout error, regardless of
the value of _abstimeout_. Furthermore, the validity of
_abstimeout_ 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, and _[errno](../man3/errno.3.html)_ 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).
**EINTR** The call was interrupted by a signal handler; see
[signal(7)](../man7/signal.7.html).
**EINVAL** _sem_ is not a valid semaphore.
**EINVAL** (**sem_timedwait**()) The value of _abstimeout.tvnsecs_ 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, see
[attributes(7)](../man7/attributes.7.html).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ **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 a **SIGALRM** signal. This handler performs a
[sem_post(3)](../man3/sem%5Fpost.3.html) 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 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);
}SEE ALSO top
[clock_gettime(2)](../man2/clock%5Fgettime.2.html), [sem_getvalue(3)](../man3/sem%5Fgetvalue.3.html), [sem_post(3)](../man3/sem%5Fpost.3.html), **timespec**(3),
[sem_overview(7)](../man7/sem%5Foverview.7.html), [time(7)](../man7/time.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 semwait(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)