sem_wait(3) Library Functions Manual sem_wait(3)
NAME
sem_wait, sem_timedwait, sem_trywait - lock a semaphore
LIBRARY
POSIX threads library (libpthread, -lpthread)
SYNOPSIS
#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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
sem_timedwait():
_POSIX_C_SOURCE >= 200112L
DESCRIPTION
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 argu-
ment points to a timespec(3) structure that specifies an absolute time-
out 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 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_time-
out. Furthermore, the validity of abs_timeout is not checked in this
case.
RETURN VALUE
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 indi-
cate the error.
ERRORS
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).
EINVAL sem is not a valid semaphore.
EINVAL (sem_timedwait()) The value of abs_timeout.tv_nsecs 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
For an explanation of the terms used in this section, see at-
tributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│sem_wait(), sem_trywait(), sem_timedwait() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2001, POSIX.1-2008.
EXAMPLES
The (somewhat trivial) program shown below operates on an unnamed sema-
phore. The program expects two command-line arguments. The first ar-
gument 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 in-
crement 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 <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
clock_gettime(2), sem_getvalue(3), sem_post(3), timespec(3), sem_over-
view(7), time(7)
Linux man-pages 6.03 2022-12-15 sem_wait(3)
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