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sem_wait
SYNOPSIS
#include <semaphore.h>
int sem_wait(sem_t *sem);
int sem_trywait(sem_t *sem);
int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout);
Link with -pthread.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
sem_timedwait(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
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 structure that specifies an absolute timeout in sec-
onds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
This structure is defined as follows:
struct timespec {
time_t tv_sec; /* Seconds */
long tv_nsec; /* Nanoseconds [0 .. 999999999] */
};
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.
than or equal to 1000 million.
ETIMEDOUT
The call timed out before the semaphore could be locked.
ATTRIBUTES
Multithreading (see pthreads(7))
The sem_wait(), sem_trywait(), and sem_timedwait() functions are
thread-safe.
CONFORMING TO
POSIX.1-2001.
NOTES
A signal handler always interrupts a blocked call to one of these func-
tions, regardless of the use of the sigaction(2) SA_RESTART flag.
EXAMPLE
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) 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 out
Program source
#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
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);
}
SEE ALSO
clock_gettime(2), sem_getvalue(3), sem_post(3), sem_overview(7),
time(7)