NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON
SEM_WAIT(3) Linux Programmer's Manual SEM_WAIT(3)
sem_wait, sem_timedwait, sem_trywait - lock a semaphore
#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 -lrt or -pthread.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
sem_timedwait(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
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 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:
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_timeout.
Furthermore, the validity of abs_timeout is not checked in this case.
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 indicate the
error.
EINTR The call was interrupted by a signal handler; see signal(7).
EINVAL sem is not a valid semaphore.
The following additional error can occur for sem_trywait():
EAGAIN The operation could not be performed without blocking (i.e., the
semaphore currently has the value zero).
The following additional errors can occur for sem_timedwait():
EINVAL The value of abs_timeout.tv_nsecs is less than 0, or greater than or
equal to 1000 million.
ETIMEDOUT
The call timed out before the semaphore could be locked.
POSIX.1-2001.
A signal handler always interrupts a blocked call to one of these functions,
regardless of the use of the sigaction(2) SA_RESTART flag.
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_getvalue() from handler; value = 1
sem_timedwait() succeeded
$ ./a.out 2 1
About to call sem_timedwait()
sem_timedwait() timed out
#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);
}
clock_gettime(2), sem_getvalue(3), sem_post(3), sem_overview(7), time(7)
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2009-02-09 SEM_WAIT(3)