SEM_WAIT(3) Linux Programmer's Manual SEM_WAIT(3)NAMEsem_wait - lock a semaphore
SYNOPSIS
#include <semaphore.h>
int sem_wait(sem_t *sem);
int sem_trywait(sem_t *sem);
#define _XOPEN_SOURCE 600
#include <semaphore.h>
int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout);
DESCRIPTIONsem_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 (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_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
EINTR The call was interrupted by a signal handler.
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.
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.
CONFORMING TO
POSIX.1-2001.
EXAMPLE
The (somewhat trivial) program shown below operates on an unnamed sema‐
phore. 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() 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
The source code of the program is as follows:
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <semaphore.h>
#include <time.h>
#include <assert.h>
#include <errno.h>
#include <signal.h>
#define die(msg) { perror(msg); exit(EXIT_FAILURE); }
sem_t sem;
static void
handler(int sig)
{
int sval;
printf("sem_post() from handler\n");
if (sem_post(&sem) == -1) die("sem_post");
if (sem_getvalue(&sem, &sval) == -1) die("sem_getvalue");
printf("sem_getvalue() from handler; value = %d\n", sval);
} /* handler */
int
main(int argc, char *argv[])
{
struct sigaction sa;
struct timespec ts;
int s;
assert(argc == 3); /* Usage: ./a.out alarm-secs wait-secs */
if (sem_init(&sem, 0, 0) == -1) die("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) die("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)
die("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 when interrupted by handler */
/* Check what happened */
if (s == -1) {
if (errno == ETIMEDOUT)
printf("sem_timedwait() timed out\n");
else
die("sem_timedwait");
} else
printf("sem_timedwait() succeeded\n");
exit(EXIT_SUCCESS);
}
SEE ALSOsem_getvalue(3), sem_post(3), sem_overview(7)Linux 2.6.16 2006-03-25 SEM_WAIT(3)
