sem_wait, sem_timedwait, sem_trywait — lock a semaphore
#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) ; |
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 ofabs_timeout
. Furthermore, the validity ofabs_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.
The call was interrupted by a signal handler.
sem
is not a
valid semaphore.
The following additional error can occur for sem_trywait
():
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
():
The value of abs_timeout.tv_nsecs
is
less than 0, or greater than or equal to 1000
million.
The call timed out before the semaphore could be locked.
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
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); }
sem_getvalue(3), sem_post(3), feature_test_macros(7), sem_overview(7)
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