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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SEM_TIMEDWAIT(3P) POSIX Programmer's Manual SEM_TIMEDWAIT(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
sem_timedwait — lock a semaphore
#include <semaphore.h>
#include <time.h>
int sem_timedwait(sem_t *restrict sem,
const struct timespec *restrict abstime);
The sem_timedwait() function shall lock the semaphore referenced
by sem as in the sem_wait() function. However, if the semaphore
cannot be locked without waiting for another process or thread to
unlock the semaphore by performing a sem_post() function, this
wait shall be terminated when the specified timeout expires.
The timeout shall expire when the absolute time specified by
abstime passes, as measured by the clock on which timeouts are
based (that is, when the value of that clock equals or exceeds
abstime), or if the absolute time specified by abstime has
already been passed at the time of the call.
The timeout shall be based on the CLOCK_REALTIME clock. The
resolution of the timeout shall be the resolution of the clock on
which it is based. The timespec data type is defined as a
structure in the <time.h> header.
Under no circumstance shall the function fail with a timeout if
the semaphore can be locked immediately. The validity of the
abstime need not be checked if the semaphore can be locked
immediately.
The sem_timedwait() function shall return zero if the calling
process successfully performed the semaphore lock operation on
the semaphore designated by sem. If the call was unsuccessful,
the state of the semaphore shall be unchanged, and the function
shall return a value of -1 and set errno to indicate the error.
The sem_timedwait() function shall fail if:
EINVAL The process or thread would have blocked, and the abstime
parameter specified a nanoseconds field value less than
zero or greater than or equal to 1000 million.
ETIMEDOUT
The semaphore could not be locked before the specified
timeout expired.
The sem_timedwait() function may fail if:
EDEADLK
A deadlock condition was detected.
EINTR A signal interrupted this function.
EINVAL The sem argument does not refer to a valid semaphore.
The following sections are informative.
The 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().
#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;
static void
handler(int sig)
{
int sav_errno = errno;
static const char info_msg[] = "sem_post() from handler\n";
write(STDOUT_FILENO, info_msg, sizeof info_msg - 1);
if (sem_post(&sem) == -1) {
static const char err_msg[] = "sem_post() failed\n";
write(STDERR_FILENO, err_msg, sizeof err_msg - 1);
_exit(EXIT_FAILURE);
}
errno = sav_errno;
}
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) {
perror("sem_init");
exit(EXIT_FAILURE);
}
/* 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) {
perror("sigaction");
exit(EXIT_FAILURE);
}
alarm(atoi(argv[1]));
/* Calculate relative interval as current time plus
number of seconds given argv[2] */
if (clock_gettime(CLOCK_REALTIME, &ts) == -1) {
perror("clock_gettime");
exit(EXIT_FAILURE);
}
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);
}
Applications using these functions may be subject to priority
inversion, as discussed in the Base Definitions volume of
POSIX.1‐2017, Section 3.291, Priority Inversion.
None.
None.
sem_post(3p), sem_trywait(3p), semctl(3p), semget(3p), semop(3p),
time(3p)
The Base Definitions volume of POSIX.1‐2017, Section 3.291,
Priority Inversion, semaphore.h(0p), time.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any
discrepancy between this version and the original IEEE and The
Open Group Standard, the original IEEE and The Open Group
Standard is the referee document. The original Standard can be
obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 SEM_TIMEDWAIT(3P)
Pages that refer to this page: semaphore.h(0p), time.h(0p), clock_getres(3p), sem_getvalue(3p), sem_init(3p), sem_open(3p), sem_post(3p), sem_trywait(3p)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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