+++ /dev/null
-'\" t
-.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
-.\"
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-.\" manual provided the copyright notice and this permission notice are
-.\" preserved on all copies.
-.\"
-.\" Permission is granted to copy and distribute modified versions of this
-.\" manual under the conditions for verbatim copying, provided that the
-.\" entire resulting derived work is distributed under the terms of a
-.\" permission notice identical to this one.
-.\"
-.\" Since the Linux kernel and libraries are constantly changing, this
-.\" manual page may be incorrect or out-of-date. The author(s) assume no
-.\" responsibility for errors or omissions, or for damages resulting from
-.\" the use of the information contained herein. The author(s) may not
-.\" have taken the same level of care in the production of this manual,
-.\" which is licensed free of charge, as they might when working
-.\" professionally.
-.\"
-.\" Formatted or processed versions of this manual, if unaccompanied by
-.\" the source, must acknowledge the copyright and authors of this work.
-.\" %%%LICENSE_END
-.\"
-.TH SEM_WAIT 3 2014-02-26 "Linux" "Linux Programmer's Manual"
-.SH NAME
-sem_wait, sem_timedwait, sem_trywait \- lock a semaphore
-.SH SYNOPSIS
-.nf
-.B #include <semaphore.h>
-.sp
-.BI "int sem_wait(sem_t *" sem );
-.sp
-.BI "int sem_trywait(sem_t *" sem );
-.sp
-.BI "int sem_timedwait(sem_t *" sem ", const struct timespec *" abs_timeout );
-.fi
-.sp
-Link with \fI\-pthread\fP.
-.sp
-.in -4n
-Feature Test Macro Requirements for glibc (see
-.BR feature_test_macros (7)):
-.in
-.sp
-.BR sem_timedwait ():
-_POSIX_C_SOURCE\ >=\ 200112L || _XOPEN_SOURCE\ >=\ 600
-.SH DESCRIPTION
-.BR sem_wait ()
-decrements (locks) the semaphore pointed to by
-.IR 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.
-
-.BR sem_trywait ()
-is the same as
-.BR sem_wait (),
-except that if the decrement cannot be immediately performed,
-then call returns an error
-.RI ( errno
-set to
-.BR EAGAIN )
-instead of blocking.
-
-.BR sem_timedwait ()
-is the same as
-.BR sem_wait (),
-except that
-.I abs_timeout
-specifies a limit on the amount of time that the call
-should block if the decrement cannot be immediately performed.
-The
-.I abs_timeout
-argument points to a structure that specifies an absolute timeout
-in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
-This structure is defined as follows:
-
-.nf
-.in +4n
-struct timespec {
- time_t tv_sec; /* Seconds */
- long tv_nsec; /* Nanoseconds [0 .. 999999999] */
-};
-.in
-.fi
-.PP
-If the timeout has already expired by the time of the call,
-and the semaphore could not be locked immediately,
-then
-.BR sem_timedwait ()
-fails with a timeout error
-.RI ( errno
-set to
-.BR ETIMEDOUT ).
-
-If the operation can be performed immediately, then
-.BR sem_timedwait ()
-never fails with a timeout error, regardless of the value of
-.IR abs_timeout .
-Furthermore, the validity of
-.I abs_timeout
-is not checked in this case.
-.SH RETURN VALUE
-All of these functions return 0 on success;
-on error, the value of the semaphore is left unchanged,
-\-1 is returned, and
-.I errno
-is set to indicate the error.
-.SH ERRORS
-.TP
-.B EINTR
-The call was interrupted by a signal handler; see
-.BR signal (7).
-.TP
-.B EINVAL
-.I sem
-is not a valid semaphore.
-.PP
-The following additional error can occur for
-.BR sem_trywait ():
-.TP
-.B EAGAIN
-The operation could not be performed without blocking (i.e., the
-semaphore currently has the value zero).
-.PP
-The following additional errors can occur for
-.BR sem_timedwait ():
-.TP
-.B EINVAL
-The value of
-.I abs_timeout.tv_nsecs
-is less than 0, or greater than or equal to 1000 million.
-.TP
-.B ETIMEDOUT
-The call timed out before the semaphore could be locked.
-.\" POSIX.1-2001 also allows EDEADLK -- "A deadlock condition
-.\" was detected", but this does not occur on Linux(?).
-.SH ATTRIBUTES
-.SS Multithreading (see pthreads(7))
-The
-.BR sem_wait (),
-.BR sem_trywait (),
-and
-.BR sem_timedwait ()
-functions are thread-safe.
-.SH CONFORMING TO
-POSIX.1-2001.
-.SH NOTES
-A signal handler always interrupts a blocked call to
-one of these functions, regardless of the use of the
-.BR sigaction (2)
-.B SA_RESTART
-flag.
-.\" sem_wait() is always interrupted on most other implementations,
-.\" but on FreeBSD 5.4 SA_RESTART does cause restarting.
-.SH EXAMPLE
-.PP
-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
-.B SIGALRM
-signal.
-This handler performs a
-.BR sem_post (3)
-to increment the semaphore that is being waited on in
-.I main()
-using
-.BR sem_timedwait ().
-The second command-line argument specifies the length
-of the timeout, in seconds, for
-.BR sem_timedwait ().
-The following shows what happens on two different runs of the program:
-
-.in +4n
-.nf
-.RB "$" " ./a.out 2 3"
-About to call sem_timedwait()
-sem_post() from handler
-sem_timedwait() succeeded
-.RB "$" " ./a.out 2 1"
-About to call sem_timedwait()
-sem_timedwait() timed out
-.fi
-.in
-.SS Program source
-\&
-.nf
-#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);
-}
-.fi
-.SH SEE ALSO
-.BR clock_gettime (2),
-.BR sem_getvalue (3),
-.BR sem_post (3),
-.BR sem_overview (7),
-.BR time (7)
-.SH COLOPHON
-This page is part of release 3.79 of the Linux
-.I man-pages
-project.
-A description of the project,
-information about reporting bugs,
-and the latest version of this page,
-can be found at
-\%http://www.kernel.org/doc/man\-pages/.
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