2006-03-25 20:42:27 +00:00
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'\" t
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|
.\" Hey Emacs! This file is -*- nroff -*- source.
|
|
|
|
.\"
|
2007-09-20 06:52:22 +00:00
|
|
|
.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
|
2006-03-25 20:42:27 +00:00
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.\"
|
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|
.\" Permission is granted to make and distribute verbatim copies of this
|
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|
.\" manual provided the copyright notice and this permission notice are
|
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|
.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
|
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|
.\" manual under the conditions for verbatim copying, provided that the
|
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|
.\" entire resulting derived work is distributed under the terms of a
|
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.\" permission notice identical to this one.
|
2007-04-12 22:42:49 +00:00
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.\"
|
2006-03-25 20:42:27 +00:00
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.\" Since the Linux kernel and libraries are constantly changing, this
|
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|
.\" manual page may be incorrect or out-of-date. The author(s) assume no
|
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|
.\" responsibility for errors or omissions, or for damages resulting from
|
2008-03-18 14:47:54 +00:00
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.\" the use of the information contained herein. The author(s) may not
|
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
|
2007-04-12 22:42:49 +00:00
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.\"
|
2006-03-25 20:42:27 +00:00
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.\" Formatted or processed versions of this manual, if unaccompanied by
|
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.\" the source, must acknowledge the copyright and authors of this work.
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|
.\"
|
2012-02-11 17:11:51 +00:00
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|
|
.TH SEM_WAIT 3 2012-02-12 "Linux" "Linux Programmer's Manual"
|
2006-03-25 20:42:27 +00:00
|
|
|
.SH NAME
|
2007-05-12 00:44:48 +00:00
|
|
|
sem_wait, sem_timedwait, sem_trywait \- lock a semaphore
|
2006-03-25 20:42:27 +00:00
|
|
|
.SH SYNOPSIS
|
|
|
|
.nf
|
|
|
|
.B #include <semaphore.h>
|
|
|
|
.sp
|
|
|
|
.BI "int sem_wait(sem_t *" sem );
|
2007-12-23 13:45:24 +00:00
|
|
|
.sp
|
2006-03-25 20:42:27 +00:00
|
|
|
.BI "int sem_trywait(sem_t *" sem );
|
2007-12-23 13:45:24 +00:00
|
|
|
.sp
|
2006-03-25 20:42:27 +00:00
|
|
|
.BI "int sem_timedwait(sem_t *" sem ", const struct timespec *" abs_timeout );
|
|
|
|
.fi
|
2007-07-08 12:11:40 +00:00
|
|
|
.sp
|
2008-03-10 08:29:29 +00:00
|
|
|
Link with \fI\-lrt\fP or \fI\-pthread\fP.
|
|
|
|
.sp
|
2007-07-08 12:11:40 +00:00
|
|
|
.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
|
2006-03-25 20:42:27 +00:00
|
|
|
.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,
|
2007-04-12 22:42:49 +00:00
|
|
|
then the call blocks until either it becomes possible to perform
|
2006-03-25 20:42:27 +00:00
|
|
|
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,
|
2007-04-12 22:42:49 +00:00
|
|
|
then call returns an error
|
|
|
|
.RI ( errno
|
2006-03-25 20:42:27 +00:00
|
|
|
set to
|
|
|
|
.BR EAGAIN )
|
|
|
|
instead of blocking.
|
|
|
|
|
|
|
|
.BR sem_timedwait ()
|
2007-04-12 22:42:49 +00:00
|
|
|
is the same as
|
2006-03-25 20:42:27 +00:00
|
|
|
.BR sem_wait (),
|
|
|
|
except that
|
|
|
|
.I abs_timeout
|
2007-04-12 22:42:49 +00:00
|
|
|
specifies a limit on the amount of time that the call
|
2006-03-25 20:42:27 +00:00
|
|
|
should block if the decrement cannot be immediately performed.
|
2007-04-12 22:42:49 +00:00
|
|
|
The
|
2006-03-25 20:42:27 +00:00
|
|
|
.I abs_timeout
|
2007-04-12 22:42:49 +00:00
|
|
|
argument points to a structure that specifies an absolute timeout
|
stime.2, time.2, utimensat.2, ctime.3, difftime.3, ftime.3, getspnam.3, mq_receive.3, mq_send.3, rtime.3, sem_wait.3, strftime.3, strptime.3, timeradd.3, rtc.4, core.5, proc.5, icmp.7, time.7: Global fix: Consistently define the Epoch
All definitions of the Epoch have been refactored to the following:
1970-01-01 00:00:00 +0000 (UTC)
That form is more consistent, logical, precise, and internationally
recognizable than the other variants.
Also, some wording has been altered as well.
Signed-off-by: Michael Witten <mfwitten@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2010-01-17 03:53:52 +00:00
|
|
|
in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
|
2006-03-25 20:42:27 +00:00
|
|
|
This structure is defined as follows:
|
|
|
|
|
|
|
|
.nf
|
2007-12-19 05:53:30 +00:00
|
|
|
.in +4n
|
2006-03-25 20:42:27 +00:00
|
|
|
struct timespec {
|
|
|
|
time_t tv_sec; /* Seconds */
|
|
|
|
long tv_nsec; /* Nanoseconds [0 .. 999999999] */
|
|
|
|
};
|
2007-12-19 05:53:30 +00:00
|
|
|
.in
|
2006-03-25 20:42:27 +00:00
|
|
|
.fi
|
|
|
|
.PP
|
|
|
|
If the timeout has already expired by the time of the call,
|
|
|
|
and the semaphore could not be locked immediately,
|
2007-04-12 22:42:49 +00:00
|
|
|
then
|
2006-03-25 20:42:27 +00:00
|
|
|
.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 .
|
2007-04-12 22:42:49 +00:00
|
|
|
Furthermore, the validity of
|
2006-03-25 20:42:27 +00:00
|
|
|
.I abs_timeout
|
|
|
|
is not checked in this case.
|
|
|
|
.SH RETURN VALUE
|
2007-04-12 22:42:49 +00:00
|
|
|
All of these functions return 0 on success;
|
2006-03-25 20:42:27 +00:00
|
|
|
on error, the value of the semaphore is left unchanged,
|
|
|
|
\-1 is returned, and
|
2007-04-12 22:42:49 +00:00
|
|
|
.I errno
|
2006-03-25 20:42:27 +00:00
|
|
|
is set to indicate the error.
|
|
|
|
.SH ERRORS
|
|
|
|
.TP
|
|
|
|
.B EINTR
|
2008-07-04 15:50:36 +00:00
|
|
|
The call was interrupted by a signal handler; see
|
|
|
|
.BR signal (7).
|
2006-03-25 20:42:27 +00:00
|
|
|
.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).
|
2006-08-03 02:25:12 +00:00
|
|
|
.PP
|
2006-03-25 20:42:27 +00:00
|
|
|
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.
|
2007-04-12 22:42:49 +00:00
|
|
|
.\" POSIX.1-2001 also allows EDEADLK -- "A deadlock condition
|
2006-03-25 20:42:27 +00:00
|
|
|
.\" was detected", but this does not occur on Linux(?).
|
2007-05-19 04:30:20 +00:00
|
|
|
.SH CONFORMING TO
|
|
|
|
POSIX.1-2001.
|
2006-03-25 20:42:27 +00:00
|
|
|
.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
|
2007-04-12 22:42:49 +00:00
|
|
|
The (somewhat trivial) program shown below operates on an
|
2006-03-25 20:42:27 +00:00
|
|
|
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
|
2007-05-12 00:30:29 +00:00
|
|
|
.BR sem_post (3)
|
2006-03-25 20:42:27 +00:00
|
|
|
to increment the semaphore that is being waited on in
|
|
|
|
.I main()
|
2007-04-12 22:42:49 +00:00
|
|
|
using
|
2006-03-25 20:42:27 +00:00
|
|
|
.BR sem_timedwait ().
|
|
|
|
The second command-line argument specifies the length
|
2007-04-12 22:42:49 +00:00
|
|
|
of the timeout, in seconds, for
|
2006-03-25 20:42:27 +00:00
|
|
|
.BR sem_timedwait ().
|
|
|
|
The following shows what happens on two different runs of the program:
|
|
|
|
|
2007-12-19 05:53:30 +00:00
|
|
|
.in +4n
|
2006-03-25 20:42:27 +00:00
|
|
|
.nf
|
eventfd.2, execve.2, getdents.2, ioprio_set.2, mprotect.2, signalfd.2, timerfd_create.2, wait.2, backtrace.3, clock_getcpuclockid.3, end.3, fmemopen.3, fopencookie.3, frexp.3, getdate.3, getgrouplist.3, getprotoent_r.3, getservent_r.3, gnu_get_libc_version.3, inet.3, inet_pton.3, makecontext.3, malloc.3, matherr.3, offsetof.3, pthread_attr_init.3, pthread_create.3, pthread_getattr_np.3, sem_wait.3, strftime.3, strtok.3, strtol.3, core.5, proc.5, cpuset.7, mq_overview.7: Global fix: Format user input in shell sessions in boldface
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2008-10-28 15:37:22 +00:00
|
|
|
.RB "$" " ./a.out 2 3"
|
2006-03-25 20:42:27 +00:00
|
|
|
About to call sem_timedwait()
|
|
|
|
sem_post() from handler
|
|
|
|
sem_timedwait() succeeded
|
eventfd.2, execve.2, getdents.2, ioprio_set.2, mprotect.2, signalfd.2, timerfd_create.2, wait.2, backtrace.3, clock_getcpuclockid.3, end.3, fmemopen.3, fopencookie.3, frexp.3, getdate.3, getgrouplist.3, getprotoent_r.3, getservent_r.3, gnu_get_libc_version.3, inet.3, inet_pton.3, makecontext.3, malloc.3, matherr.3, offsetof.3, pthread_attr_init.3, pthread_create.3, pthread_getattr_np.3, sem_wait.3, strftime.3, strtok.3, strtol.3, core.5, proc.5, cpuset.7, mq_overview.7: Global fix: Format user input in shell sessions in boldface
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2008-10-28 15:37:22 +00:00
|
|
|
.RB "$" " ./a.out 2 1"
|
2006-03-25 20:42:27 +00:00
|
|
|
About to call sem_timedwait()
|
|
|
|
sem_timedwait() timed out
|
|
|
|
.fi
|
2007-12-19 05:53:30 +00:00
|
|
|
.in
|
eventfd.2, getdents.2, mprotect.2, signalfd.2, timerfd_create.2, wait.2, backtrace.3, clock_getcpuclockid.3, end.3, fmemopen.3, fopencookie.3, getdate.3, getgrouplist.3, getprotoent_r.3, getservent_r.3, gnu_get_libc_version.3, inet.3, inet_pton.3, makecontext.3, matherr.3, offsetof.3, pthread_attr_init.3, pthread_create.3, pthread_getattr_np.3, sem_wait.3, strtol.3, core.5: global fix: Add ".SS Program source" to EXAMPLE
Add ".SS Program source" to clearly distinguish shell session and
descriptive text from actual program code.
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2008-10-28 15:51:05 +00:00
|
|
|
.SS Program source
|
eventfd.2, getdents.2, mprotect.2, signalfd.2, timerfd_create.2, wait.2, backtrace.3, clock_getcpuclockid.3, end.3, fmemopen.3, fopencookie.3, frexp.3, getaddrinfo.3, getdate.3, getgrouplist.3, getprotoent_r.3, getservent_r.3, gnu_get_libc_version.3, inet.3, inet_pton.3, makecontext.3, matherr.3, offsetof.3, pthread_attr_init.3, pthread_create.3, pthread_getattr_np.3, sem_wait.3, strftime.3, strtok.3, strtol.3, core.5: srcfix
s/\.R " "/\\\&/ as a way of getting a blank line after a .SS heading.
(Suggested by Sam Varshavchik <mrsam@courier-mta.com>)
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2008-11-05 01:42:45 +00:00
|
|
|
\&
|
2006-03-25 20:42:27 +00:00
|
|
|
.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;
|
|
|
|
|
2007-11-19 03:34:00 +00:00
|
|
|
#define handle_error(msg) \\
|
|
|
|
do { perror(msg); exit(EXIT_FAILURE); } while (0)
|
|
|
|
|
2006-03-25 20:42:27 +00:00
|
|
|
static void
|
|
|
|
handler(int sig)
|
|
|
|
{
|
2007-07-19 07:14:03 +00:00
|
|
|
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);
|
|
|
|
}
|
|
|
|
}
|
2006-03-25 20:42:27 +00:00
|
|
|
|
|
|
|
int
|
|
|
|
main(int argc, char *argv[])
|
|
|
|
{
|
|
|
|
struct sigaction sa;
|
|
|
|
struct timespec ts;
|
|
|
|
int s;
|
|
|
|
|
2007-07-19 07:14:03 +00:00
|
|
|
if (argc != 3) {
|
|
|
|
fprintf(stderr, "Usage: %s <alarm\-secs> <wait\-secs>\\n",
|
|
|
|
argv[0]);
|
|
|
|
exit(EXIT_FAILURE);
|
|
|
|
}
|
2006-03-25 20:42:27 +00:00
|
|
|
|
2007-11-19 03:34:00 +00:00
|
|
|
if (sem_init(&sem, 0, 0) == \-1)
|
|
|
|
handle_error("sem_init");
|
2006-03-25 20:42:27 +00:00
|
|
|
|
|
|
|
/* Establish SIGALRM handler; set alarm timer using argv[1] */
|
|
|
|
|
|
|
|
sa.sa_handler = handler;
|
|
|
|
sigemptyset(&sa.sa_mask);
|
|
|
|
sa.sa_flags = 0;
|
2007-11-19 08:16:37 +00:00
|
|
|
if (sigaction(SIGALRM, &sa, NULL) == \-1)
|
2007-11-19 03:34:00 +00:00
|
|
|
handle_error("sigaction");
|
2006-03-25 20:42:27 +00:00
|
|
|
|
|
|
|
alarm(atoi(argv[1]));
|
|
|
|
|
|
|
|
/* Calculate relative interval as current time plus
|
|
|
|
number of seconds given argv[2] */
|
|
|
|
|
2007-11-19 03:34:00 +00:00
|
|
|
if (clock_gettime(CLOCK_REALTIME, &ts) == \-1)
|
|
|
|
handle_error("clock_gettime");
|
|
|
|
|
2006-03-25 20:42:27 +00:00
|
|
|
ts.tv_sec += atoi(argv[2]);
|
|
|
|
|
|
|
|
printf("main() about to call sem_timedwait()\\n");
|
2007-06-20 21:39:45 +00:00
|
|
|
while ((s = sem_timedwait(&sem, &ts)) == \-1 && errno == EINTR)
|
2007-07-19 07:14:03 +00:00
|
|
|
continue; /* Restart if interrupted by handler */
|
2006-03-25 20:42:27 +00:00
|
|
|
|
|
|
|
/* Check what happened */
|
|
|
|
|
2007-06-20 21:39:45 +00:00
|
|
|
if (s == \-1) {
|
2006-03-25 20:42:27 +00:00
|
|
|
if (errno == ETIMEDOUT)
|
|
|
|
printf("sem_timedwait() timed out\\n");
|
|
|
|
else
|
2007-07-19 07:14:03 +00:00
|
|
|
perror("sem_timedwait");
|
2006-03-25 20:42:27 +00:00
|
|
|
} else
|
|
|
|
printf("sem_timedwait() succeeded\\n");
|
|
|
|
|
2007-07-19 07:14:03 +00:00
|
|
|
exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
|
2006-03-25 20:42:27 +00:00
|
|
|
}
|
|
|
|
.fi
|
|
|
|
.SH "SEE ALSO"
|
2009-02-07 21:12:29 +00:00
|
|
|
.BR clock_gettime (2),
|
2006-03-25 20:42:27 +00:00
|
|
|
.BR sem_getvalue (3),
|
|
|
|
.BR sem_post (3),
|
2008-06-23 08:48:25 +00:00
|
|
|
.BR sem_overview (7),
|
|
|
|
.BR time (7)
|