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sem_timedwait.3p (6526B)

'\" et
.TH SEM_TIMEDWAIT "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
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.
.\"
.SH NAME
sem_timedwait
\(em lock a semaphore
.SH SYNOPSIS
.LP
.nf
#include <semaphore.h>
#include <time.h>
.P
int sem_timedwait(sem_t *restrict \fIsem\fP,
    const struct timespec *restrict \fIabstime\fP);
.fi
.SH DESCRIPTION
The
\fIsem_timedwait\fR()
function shall lock the semaphore referenced by
.IR sem
as in the
\fIsem_wait\fR()
function. However, if the semaphore cannot be locked without waiting
for another process or thread to unlock the semaphore by performing a
\fIsem_post\fR()
function, this wait shall be terminated when the specified timeout
expires.
.P
The timeout shall expire when the absolute time specified by
.IR abstime
passes, as measured by the clock on which timeouts are based (that is,
when the value of that clock equals or exceeds
.IR abstime ),
or if the absolute time specified by
.IR abstime
has already been passed at the time of the call.
.P
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
.BR timespec
data type is defined as a structure in the
.IR <time.h> 
header.
.P
Under no circumstance shall the function fail with a timeout if the
semaphore can be locked immediately. The validity of the
.IR abstime
need not be checked if the semaphore can be locked immediately.
.SH "RETURN VALUE"
The
\fIsem_timedwait\fR()
function shall return zero if the calling process successfully
performed the semaphore lock operation on the semaphore designated by
.IR 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
.IR errno
to indicate the error.
.SH ERRORS
The
\fIsem_timedwait\fR()
function shall fail if:
.TP
.BR EINVAL
The process or thread would have blocked, and the
.IR abstime
parameter specified a nanoseconds field value less than zero or greater
than or equal to 1\|000 million.
.TP
.BR ETIMEDOUT
The semaphore could not be locked before the specified timeout expired.
.P
The
\fIsem_timedwait\fR()
function may fail if:
.TP
.BR EDEADLK
A deadlock condition was detected.
.TP
.BR EINTR
A signal interrupted this function.
.TP
.BR EINVAL
The
.IR sem
argument does not refer to a valid semaphore.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
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
.IR sem_post (3)
to increment the semaphore that is being waited on in
\fImain\fR()
using
\fIsem_timedwait\fR().
The second command-line argument specifies the length of the timeout,
in seconds, for
\fIsem_timedwait\fR().
.sp
.RS 4
.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>
.P
sem_t sem;
.P
static void
handler(int sig)
{
    int sav_errno = errno;
    static const char info_msg[] = "sem_post() from handler\en";
    write(STDOUT_FILENO, info_msg, sizeof info_msg - 1);
    if (sem_post(&sem) == -1) {
        static const char err_msg[] = "sem_post() failed\en";
        write(STDERR_FILENO, err_msg, sizeof err_msg - 1);
        _exit(EXIT_FAILURE);
    }
    errno = sav_errno;
}
.P
int
main(int argc, char *argv[])
{
    struct sigaction sa;
    struct timespec ts;
    int s;
.P
    if (argc != 3) {
        fprintf(stderr, "Usage: %s <alarm-secs> <wait-secs>\en",
            argv[0]);
        exit(EXIT_FAILURE);
    }
.P
    if (sem_init(&sem, 0, 0) == -1) {
        perror("sem_init");
        exit(EXIT_FAILURE);
    }
.P
    /* Establish SIGALRM handler; set alarm timer using argv[1] */
.P
    sa.sa_handler = handler;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = 0;
    if (sigaction(SIGALRM, &sa, NULL) == -1) {
        perror("sigaction");
        exit(EXIT_FAILURE);
    }
.P
    alarm(atoi(argv[1]));
.P
    /* Calculate relative interval as current time plus
       number of seconds given argv[2] */
.P
    if (clock_gettime(CLOCK_REALTIME, &ts) == -1) {
        perror("clock_gettime");
        exit(EXIT_FAILURE);
    }
    ts.tv_sec += atoi(argv[2]);
.P
    printf("main() about to call sem_timedwait()\en");
    while ((s = sem_timedwait(&sem, &ts)) == -1 && errno == EINTR)
        continue;       /* Restart if interrupted by handler */
.P
    /* Check what happened */
.P
    if (s == -1) {
        if (errno == ETIMEDOUT)
            printf("sem_timedwait() timed out\en");
        else
            perror("sem_timedwait");
    } else
        printf("sem_timedwait() succeeded\en");
.P
    exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
}
.fi
.P
.RE
.SH "APPLICATION USAGE"
Applications using these functions may be subject to priority
inversion, as discussed in the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 3.291" ", " "Priority Inversion".
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIsem_post\fR\^(\|)",
.IR "\fIsem_trywait\fR\^(\|)",
.IR "\fIsemctl\fR\^(\|)",
.IR "\fIsemget\fR\^(\|)",
.IR "\fIsemop\fR\^(\|)",
.IR "\fItime\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 3.291" ", " "Priority Inversion",
.IR "\fB<semaphore.h>\fP",
.IR "\fB<time.h>\fP"
.\"
.SH COPYRIGHT
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 .
.PP
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 .