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.\"
.\" Sun Microsystems, Inc. gratefully acknowledges The Open Group for
.\" permission to reproduce portions of its copyrighted documentation.
.\" Original documentation from The Open Group can be obtained online at
.\" http://www.opengroup.org/bookstore/.
.\"
.\" The Institute of Electrical and Electronics Engineers and The Open
.\" Group, have given us permission to reprint portions of their
.\" documentation.
.\"
.\" In the following statement, the phrase ``this text'' refers to portions
.\" of the system documentation.
.\"
.\" Portions of this text are reprinted and reproduced in electronic form
.\" in the SunOS Reference Manual, from IEEE Std 1003.1, 2004 Edition,
.\" Standard for Information Technology -- Portable Operating System
.\" Interface (POSIX), The Open Group Base Specifications Issue 6,
.\" Copyright (C) 2001-2004 by the Institute of Electrical and Electronics
.\" Engineers, Inc and The Open Group. In the event of any discrepancy
.\" between these versions 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.
.\"
.\" This notice shall appear on any product containing this material.
.\"
.\" The contents of this file are subject to the terms of the
.\" Common Development and Distribution License (the "License").
.\" You may not use this file except in compliance with the License.
.\"
.\" You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
.\" or http://www.opensolaris.org/os/licensing.
.\" See the License for the specific language governing permissions
.\" and limitations under the License.
.\"
.\" When distributing Covered Code, include this CDDL HEADER in each
.\" file and include the License file at usr/src/OPENSOLARIS.LICENSE.
.\" If applicable, add the following below this CDDL HEADER, with the
.\" fields enclosed by brackets "[]" replaced with your own identifying
.\" information: Portions Copyright [yyyy] [name of copyright owner]
.\"
.\"
.\" Copyright 1989 AT&T
.\" Portions Copyright (c) 1992, X/Open Company Limited. All Rights Reserved.
.\" Copyright (c) 2008, Sun Microsystems, Inc. All Rights Reserved.
.\"
.TH SEM_WAIT 3C "Feb 5, 2008"
.SH NAME
sem_wait, sem_trywait \- acquire or wait for a semaphore
.SH SYNOPSIS
.LP
.nf
#include <semaphore.h>
\fBint\fR \fBsem_wait\fR(\fBsem_t *\fR\fIsem\fR);
.fi
.LP
.nf
\fBint\fR \fBsem_trywait\fR(\fBsem_t *\fR\fIsem\fR);
.fi
.SH DESCRIPTION
.sp
.LP
The \fBsem_wait()\fR function locks the semaphore referenced by \fIsem\fR by
performing a semaphore lock operation on that semaphore. If the semaphore value
is currently zero, then the calling thread will not return from the call to
\fBsem_wait()\fR until it either locks the semaphore or the call is interrupted
by a signal. The \fBsem_trywait()\fR function locks the semaphore referenced by
\fIsem\fR only if the semaphore is currently not locked; that is, if the
semaphore value is currently positive. Otherwise, it does not lock the
semaphore.
.sp
.LP
Upon successful return, the state of the semaphore is locked and remains locked
until the \fBsem_post\fR(3C) function is executed and returns successfully.
.sp
.LP
The \fBsem_wait()\fR function is interruptible by the delivery of a signal.
.SH RETURN VALUES
.sp
.LP
The \fBsem_wait()\fR and \fBsem_trywait()\fR functions return \fB0\fR if the
calling process successfully performed the semaphore lock operation on the
semaphore designated by \fIsem\fR. If the call was unsuccessful, the state of
the semaphore is unchanged, and the function returns \fB\(mi1\fR and sets
\fBerrno\fR to indicate the error.
.SH ERRORS
.sp
.LP
The \fBsem_wait()\fR and \fBsem_trywait()\fR functions will fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR \fR
.ad
.RS 11n
The \fIsem\fR function does not refer to a valid semaphore.
.RE
.sp
.ne 2
.na
\fB\fBENOSYS\fR \fR
.ad
.RS 11n
The \fBsem_wait()\fR and \fBsem_trywait()\fR functions are not supported by the
system.
.RE
.sp
.LP
The \fBsem_trywait()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR \fR
.ad
.RS 11n
The semaphore was already locked, so it cannot be immediately locked by the
\fBsem_trywait()\fR operation.
.RE
.sp
.LP
The \fBsem_wait()\fR and \fBsem_trywait()\fR functions may fail if:
.sp
.ne 2
.na
\fB\fBEDEADLK\fR \fR
.ad
.RS 12n
A deadlock condition was detected; that is, two separate processes are waiting
for an available resource to be released via a semaphore "held" by the other
process.
.RE
.sp
.ne 2
.na
\fB\fBEINTR\fR \fR
.ad
.RS 12n
A signal interrupted this function.
.RE
.SH USAGE
.sp
.LP
Realtime applications may encounter priority inversion when using semaphores.
The problem occurs when a high priority thread "locks" (that is, waits on) a
semaphore that is about to be "unlocked" (that is, posted) by a low priority
thread, but the low priority thread is preempted by a medium priority thread.
This scenario leads to priority inversion; a high priority thread is blocked by
lower priority threads for an unlimited period of time. During system design,
realtime programmers must take into account the possibility of this kind of
priority inversion. They can deal with it in a number of ways, such as by
having critical sections that are guarded by semaphores execute at a high
priority, so that a thread cannot be preempted while executing in its critical
section.
.SH EXAMPLES
.LP
\fBExample 1 \fRThe customer waiting-line in a bank may be analogous to the
synchronization scheme of a semaphore utilizing \fBsem_wait()\fR and
\fBsem_trywait()\fR:
.sp
.in +2
.nf
#include <errno.h>
#define TELLERS 10
sem_t bank_line; /* semaphore */
int banking_hours(), deposit_withdrawal;
void *customer(), do_business(), skip_banking_today();
thread_t tid;
\&...
sem_init(&bank_line,TRUE,TELLERS); /* 10 tellers
available */
while(banking_hours())
thr_create(NULL, NULL, customer,
(void *)deposit_withdrawal, THREAD_NEW_LWP, &tid);
\&...
void *
customer(deposit_withdrawal)
void *deposit_withdrawal;
{
int this_customer, in_a_hurry = 50;
this_customer = rand() % 100;
if (this_customer == in_a_hurry) {
if (sem_trywait(&bank_line) != 0)
if (errno == EAGAIN) { /* no teller available */
skip_banking_today(this_customer);
return;
} /*else go immediately to available teller
& decrement bank_line*/
}
else
sem_wait(&bank_line); /* wait for next teller,
then proceed, and decrement bank_line */
do_business((int *)deposit_withdrawal);
sem_getvalue(&bank_line,&num_tellers);
sem_post(&bank_line); /* increment bank_line;
this_customer's teller is now available */
}
.fi
.in -2
.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp
.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE ATTRIBUTE VALUE
_
Interface Stability Committed
_
MT-Level MT-Safe
_
Standard See \fBstandards\fR(5).
.TE
.SH SEE ALSO
.sp
.LP
\fBsem_post\fR(3C), \fBattributes\fR(5), \fBstandards\fR(5)
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