.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "SEM_TRYWAIT" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" sem_trywait 
.SH NAME
sem_trywait, sem_wait \- lock a semaphore (\fBREALTIME\fP)
.SH SYNOPSIS
.LP
\fB#include <semaphore.h>
.br
.sp
int sem_trywait(sem_t *\fP\fIsem\fP\fB);
.br
int sem_wait(sem_t *\fP\fIsem\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIsem_trywait\fP() function shall lock the semaphore referenced
by \fIsem\fP only if the semaphore is currently not
locked; that is, if the semaphore value is currently positive. Otherwise,
it shall not lock the semaphore.
.LP
The \fIsem_wait\fP() function shall lock the semaphore referenced
by \fIsem\fP by performing a semaphore lock operation on
that semaphore. If the semaphore value is currently zero, then the
calling thread shall not return from the call to
\fIsem_wait\fP() until it either locks the semaphore or the call is
interrupted by a signal.
.LP
Upon successful return, the state of the semaphore shall be locked
and shall remain locked until the \fIsem_post\fP() function is executed
and returns successfully.
.LP
The \fIsem_wait\fP() function is interruptible by the delivery of
a signal.
.SH RETURN VALUE
.LP
The \fIsem_trywait\fP() and \fIsem_wait\fP() functions shall return
zero if the calling process successfully performed the
semaphore lock operation on the semaphore designated by \fIsem\fP.
If the call was unsuccessful, the state of the semaphore shall
be unchanged, and the function shall return a value of -1 and set
\fIerrno\fP to indicate the error.
.SH ERRORS
.LP
The \fIsem_trywait\fP() and \fIsem_wait\fP() functions shall fail
if:
.TP 7
.B EAGAIN
The semaphore was already locked, so it cannot be immediately locked
by the \fIsem_trywait\fP() operation (
\fIsem_trywait\fP() only).
.TP 7
.B EINVAL
The \fIsem\fP argument does not refer to a valid semaphore.
.sp
.LP
The \fIsem_trywait\fP() and \fIsem_wait\fP() functions may fail if:
.TP 7
.B EDEADLK
A deadlock condition was detected.
.TP 7
.B EINTR
A signal interrupted this function.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
Applications using these functions may be subject to priority inversion,
as discussed in the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, Section 3.285, Priority Inversion.
.LP
The \fIsem_trywait\fP() and \fIsem_wait\fP() functions are part of
the Semaphores option and need not be provided on all
implementations.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIsemctl\fP() , \fIsemget\fP() , \fIsemop\fP()
, \fIsem_post\fP() , \fIsem_timedwait\fP() , the Base Definitions
volume of IEEE\ Std\ 1003.1-2001, \fI<semaphore.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .