mirror of https://github.com/mkerrisk/man-pages
213 lines
6.8 KiB
Plaintext
213 lines
6.8 KiB
Plaintext
.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "SEMGET" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" semget
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.SH NAME
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semget \- get set of XSI semaphores
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.SH SYNOPSIS
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.LP
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\fB#include <sys/sem.h>
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.br
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.sp
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int semget(key_t\fP \fIkey\fP\fB, int\fP \fInsems\fP\fB, int\fP \fIsemflg\fP\fB);
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\fP
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\fB
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.br
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\fP
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.SH DESCRIPTION
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.LP
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The \fIsemget\fP() function operates on XSI semaphores (see the Base
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Definitions volume of IEEE\ Std\ 1003.1-2001, Section 4.15, Semaphore).
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It is unspecified whether this function interoperates
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with the realtime interprocess communication facilities defined in
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\fIRealtime\fP .
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.LP
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The \fIsemget\fP() function shall return the semaphore identifier
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associated with \fIkey\fP.
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.LP
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A semaphore identifier with its associated \fBsemid_ds\fP data structure
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and its associated set of \fInsems\fP semaphores (see
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\fI<sys/sem.h>\fP) is created for \fIkey\fP if one of the following
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is true:
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.IP " *" 3
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The argument \fIkey\fP is equal to IPC_PRIVATE.
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.LP
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.IP " *" 3
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The argument \fIkey\fP does not already have a semaphore identifier
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associated with it and (\fIsemflg\fP &IPC_CREAT) is
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non-zero.
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.LP
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.LP
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Upon creation, the \fBsemid_ds\fP data structure associated with the
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new semaphore identifier is initialized as follows:
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.IP " *" 3
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In the operation permissions structure \fIsem_perm.cuid\fP, \fIsem_perm.uid\fP,
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\fIsem_perm.cgid\fP, and \fIsem_perm.gid\fP
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shall be set equal to the effective user ID and effective group ID,
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respectively, of the calling process.
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.LP
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.IP " *" 3
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The low-order 9 bits of \fIsem_perm.mode\fP shall be set equal to
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the low-order 9 bits of \fIsemflg\fP.
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.LP
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.IP " *" 3
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The variable \fIsem_nsems\fP shall be set equal to the value of \fInsems\fP.
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.LP
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.IP " *" 3
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The variable \fIsem_otime\fP shall be set equal to 0 and \fIsem_ctime\fP
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shall be set equal to the current time.
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.LP
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.IP " *" 3
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The data structure associated with each semaphore in the set shall
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not be initialized. The \fIsemctl\fP() function with the command SETVAL
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or SETALL can be used to initialize each
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semaphore.
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.LP
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.SH RETURN VALUE
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.LP
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Upon successful completion, \fIsemget\fP() shall return a non-negative
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integer, namely a semaphore identifier; otherwise, it
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shall return -1 and set \fIerrno\fP to indicate the error.
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.SH ERRORS
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.LP
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The \fIsemget\fP() function shall fail if:
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.TP 7
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.B EACCES
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A semaphore identifier exists for \fIkey\fP, but operation permission
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as specified by the low-order 9 bits of \fIsemflg\fP
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would not be granted; see \fIXSI Interprocess Communication\fP .
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.TP 7
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.B EEXIST
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A semaphore identifier exists for the argument \fIkey\fP but ((\fIsemflg\fP
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&IPC_CREAT) &&(\fIsemflg\fP
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&IPC_EXCL)) is non-zero.
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.TP 7
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.B EINVAL
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The value of \fInsems\fP is either less than or equal to 0 or greater
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than the system-imposed limit, or a semaphore identifier
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exists for the argument \fIkey\fP, but the number of semaphores in
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the set associated with it is less than \fInsems\fP and
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\fInsems\fP is not equal to 0.
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.TP 7
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.B ENOENT
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A semaphore identifier does not exist for the argument \fIkey\fP and
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(\fIsemflg\fP &IPC_CREAT) is equal to 0.
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.TP 7
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.B ENOSPC
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A semaphore identifier is to be created but the system-imposed limit
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on the maximum number of allowed semaphores system-wide
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would be exceeded.
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.sp
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.LP
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\fIThe following sections are informative.\fP
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.SH EXAMPLES
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.SS Creating a Semaphore Identifier
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.LP
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The following example gets a unique semaphore key using the \fIftok\fP()
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function, then
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gets a semaphore ID associated with that key using the \fIsemget\fP()
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function (the first call also tests to make sure the
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semaphore exists). If the semaphore does not exist, the program creates
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it, as shown by the second call to \fIsemget\fP(). In
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creating the semaphore for the queuing process, the program attempts
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to create one semaphore with read/write permission for all. It
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also uses the IPC_EXCL flag, which forces \fIsemget\fP() to fail if
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the semaphore already exists.
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.LP
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After creating the semaphore, the program uses a call to \fIsemop\fP()
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to initialize it
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to the values in the \fIsbuf\fP array. The number of processes that
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can execute concurrently without queuing is initially set to
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2. The final call to \fIsemget\fP() creates a semaphore identifier
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that can be used later in the program.
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.sp
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.RS
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.nf
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\fB#include <sys/types.h>
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#include <stdio.h>
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#include <sys/ipc.h>
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#include <sys/sem.h>
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#include <sys/stat.h>
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#include <errno.h>
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#include <unistd.h>
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#include <stdlib.h>
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#include <pwd.h>
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#include <fcntl.h>
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#include <limits.h>
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\&...
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key_t semkey;
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int semid, pfd, fv;
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struct sembuf sbuf;
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char *lgn;
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char filename[PATH_MAX+1];
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struct stat outstat;
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struct passwd *pw;
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\&...
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/* Get unique key for semaphore. */
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if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
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perror("IPC error: ftok"); exit(1);
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}
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.sp
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/* Get semaphore ID associated with this key. */
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if ((semid = semget(semkey, 0, 0)) == -1) {
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.sp
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/* Semaphore does not exist - Create. */
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if ((semid = semget(semkey, 1, IPC_CREAT | IPC_EXCL | S_IRUSR |
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S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)) != -1)
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{
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/* Initialize the semaphore. */
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sbuf.sem_num = 0;
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sbuf.sem_op = 2; /* This is the number of runs
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without queuing. */
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sbuf.sem_flg = 0;
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if (semop(semid, &sbuf, 1) == -1) {
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perror("IPC error: semop"); exit(1);
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}
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}
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else if (errno == EEXIST) {
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if ((semid = semget(semkey, 0, 0)) == -1) {
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perror("IPC error 1: semget"); exit(1);
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}
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}
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else {
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perror("IPC error 2: semget"); exit(1);
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}
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}
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\&...
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\fP
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.fi
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.RE
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.SH APPLICATION USAGE
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.LP
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The POSIX Realtime Extension defines alternative interfaces for interprocess
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communication. Application developers who need to
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use IPC should design their applications so that modules using the
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IPC routines described in \fIXSI Interprocess Communication\fP can
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be easily modified to use the alternative
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interfaces.
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.SH RATIONALE
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.LP
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None.
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.SH FUTURE DIRECTIONS
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.LP
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None.
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.SH SEE ALSO
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.LP
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\fIXSI Interprocess Communication\fP , \fIRealtime\fP , \fIsemctl\fP()
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, \fIsemop\fP() , \fIsem_close\fP() , \fIsem_destroy\fP() , \fIsem_getvalue\fP()
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, \fIsem_init\fP() , \fIsem_open\fP() , \fIsem_post\fP() , \fIsem_unlink\fP()
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, \fIsem_wait\fP() , the Base Definitions volume of IEEE\ Std\ 1003.1-2001,
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\fI<sys/sem.h>\fP
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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