SEMGET(3P) POSIX Programmer's Manual SEMGET(3P)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.
NAMEsemget — get set of XSI semaphores
SYNOPSIS
#include <sys/sem.h>
int semget(key_t key, int nsems, int semflg);
DESCRIPTION
The semget() function operates on XSI semaphores (see the Base Defini‐
tions volume of POSIX.1‐2008, Section 4.16, Semaphore). It is unspeci‐
fied whether this function interoperates with the realtime interprocess
communication facilities defined in Section 2.8, Realtime.
The semget() function shall return the semaphore identifier associated
with key.
A semaphore identifier with its associated semid_ds data structure and
its associated set of nsems semaphores (see <sys/sem.h>) is created for
key if one of the following is true:
* The argument key is equal to IPC_PRIVATE.
* The argument key does not already have a semaphore identifier asso‐
ciated with it and (semflg &IPC_CREAT) is non-zero.
Upon creation, the semid_ds data structure associated with the new sem‐
aphore identifier is initialized as follows:
* In the operation permissions structure sem_perm.cuid, sem_perm.uid,
sem_perm.cgid, and sem_perm.gid shall be set to the effective user
ID and effective group ID, respectively, of the calling process.
* The low-order 9 bits of sem_perm.mode shall be set to the low-order
9 bits of semflg.
* The variable sem_nsems shall be set to the value of nsems.
* The variable sem_otime shall be set to 0 and sem_ctime shall be set
to the current time, as described in Section 2.7.1, IPC General
Description.
* The data structure associated with each semaphore in the set need
not be initialized. The semctl() function with the command SETVAL
or SETALL can be used to initialize each semaphore.
RETURN VALUE
Upon successful completion, semget() shall return a non-negative inte‐
ger, namely a semaphore identifier; otherwise, it shall return −1 and
set errno to indicate the error.
ERRORS
The semget() function shall fail if:
EACCES A semaphore identifier exists for key, but operation permission
as specified by the low-order 9 bits of semflg would not be
granted; see Section 2.7, XSI Interprocess Communication.
EEXIST A semaphore identifier exists for the argument key but ((semflg
&IPC_CREAT) &&(semflg &IPC_EXCL)) is non-zero.
EINVAL The value of nsems is either less than or equal to 0 or greater
than the system-imposed limit, or a semaphore identifier exists
for the argument key, but the number of semaphores in the set
associated with it is less than nsems and nsems is not equal to
0.
ENOENT A semaphore identifier does not exist for the argument key and
(semflg &IPC_CREAT) is equal to 0.
ENOSPC A semaphore identifier is to be created but the system-imposed
limit on the maximum number of allowed semaphores system-wide
would be exceeded.
The following sections are informative.
EXAMPLES
Refer to semop().
APPLICATION USAGE
The POSIX Realtime Extension defines alternative interfaces for inter‐
process communication. Application developers who need to use IPC
should design their applications so that modules using the IPC routines
described in Section 2.7, XSI Interprocess Communication can be easily
modified to use the alternative interfaces.
RATIONALE
None.
FUTURE DIRECTIONS
A future version may require that the value of the semval, sempid, sem‐
ncnt, and semzcnt members of all semaphores in a semaphore set be ini‐
tialized to zero when a call to semget() creates a semaphore set. Many
semaphore implementations already do this and it greatly simplifies
what an application must do to initialize a semaphore set.
SEE ALSO
Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
ftok(), semctl(), semop(), sem_close(), sem_destroy(), sem_getvalue(),
sem_init(), sem_open(), sem_post(), sem_trywait(), sem_unlink()
The Base Definitions volume of POSIX.1‐2008, Section 4.16, Semaphore,
<sys_sem.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
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IEEE/The Open Group 2013 SEMGET(3P)
