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.
The semget() function operates on XSI semaphores (see the Base
Definitions volume of POSIX.1‐2008, Section 4.16, Semaphore). It is
unspecified whether this function interoperates with the realtime
interprocess communication facilities defined in Section 2.8,
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
associated with it and (semflg &IPC_CREAT) is non-zero.
Upon creation, the semid_ds data structure associated with the new
semaphore 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
* 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, IPCGeneral 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.
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 InterprocessCommunication.
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.
The POSIX Realtime Extension defines alternative interfaces for
interprocess 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.
A future version may require that the value of the semval, sempid,
semncnt, and semzcnt members of all semaphores in a semaphore set be
initialized 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.
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 Electrical 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
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 SEMGET(3P)