Section: POSIX Programmer's Manual (3P)
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.
--- get set of XSI semaphores
int semget(key_t key, int nsems, int semflg);
function operates on XSI semaphores (see the Base Definitions volume of POSIX.1-2017,
It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in
function shall return the semaphore identifier associated with
A semaphore identifier with its associated
data structure and its associated set of
is created for
if one of the following is true:
is equal to IPC_PRIVATE.
does not already have a semaphore identifier associated with it and
(semflg &IPC_CREAT) is non-zero.
Upon creation, the
data structure associated with the new semaphore identifier is
initialized as follows:
In the operation permissions structure
shall be set to the effective user ID and effective group ID,
respectively, of the calling process.
The low-order 9 bits of
shall be set to the low-order 9 bits of
shall be set to the value of
shall be set to 0 and
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
function with the command SETVAL or SETALL
can be used to initialize each semaphore.
Upon successful completion,
shall return a non-negative integer, namely a semaphore identifier;
otherwise, it shall return -1 and set
to indicate the error.
function shall fail if:
A semaphore identifier exists for
but operation permission as specified by the low-order 9 bits of
would not be granted; see
Section 2.7, XSI Interprocess Communication.
A semaphore identifier exists for the argument
but ((semflg &IPC_CREAT) &&(semflg &IPC_EXCL))
The value of
is either less than or equal to 0 or greater than the system-imposed
limit, or a semaphore identifier exists for the argument
but the number of semaphores in the set associated with it is less than
is not equal to 0.
A semaphore identifier does not exist for the argument
and (semflg &IPC_CREAT) is equal to 0.
A semaphore identifier is to be created but the system-imposed limit on
the maximum number of allowed semaphores system-wide would be
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
, XSI Interprocess Communication
can be easily modified to use the alternative interfaces.
A future version may require that the value of the
members of all semaphores in a semaphore set be initialized to zero when
a call to
creates a semaphore set. Many semaphore implementations already do this
and it greatly simplifies what an application must do to initialize a
, XSI Interprocess Communication,
The Base Definitions volume of POSIX.1-2017,
Section 4.17, Semaphore,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 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
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