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 shmget() function operates on XSI shared memory (see the Base
Definitions volume of POSIX.1‐2008, Section 3.342, Shared MemoryObject). It is unspecified whether this function interoperates with
the realtime interprocess communication facilities defined in Section2.8, Realtime.
The shmget() function shall return the shared memory identifier
associated with key.
A shared memory identifier, associated data structure, and shared
memory segment of at least size bytes (see <sys/shm.h>) are 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 shared memory identifier
associated with it and (shmflg &IPC_CREAT) is non-zero.
Upon creation, the data structure associated with the new shared
memory identifier shall be initialized as follows:
* The values of shm_perm.cuid, shm_perm.uid, shm_perm.cgid, and
shm_perm.gid are set to the effective user ID and effective group
ID, respectively, of the calling process.
* The low-order nine bits of shm_perm.mode are set to the low-order
nine bits of shmflg.
* The value of shm_segsz is set to the value of size.
* The values of shm_lpid, shm_nattch, shm_atime, and shm_dtime are
set to 0.
* The value of shm_ctime is set to the current time, as described
in Section 2.7.1, IPC General Description.
When the shared memory segment is created, it shall be initialized
with all zero values.
The shmget() function shall fail if:
EACCES A shared memory identifier exists for key but operation
permission as specified by the low-order nine bits of shmflg
would not be granted; see Section 2.7, XSI InterprocessCommunication.
EEXIST A shared memory identifier exists for the argument key but
(shmflg &IPC_CREAT) &&(shmflg &IPC_EXCL) is non-zero.
EINVAL A shared memory segment is to be created and the value of size
is less than the system-imposed minimum or greater than the
EINVAL No shared memory segment is to be created and a shared memory
segment exists for key but the size of the segment associated
with it is less than size.
ENOENT A shared memory identifier does not exist for the argument key
and (shmflg &IPC_CREAT) is 0.
ENOMEM A shared memory identifier and associated shared memory
segment shall be created, but the amount of available physical
memory is not sufficient to fill the request.
ENOSPC A shared memory identifier is to be created, but the system-
imposed limit on the maximum number of allowed shared memory
identifiers 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.
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 SHMGET(3P)