The shmget() function operates on XSI shared memory (see the
Base Definitions volume of IEEE Std 1003.1-2001,
Section 3.340, Shared Memory Object). It is unspecified whether this
interoperates with the realtime interprocess communication facilities
defined in 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)
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 equal to the
effective user ID and effective group ID, respectively, of the calling
The low-order nine bits of shm_perm.mode are set equal to the
low-order nine bits of shmflg.
The value of shm_segsz is set equal to the value of size.
The values of shm_lpid, shm_nattch, shm_atime, and
shm_dtime are set equal to 0.
The value of shm_ctime is set equal to the current time.
When the shared memory segment is created, it shall be initialized
with all zero values.
Upon successful completion, shmget() shall return a non-negative
integer, namely a shared memory identifier; otherwise,
it shall return -1 and set errno to indicate the error.
The shmget() function shall fail if:
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 XSI Interprocess Communication
A shared memory identifier exists for the argument key but (shmflg
&IPC_EXCL) is non-zero.
A shared memory segment is to be created and the value of size is
less than the system-imposed minimum or greater than the
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 and size is not 0.
A shared memory identifier does not exist for the argument key
and (shmflg &IPC_CREAT) is 0.
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.
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 XSI Interprocess Communication can
be easily modified to use the alternative
XSI Interprocess Communication , Realtime , shmat()
, shmctl() , shmdt() , shm_open() , shm_unlink()
, the Base Definitions volume of IEEE Std 1003.1-2001, <sys/shm.h>
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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