SHMGET(P) POSIX Programmer's Manual SHMGET(P)
shmget - get an XSI shared memory segment
int shmget(key_t key, size_t size, int shmflg);
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
function interoperates with the realtime interprocess communication facilities defined in
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
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
* 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:
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 XSI Interprocess Commu-
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 system-imposed maximum.
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 and size is
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 communica-
tion. 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 interfaces.
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 SHMGET(P)