SHMGET(P) POSIX Programmer's Manual SHMGET(P)NAMEshmget - get an XSI shared memory segment
SYNOPSIS
#include <sys/shm.h>
int shmget(key_t key, size_t size, int shmflg);
DESCRIPTION
The shmget() function operates on XSI shared memory (see the Base Defi‐
nitions 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 Realtime .
The shmget() function shall return the shared memory identifier associ‐
ated with key.
A shared memory identifier, associated data structure, and shared mem‐
ory 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 process.
* 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.
RETURN VALUE
Upon successful completion, shmget() shall return a non-negative inte‐
ger, namely a shared memory identifier; otherwise, it shall return -1
and set errno to indicate the error.
ERRORS
The shmget() function shall fail if:
EACCES A shared memory identifier exists for key but operation permis‐
sion as specified by the low-order nine bits of shmflg would not
be granted; see XSI Interprocess Communication .
EEXIST A shared memory identifier exists for the argument key but (shm‐
flg &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 sys‐
tem-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 not 0.
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.
EXAMPLES
None.
APPLICATION USAGE
The POSIX Realtime Extension defines alternative interfaces for inter‐
process 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 interfaces.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
XSI Interprocess Communication , Realtime , shmat() , shmctl() ,
shmdt() , shm_open() , shm_unlink() , the Base Definitions volume of
IEEE Std 1003.1-2001, <sys/shm.h>
COPYRIGHT
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 http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 SHMGET(P)
