SHMGET(2) BSD System Calls Manual SHMGET(2)NAME
shmget -- get shared memory area identifier
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int
shmget(key_t key, int size, int shmflg);
DESCRIPTION
shmget() returns the shared memory identifier associated with the key key.
A shared memory segment is created if either key is equal to IPC_PRIVATE, or key does not have a shared memory segment identifier associated
with it, and the IPC_CREAT bit is set in shmflg.
If a new shared memory segment is created, the data structure associated with it (the shmid_ds structure, see shmctl(2)) is initialized as
follows:
o shm_perm.cuid and shm_perm.uid are set to the effective uid of the calling process.
o shm_perm.gid and shm_perm.cgid are set to the effective gid of the calling process.
o shm_perm.mode is set to the lower 9 bits of shmflg.
o shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0
o shm_ctime is set to the current time.
o shm_segsz is set to the value of size.
RETURN VALUES
Upon successful completion a positive shared memory segment identifier is returned. Otherwise, -1 is returned and the global variable errno
is set to indicate the error.
ERRORS
[EACESS] A shared memory segment is already associated with key and the caller has no permission to access it.
[EEXIST] Both IPC_CREAT and IPC_EXCL are set in shmflg, and a shared memory segment is already associated with key.
[ENOSPC] A new shared memory indentifier could not be created because the system limit for the number of shared memory identifiers
has been reached.
[ENOENT] IPC_CREAT was not set in shmflg and no shared memory segment associated with key was found.
[ENOMEM] There is not enough memory left to created a shared memory segment of the requested size.
SEE ALSO shmctl(2), shmat(2), shmdt(2)BSD August 17, 1995 BSD
Check Out this Related Man Page
SHMGET(P) POSIX Programmer's Manual SHMGET(P)
NAME
shmget - 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 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 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 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 integer, 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 permission 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 (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 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 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 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 Technol-
ogy -- 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)