SHMGET(S) UNIX System V SHMGET(S)
Name
shmget - get shared memory segment identifier
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int shmget (key, size, shmflg)
key_t key;
int size, shmflg;
Description
The shmget system call returns the shared memory identifier
associated with key.
A shared memory identifier and associated data structure and
shared memory segment of at least size bytes (see intro(S))
are created for key if one of the following is true:
key is equal to IPC_PRIVATE.
key does not already have a shared memory identifier
associated with it, and (shmflg & IPC_CREAT) is
``true''.
Upon creation, the data structure associated with the new
shared memory identifier is initialized as follows:
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 9 bits of shm_perm.mode are set equal to
the low-order 9 bits of shmflg. shm_segsz is set equal
to the value of size.
shm_lpid, shm_nattch, shm_atime, and shm_dtime are set
equal to 0.
shm_ctime is set equal to the current time.
The shmget system call will fail if one or more of the
following is true:
[EINVAL] size is less than the system-imposed minimum
or greater than the system-imposed maximum.
[EACCES] A shared memory identifier exists for key,
but operation permission (see intro(S)) as
specified by the low-order 9 bits of shmflg
would not be granted.
[EINVAL] A shared memory identifier exists for key,
but the size of the segment associated with
it is less than size, and size is not equal
to zero.
[ENOENT] A shared memory identifier does not exist for
key, and (shmflg & IPC_CREAT) is ``false''.
[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.
[ENOMEM] A shared memory identifier and associated
shared memory segment are to be created, but
the amount of available memory is not
sufficient to fill the request.
[EEXIST] A shared memory identifier exists for key but
(shmflg & IPC_CREAT) and (shmflg& IPC_EXCL)
are ``true''.
See Also
intro(S), shmctl(S), shmop(S)
Diagnostics
Upon successful completion, a non-negative integer, namely a
shared memory identifier is returned. Otherwise, a value of
-1 is returned, and errno is set to indicate the error.
Notes
The user must explicitly remove shared memory segments after
the last reference to them has been removed.
Standards Conformance
shmget is conformant with:
AT&T SVID Issue 2, Select Code 307-127;
and The X/Open Portability Guide II of January 1987.
(printed 6/20/89)