SHMGET(2) System Calls SHMGET(2)

NAME

shmget - get shared memory segment identifier

SYNOPSIS

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

int shmget(key_t key, size_t size, int shmflg);

DESCRIPTION

The shmget() function 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(2)) are created for
key if one of the following are true:

o The key argument is equal to IPC_PRIVATE.

o The key argument 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:

o 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.

o The access permission bits of shm_perm.mode are set equal
to the access permission bits of shmflg. shm_segsz is set
equal to the value of size.

o The values of shm_lpid, shm_nattch shm_atime, and
shm_dtime are set equal to 0.

o The shm_ctime is set equal to the current time.


Shared memory segments must be explicitly removed after the last
reference to them has been removed.

RETURN VALUES

Upon successful completion, a non-negative integer representing a
shared memory identifier is returned. Otherwise, -1 is returned and
errno is set to indicate the error.

ERRORS

The shmget() function will fail if:

EACCES
A shared memory identifier exists for key but operation
permission (see Intro(2)) as specified by the low-order 9
bits of shmflg would not be granted.


EEXIST
A shared memory identifier exists for key but both
(shmflg&IPC_CREAT) and (shmflg&IPC_EXCL) are true.


EINVAL
The size argument is less than the system-imposed minimum
or greater than the system-imposed maximum. See NOTES.

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 0.


ENOENT
A shared memory identifier does not exist for key and
(shmflg&IPC_CREAT) is false.


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.


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. See
NOTES.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+

SEE ALSO

Intro(2), setrctl(2), shmctl(2), shmop(2), ftok(3C), getpagesize(3C),
attributes(7), standards(7), rctladm(8)

NOTES

The project.max-shm-memory resource control restricts the total
amount of shared memory a project can allocate. The zone.max-shm-
memory resource control restricts the total amount of shared memory
that can be allocated by a zone. The system-imposed maximum on the
size of a shared memory segment is therefore a function of the sizes
of any other shared memory segments the calling project might have
allocated that are still in use, as well as any other shared memory
segments allocated and still in use by processes in the zone. For
accounting purposes, segment sizes are rounded up to the nearest
multiple of the system page size. See getpagesize(3C).


The system-imposed limit on the number of shared memory identifiers
is maintained on a per-project basis using the project.max-shm-ids
resource control. The zone.max-shm-ids resource control restricts the
total number of shared memory identifiers that can be allocated by a
zone.


See rctladm(8) and setrctl(2) for information about using resource
controls.

August 14, 2006 SHMGET(2)