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)