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, size, shmflg)
key_t key;
int size, shmflg;

DESCRIPTION

shmget() 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:

•   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.

A shared memory identifier (shmid) is a unique positive integer created by a shmget(2) system call. Each shmid has a segment of memory (referred to as a shared memory segment) and a data structure associated with it. The data structure is referred to as shmid_ds and contains the following members:

    struct	ipc_perm shm_perm;	/* operation permission struct */
    int	shm_segsz;	/* size of segment */
    ushort	shm_cpid;	/* creator pid */
    ushort	shm_lpid;	/* pid of last operation */
    short	shm_nattch;	/* number of current attaches */
    time_t	shm_atime;	/* last attach time */
    time_t	shm_dtime;	/* last detach time */
    time_t	shm_ctime;	/* last change time */
    		/* Times measured in secs since */
    		/* 00:00:00 GMT, Jan. 1, 1970 */

shm_perm is an ipc_perm structure that specifies the shared memory operation permission (see below). This structure includes the following members:

    ushort	cuid;	/* creator user id */
    ushort	cgid;	/* creator group id */
    ushort	uid;	/* user id */
    ushort	gid;	/* group id */
    ushort	mode;	/* r/w permission */

shm_segsz specifies the size of the shared memory segment. shm_cpid is the process ID of the process that created the shared memory identifier. shm_lpid is the process ID of the last process that performed a shmop(2) operation. shm_nattch is the number of processes that currently have this segment attached. shm_atime is the time of the last shmat operation, shm_dtime is the time of the last shmdt operation, and shm_ctime is the time of the last shmctl(2) operation that changed one of the members of the above structure.

RETURN VALUES
shmget() returns a non-negative shared memory identifier on success. On failure, it returns -1 and sets errno to indicate the error.

ERRORS

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 ( (shmflg & IPC_CREAT) && ( shmflg & IPC_EXCL) ) is ``true''.

EINVAL
    size is less than the system-imposed minimum or greater than the system-imposed maximum.

    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''.

ENOMEM
    A shared memory identifier and associated shared memory segment are to 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.

SEE ALSO
ipcrm(1), ipcs(1), intro(2), shmctl(2), shmop(2)