SHMGET(2) — SYSTEM CALLS

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.

RETURN VALUE

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.

ERRORS

shmget() will fail if one or more of the following are 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(2)) 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 physical memory is not sufficient to fill the request.

EEXIST A shared memory identifier exists for key but ( (shmflg & IPC_CREAT) && ( shmflg & IPC_EXCL) ) is “true”.

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