NAME

shmget − get shared memory segment

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 size 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. 

shmget will fail if one or more of the following are true:

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

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.  ­[EACCES]

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.  ­[EINVAL]

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

A shared memory identifier is to be created but the system-imposed limit on the maximum number of allowed shared memory identifiers systemwide would be exceeded.  ­[ENOSPC]

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.  ­[ENOMEM]

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

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. 

SEE ALSO

shmctl(2), shmop(2). 

Version 3.6  —  December 20, 1987