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 glossary) are created for key if one of the following is true:

Key is equal to IPC_PRIVATE.  This call creates a new identifier, subject to available resources.  The identifier will never be returned by another call to shmget until it has been released by a call to shmctl. The identifier should be used among the calling process and its descendents; however, it is not a requirement. The resource can be accessed by any process having the proper permissions.

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. 

EXAMPLES

The following call to shmget returns a unique shmid for the newly created shared memory segment of 4096 bytes:

int myshmid; myshmid = shmget (IPC_PRIVATE, 4096, 0600);

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 glossary) 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”. 

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

ipcrm(1), ipcs(1), shmctl(2), shmop(2), stdipc(3C). 

STANDARDS CONFORMANCE

shmget: SVID2, XPG2, XPG3

Hewlett-Packard Company  —  HP-UX Release 7.0: Sept 1989