NAME

shmget − get shared memory segment

SYNOPSIS

#include <sys/shm.h>

int shmget(key_t key, size_t size, int 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(9)) 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. 

• shm_perm.cuid, 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);

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() fails if any of the following conditions are encountered:

[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(9)) 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”.

SEE ALSO

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

STANDARDS CONFORMANCE

shmget(): SVID2, XPG2, XPG3, XPG4

Hewlett-Packard Company  —  HP-UX Release 9.0: August 1992