The shmget() function 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:

• The key argument is equal to IPC_PRIVATE.
• The key argument 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:

• The values of 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 access permission bits of shm_perm.mode are set equal to the access permission bits of shmflg. shm_segsz is set equal to the value of size.
• The values of shm_lpid, shm_nattch shm_atime, and shm_dtime are set equal to 0.
• The shm_ctime is set equal to the current time.

Shared memory segments must be explicitly removed after the last reference to them has been removed.

Upon successful completion, a non-negative integer representing a shared memory identifier is returned. Otherwise, -1 is returned and errno is set to indicate the error.

The shmget() function will fail if:

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 both (shmflg&IPC_CREATE) and (shmflg&IPC_EXCL) are true.

EINVAL

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

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 0.

ENOENT

A shared memory identifier does not exist for key and (shmflg&IPC_CREATE) is false.

ENOMEM

A shared memory identifier and associated shared memory segment are to be created but the amount of available 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.

intro(2), shmctl(2), shmop(2), ftok(3C)