The shmctl() function provides a variety of shared memory control operations as specified by cmd. The permission required for a shared memory control operation is given as {token}, where token is the type of permission needed. The types of permission are interpreted as follows:

00400 READ by user 00200 WRITE by user 00040 READ by group 00020 WRITE by group 00004 READ by others 00002 WRITE by others

See the Shared Memory Operation Permissions section of intro(2) for more information.

The following operations require the specified tokens:

IPC_STAT

Place the current value of each member of the data structure associated with shmid into the structure pointed to by buf. The contents of this structure are defined in intro(2). {READ}

IPC_SET

Set the value of the following members of the data structure associated with shmid to the corresponding value found in the structure pointed to by buf:

shm_perm.uid shm_perm.gid shm_perm.mode /* access permission bits only */

This command can be executed only by a process that has an effective user ID equal to that of super-user, or to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid.

IPC_RMID

Remove the shared memory identifier specified by shmid from the system and destroy the shared memory segment and data structure associated with it. This command can be executed only by a process that has an effective user ID equal to that of super-user, or to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid.

SHM_LOCK

Lock the shared memory segment specified by shmid in memory. This command can be executed only by a process that has an effective user ID equal to super-user.

SHM_UNLOCK

Unlock the shared memory segment specified by shmid. This command can be executed only by a process that has an effective user ID equal to super-user.

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

Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.

The shmctl() function will fail if:

EACCES

The cmd argument is equal to IPC_STAT and {READ} operation permission is denied to the calling process.

EFAULT

The buf argument points to an illegal address.

EINVAL

The shmid argument is not a valid shared memory identifier; or the cmd argument is not a valid command or is IPC_SET and shm_perm.uid or shm_perm.gid is not valid.

ENOMEM

The cmd argument is equal to SHM_LOCK and there is not enough memory.

EOVERFLOW

The cmd argument is IPC_STAT and uid or gid is too large to be stored in the structure pointed to by buf.

EPERM

The cmd argument is equal to IPC_RMID or IPC_SET and the effective user ID of the calling process is not super-user and it is not equal to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid.

EPERM

The cmd argument is equal to SHM_LOCK or SHM_UNLOCK and the effective user ID of the calling process is not equal to that of super-user.

ipcs(1), intro(2), shmget(2), shmop(2)