shmctl() provides a variety of shared memory control operations as specified by cmd. The following cmds are available:

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 /** only low 9 bits **/

IPC_RMID

Remove the shared memory identifier specified by shmid from the system. If no processes are currently mapped to the corresponding shared memory segment, then the segment is removed and the associated resources are reclaimed. Otherwise, the segment will persist, although shmget(2) will not be able to locate it, until it is no longer mapped by any process. This cmd can only be executed 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.

In the shmop(2) and shmctl(2) system call descriptions, the permission required for an operation is given as "[token]", where (token) is the type of permission needed interpreted as follows:

00400	Read by user
00200	Write by user
00060	Read, Write by group
00006	Read, Write by others

Read and Write permissions on a shmid are granted to a process if one or more of the following are true: The effective user ID of the process is super-user. The effective user ID of the process matches shm_perm.[c]uid in the data structure associated with shmid and the appropriate bit of the (user) portion (0600) of shm_perm.mode is set. The effective user ID of the process does not match shm_perm.[c]uid and the effective group ID of the process matches shm_perm.[c]gid and the appropriate bit of the (group) portion (060) of shm_perm.mode is set.
The effective user ID of the process does not match shm_perm.[c]uid and the effective group ID of the process does not match shm_perm.[c]gid and the appropriate bit of the (other) portion (06) of shm_perm.mode is set.

Otherwise, the corresponding permissions are denied.

shmctl() returns:

0

on success.

-1

on failure and sets errno to indicate the error.

EACCES

cmd is equal to IPC_STAT and [READ] operation permission is denied to the calling process (see intro(2)).

EFAULT

buf points to an illegal address.

EINVAL

shmid is not a valid shared memory identifier. cmd is not a valid command.

EPERM

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