shmctl

shmctl is a System V IPC control operation for shared memory segments. It is invoked with shmctl(shmid, cmd, buf) where shmid identifies a segment created by shmget, cmd selects the operation, and buf points to a shmid_ds structure used to read or modify metadata.

The standard commands are IPC_STAT, IPC_SET, and IPC_RMID. IPC_STAT copies the current metadata of the segment into the user-supplied shmid_ds buffer. IPC_SET updates the segment’s metadata from the provided shmid_ds, typically to change ownership and permission bits. IPC_RMID marks the segment to be destroyed; the segment is not removed until there are no more attachments, or immediately if no processes are attached.

The shmid_ds structure contains information about a segment. It includes a nested shm_perm structure with owner user id (uid), owner group id (gid), creator uid, creator gid, and access mode bits. It also records the size of the segment (shm_segsz), and timing and process fields: shm_atime, shm_dtime, shm_ctime, shm_cpid, shm_lpid, and the count of attached processes (shm_nattch). The exact layout may vary across platforms, but these fields are common on Linux and other Unixes.

Usage notes: shmctl requires appropriate permissions for IPC_STAT or IPC_SET; IPC_RMID removes the segment but does not free resources while processes still attach. Typical error codes include EACCES for permission problems, EINVAL for an invalid shmid or command, EFAULT for a bad memory buffer, and EIDRM if the segment has been removed.

Overall, shmctl provides a mechanism to inspect, adjust, and reclaim System V shared memory segments in a controlled manner.