The
shmctl() system call performs control operations on the shared memory segment specified by
shmid.
Each shared memory segment has a
shmid_ds structure associated with it which contains the following members:
struct ipc_perm shm_perm; /* operation permissions */
size_t shm_segsz; /* size of segment in bytes */
pid_t shm_lpid; /* pid of last shm op */
pid_t shm_cpid; /* pid of creator */
shmatt_t shm_nattch; /* # of current attaches */
time_t shm_atime; /* last shmat() time */
time_t shm_dtime; /* last shmdt() time */
time_t shm_ctime; /* last change by shmctl() */
The
ipc_perm structure used inside the
shmid_ds structure is defined in
<sys/ipc.h> and contains the following members:
uid_t cuid; /* creator user id */
gid_t cgid; /* creator group id */
uid_t uid; /* user id */
gid_t gid; /* group id */
mode_t mode; /* permission (lower 9 bits) */
The operation to be performed by
shmctl() is specified in
cmd and is one of:
IPC_STAT
Gather information about the shared memory segment and place it in the structure pointed to by buf.
IPC_SET
Set the value of the shm_perm.uid, shm_perm.gid and shm_perm.mode fields in the structure associated with shmid. The values are taken from the corresponding fields in the structure pointed to by buf. This operation can only be executed by the super-user, or a process that has an effective user id equal to either shm_perm.cuid or shm_perm.uid in the data structure associated with the shared memory segment.
IPC_RMID
Remove the shared memory segment specified by shmid and destroy the data associated with it. Only the super-user or a process with an effective uid equal to the shm_perm.cuid or shm_perm.uid values in the data structure associated with the segment can do this.
SHM_LOCK
Lock the shared memory segment specified by shmid in memory. This operation can only be executed by the super-user.
SHM_UNLOCK
Unlock the shared memory segment specified by shmid. This operation can only be executed by the super-user.
The read and write permissions on a shared memory identifier are determined by the
shm_perm.mode field in the same way as is done with files (see
chmod(2)), but the effective uid can match either the
shm_perm.cuid field or the
shm_perm.uid field, and the effective gid can match either
shm_perm.cgid or
shm_perm.gid.