SHMCTL(2) System Calls Manual SHMCTL(2)
shmctl -- shared memory control operations
Standard C Library (libc, -lc)
shmctl(int shmid, int cmd, struct shmid_ds *buf);
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
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.
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.
Upon successful completion, a value of 0 is returned. Otherwise, -1 is
returned and the global variable errno is set to indicate the error.
shmctl() will fail if:
[EACCES] The command is IPC_STAT and the caller has no read
permission for this shared memory segment.
[EFAULT] buf specifies an invalid address.
[EINVAL] shmid is not a valid shared memory segment identifier.
cmd is not a valid command.
[ENOMEM] The cmd is equal to SHM_LOCK and there is not enough
[EPERM] cmd is equal to IPC_SET or IPC_RMID and the caller is
not the super-user, nor does the effective uid match
either the shm_perm.uid or shm_perm.cuid fields of the
data structure associated with the shared memory
An attempt was made to increase the value of
shm_qbytes through IPC_SET but the caller is not the
The cmd is equal to SHM_LOCK or SHM_UNLOCK and the
caller is not the super-user.
ipcrm(1), ipcs(1), shmat(2), shmget(2)
The shmctl system call conforms to X/Open System Interfaces and Headers
Issue 5 (``XSH5'').
Shared memory segments appeared in the first release of AT&T System V
NetBSD 6.1.5 November 25, 2006 NetBSD 6.1.5