SHMCTL(S) UNIX System V SHMCTL(S)
Name
shmctl - shared memory control operations
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int shmctl (shmid, cmd, buf)
int shmid, cmd;
struct shmid_ds *buf;
Description
The shmctl system call 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(S). {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 */
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.
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 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.
SHM_LOCK
Lock the shared memory segment specified by shmid in
memory. This cmd can only be executed by a process
that has an effective user ID equal to super-user.
SHM_UNLOCK
Unlock the shared memory segment specified by shmid.
This cmd can only be executed by a process that has an
effective user ID equal to super-user.
The shmctl system call will fail if one or more of the
following is true:
[EINVAL] shmid is not a valid shared memory identifier.
[EINVAL] cmd is not a valid command.
[EACCES] cmd is equal to IPC_STAT and {READ} operation
permission is denied to the calling process (see
intro(S)).
[EPERM] cmd is equal to IPC_RMID or IPC_SET, and the
effective user ID of the calling process is not
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.
[EPERM] cmd 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.
[EFAULT] buf points to an illegal address.
[ENOMEM] cmd is equal to SHM_LOCK, and there is not
enough memory.
See Also
shmget(S), shmop(S)
Diagnostics
Upon successful completion, a value of 0 is returned.
Otherwise, a value of -1 is returned, and errno is set to
indicate the error.
Notes
The user must explicitly remove shared memory segments after
the last reference to them has been removed.
Standards Conformance
shmctl is conformant with:
AT&T SVID Issue 2, Select Code 307-127;
and The X/Open Portability Guide II of January 1987.
(printed 6/20/89)