shmctl - shared memory control operations
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int shmctl(int shmid, int cmd, struct shmid_ds *buf);
The shmctl() function provides a variety of shared memory
control operations as specified by cmd. The permission required for a
shared memory control operation is given as {token}, where
token is the type of permission needed. The types of permission are
interpreted as follows:
00400 READ by user
00200 WRITE by user
00040 READ by group
00020 WRITE by group
00004 READ by others
00002 WRITE by others
See the Shared Memory Operation Permissions section of
Intro(2) for more information.
The following operations require the specified tokens:
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 /* access permission bits only */
This command can be executed only by a process that has
appropriate privileges or an effective user ID equal 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. The segment referenced by the identifier will be
destroyed when all processes with the segment attached have either detached
the segment or exited. If the segment is not attached to any process when
IPC_RMID is invoked, it will be destroyed immediately. This command can
be executed only by a process that has appropriate privileges or an effective
user ID equal 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 command can be executed only by a process that has appropriate
privileges.
SHM_UNLOCK
Unlock the shared memory segment specified by
shmid. This command can be executed only by a process that has
appropriate privileges.
A shared memory segment must be explicitly removed using
IPC_RMID before the system can deallocate it and the resources it
uses.
Upon successful completion, 0 is returned. Otherwise,
−1 is returned and errno is set to indicate the
error.
The shmctl() function will fail if:
EACCES
The cmd argument is equal to IPC_STAT and
{READ} operation permission is denied to the calling process.
EFAULT
The buf argument points to an illegal
address.
EINVAL
The shmid argument is not a valid shared memory
identifier; or the cmd argument is not a valid command or is
IPC_SET and shm_perm.uid or shm_perm.gid is not
valid.
ENOMEM
The cmd argument is equal to SHM_LOCK and
there is not enough memory, or the operation would exceed a limit or resource
control on locked memory.
EOVERFLOW
The cmd argument is IPC_STAT and uid
or gid is too large to be stored in the structure pointed to by
buf.
EPERM
The
cmd argument is equal to
IPC_RMID or
IPC_SET, the effective user ID of the calling process is not equal to
the value of
shm_perm.cuid or
shm_perm.uid in the data structure
associated with
shmid, and {
PRIV_IPC_OWNER} is not asserted in
the effective set of the calling process.
The cmd argument is equal to SHM_LOCK or
SHM_UNLOCK and {PRIV_PROC_LOCK_MEMORY} is not asserted in the
effective set of the calling process.
See attributes(7) for descriptions of the following
attributes:
| ATTRIBUTE
TYPE |
ATTRIBUTE VALUE |
| Interface Stability |
Standard |