SHMCTL(2) System Calls SHMCTL(2)


NAME


shmctl - shared memory control operations

SYNOPSIS


#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

int shmctl(int shmid, int cmd, struct shmid_ds *buf);


DESCRIPTION


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.

RETURN VALUES


Upon successful completion, 0 is returned. Otherwise, -1 is returned and
errno is set to indicate the error.

ERRORS


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.


ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+

SEE ALSO


ipcs(1), Intro(2), shmget(2), shmop(2), attributes(5), privileges(5),
standards(5)


April 10, 2007 SHMCTL(2)