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(7) for descriptions of the following attributes:


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

SEE ALSO

ipcs(1), Intro(2), shmget(2), shmop(2), attributes(7), privileges(7),
standards(7)

April 10, 2007 SHMCTL(2)