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:

See Also

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