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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

shmctl() 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    /* only access permission bits */

This command can be executed only 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 command can be executed only 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 command can be executed only by a process that has an effective user ID equal to super-user.
SHM_UNLOCK
Unlock the shared memory segment specified by shmid. This command can be executed only by a process that has an effective user ID equal to super-user.

Return Values

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

Errors

shmctl() fails if one or more of the following are true:

EACCES
cmd is equal to IPC_STAT and {READ} operation permission is denied to the calling process.
EFAULT
buf points to an illegal address.
EINVAL
shmid is not a valid shared memory identifier.
EINVAL
cmd is not a valid command.
EINVAL
cmd is IPC_SET and shm_perm.uid or shm_perm.gid is not valid.
ENOMEM
cmd is equal to SHM_LOCK and there is not enough memory.
EOVERFLOW
cmd is IPC_STAT and uid or gid is too large to be stored in the structure pointed to by buf.
EPERM
cmd is equal to IPC_RMID or IPC_SET and the effective user of the calling process is not 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.

See Also

ipcs(1) , intro(2) , shmget(2) , shmop(2)

Notes

The user must explicitly remove shared memory segments after the last reference to them has been removed.

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