NAME

shmctl − shared memory control operations

SYNOPSIS

#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 following cmds are available:

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 the glossary. 

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 low 9 bits */

This cmd can only be executed by a process that has an effective user ID equal to either that of a user having appropriate privileges or to the value of either shm_perm.uid or shm_perm.cuid 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.  If the segment is attached to one or more processes, then the segment key is changed to IPC_PRIVATE and the segment is marked removed.  The segment disappears when the last attached process detaches it.  This cmd can only be executed by a process that has an effective user ID equal to either that of a user with appropriate privileges or to the value of either shm_perm.uid or shm_perm.cuid in the data structure associated with shmid.

SHM_LOCK Lock the shared memory segment specified by shmid in memory.  This cmd can only be executed by a process that either has an effective user ID equal to that of a user having appropriate privileges or has an effective user ID equal to the value of either shm_perm.uid or shm_perm.cuid in the data structure associated with shmid and has PRIV_MLOCK privilege (see setprivgrp() description, getprivgrp(2)).

SHM_UNLOCK Unlock the shared memory segment specified by shmid. This cmd can only be executed by a process that either has an effective user ID equal to a user having appropriate privileges or has an effective user ID equal to the value of either shm_perm.uid or shm_perm.cuid in the data structure associated with shmid and has PRIV_MLOCK privilege (see setprivgrp() description, getprivgrp(2)).

RETURN VALUE

shmctl() returns a value of 0 upon successful completion; otherwise, a value of −1 is returned and errno is set to indicate the error. 

ERRORS

shmctl() fails if any of the following conditions are encountered (see DEPENDENCIES):

[EINVAL] shmid is not a valid shared memory identifier. 

[EINVAL] cmd is not a valid command. 

[EACCES] cmd is equal to IPC_STAT and Read operation permission is denied to the calling process (see shared memory operation permissions in glossary(9)).

[EPERM] cmd is equal to IPC_RMID, IPC_SET, SHM_LOCK, or SHM_UNLOCK and the effective user ID of the calling process is not equal to that of a user having appropriate privileges and it is not equal to the value of either shm_perm.uid or shm_perm.cuid 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 a user having appropriate privileges and the calling process does not have PRIV_MLOCK privilege (see setprivgrp() description, getprivgrp(2)).

[EINVAL] cmd is equal to SHM_UNLOCK and the shared-memory segment specified by shmid is not locked in memory. 

[EFAULT] buf points to an illegal address.  The reliable detection of this error is implementation dependent. 

[ENOMEM] cmd is equal to SHM_LOCK and there is not sufficient lockable memory to fill the request. 

EXAMPLES

The following call to shmctl() locks in memory the shared memory segment represented by myshmid. This example assumes the process has a valid shmid, which can be obtained by calling shmget(2).

shmctl (myshmid, SHM_LOCK, 0);

The following call to shmctl() removes the shared memory segment represented by myshmid. This example assumes the process has a valid shmid, which can be obtained by calling shmget() (see shmget(2).

shmctl (myshmid, IPC_RMID, 0);

DEPENDENCIES

Series 300/400

An additional error condition can occur on Series 300/400 systems:

[EACCES] shmid is the id of a shared memory segment currently being used by the system to implement other features (see graphics(7) and iomap(7)).

AUTHOR

shmctl() was developed by AT&T and HP. 

SEE ALSO

ipcrm(1), ipcs(1), shmget(2), shmop(2), stdipc(3C). 

STANDARDS CONFORMANCE

shmctl(): SVID2, XPG2, XPG3, XPG4

Hewlett-Packard Company  —  HP-UX Release 9.0: August 1992