NAME

shmctl − shared memory control operations

SYNOPSIS

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

int shmctl (shmid, cmd, buf)
int shmid, 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 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 low 9 bits ∗/

This cmd can only be executed 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 cmd can only be executed 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 cmd can only be executed by a process that has an effective user ID equal to super user. 

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

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

[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 intro(2)].

[EPERM] cmd is equal to IPC_RMID or IPC_SET and the effective user ID of the calling process is not 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.

[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. 

[EFAULT] buf points to an illegal address. 

[ENOMEM] cmd is equal to SHM_LOCK and there is not enough memory. 

SEE ALSO

shmget(2), shmop(2)

DIAGNOSTICS

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

NOTES

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

March 13, 1992