shmctl(2) 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 shmidds *buf);
DESCRIPTION
shmctl provides a variety of shared memory control operations as
specified by cmd. The following cmds are available:
IPCSTAT 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}
IPCSET 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:
shmperm.uid
shmperm.gid
shmperm.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 shmperm.cuid or shmperm.uid in the data
structure associated with shmid.
IPCRMID 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
shmperm.cuid or shmperm.uid in the data structure
associated with shmid.
SHMLOCK 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.
SHMUNLOCK 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.
shmctl fails if one or more of the following are true:
8/91 Page 1
shmctl(2) shmctl(2)
EACCES cmd is equal to IPCSTAT and {READ} operation permission
is denied to the calling process [see intro(2)].
EINVAL shmid is not a valid shared memory identifier.
EINVAL cmd is not a valid command.
EINVAL cmd is IPCSET and shmperm.uid or shmperm.gid is not
valid.
EOVERFLOW cmd is IPCSTAT and uid or gid is too large to be stored
in the structure pointed to by buf.
EPERM cmd is equal to IPCRMID or IPCSET and the effective
user ID of the calling process is not equal to that of
super-user, or to the value of shmperm.cuid or
shmperm.uid in the data structure associated with shmid.
EPERM cmd is equal to SHMLOCK or SHMUNLOCK 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 SHMLOCK 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.
Page 2 8/91