shmctl(2)shmctl(2)NAMEshmctl - 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 */);
DESCRIPTIONshmctl 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 access permission bits */
This command can be executed only by a process that has an
effective user ID equal to the value of shm_perm.cuid or
shm_perm.uid in the data structure associated with shmid or
to a process that has the super-user privilege.
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 the
value of shm_perm.cuid or shm_perm.uid in the data structure
associated with shmid or to a process that has the super-user
privilege.
SHM_LOCK Lock the shared memory segment specified by shmid in memory.
This command can be executed only by a process that has the
super-user privilege. Locking occurs per address space;
multiple processes or sprocs mapping the area at different
addresses each need to issue the lock (this is primarily an
issue with the per-process page tables).
SHM_UNLOCK Unlock the shared memory segment specified by shmid. This
command can be executed only by a process that has the
super-user privilege.
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shmctl(2)shmctl(2)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 [see intro(2)].
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.
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 is
not equal to the value of shm_perm.cuid or shm_perm.uid in
the data structure associated with shmid and the process does
not have the super-user privilege.
EPERM cmd is equal to SHM_LOCK or SHM_UNLOCK and the calling
process does not have the super-user privilege.
EFAULT buf points to an illegal address.
ENOMEM cmd is equal to SHM_LOCK and there is not enough memory.
SEE ALSOshmget(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.
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