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 the value of shmperm.cuid or
                 shmperm.uid in the data structure associated with shmid or
                 to a process that has the super-user privilege.

     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 the
                 value of shmperm.cuid or shmperm.uid in the data structure
                 associated with shmid or to a process that has the super-user
                 privilege.

     SHMLOCK    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).

     SHMUNLOCK  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 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 is
                 not equal to the value of shmperm.cuid or shmperm.uid in
                 the data structure associated with shmid and the process does
                 not have the super-user privilege.

     EPERM       cmd is equal to SHMLOCK or SHMUNLOCK and the calling
                 process does not have the super-user privilege.

     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.



















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