NAME

       shmctl - XSI shared memory control operations

SYNOPSIS

       #include <sys/shm.h>

       int shmctl(int shmid, int cmd, struct shmid_ds *buf);

DESCRIPTION

       The  shmctl()  function  operates on XSI shared memory (see the Base Definitions volume of
       IEEE Std 1003.1-2001, Section 3.340, Shared Memory Object). It is unspecified whether this
       function  interoperates with the realtime interprocess communication facilities defined in
       Realtime .

       The shmctl() function provides a variety of shared memory control operations as  specified
       by cmd. The following values for cmd are available:

       IPC_STAT
              Place  the  current  value of each member of the shmid_ds data structure associated
              with shmid into the structure pointed to by buf. The contents of the structure  are
              defined in <sys/shm.h>.

       IPC_SET
              Set  the  value  of the following members of the shmid_ds 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    Low-order nine bits.

       IPC_SET can only be executed by a process that has an effective user ID  equal  to  either
       that  of  a  process  with  appropriate  privileges  or  to  the value of shm_perm.cuid or
       shm_perm.uid in the shmid_ds 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 shmid_ds data structure associated with it. IPC_RMID
              can only be executed by a process that has an effective user  ID  equal  to  either
              that  of  a process with appropriate privileges or to the value of shm_perm.cuid or
              shm_perm.uid in the shmid_ds data structure associated with shmid.

RETURN VALUE

       Upon successful completion, shmctl() shall return 0; otherwise, it shall return -1 and set
       errno to indicate the error.

ERRORS

       The shmctl() function shall fail if:

       EACCES The  argument  cmd  is equal to IPC_STAT and the calling process does not have read
              permission; see XSI Interprocess Communication .

       EINVAL The value of shmid is not a valid shared memory identifier, or the value of cmd  is
              not a valid command.

       EPERM  The  argument  cmd is equal to IPC_RMID or IPC_SET and the effective user ID of the
              calling process is not equal to that of a process with appropriate  privileges  and
              it is not equal to the value of shm_perm.cuid or shm_perm.uid in the data structure
              associated with shmid.

       The shmctl() function may fail if:

       EOVERFLOW
              The cmd argument is IPC_STAT and the gid or uid value is too large to be stored  in
              the structure pointed to by the buf argument.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       The   POSIX   Realtime   Extension   defines   alternative   interfaces  for  interprocess
       communication. Application developers who need to use IPC should design their applications
       so  that modules using the IPC routines described in XSI Interprocess Communication can be
       easily modified to use the alternative interfaces.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       XSI Interprocess Communication , Realtime , shmat() , shmdt() , shmget()  ,  shm_open()  ,
       shm_unlink() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/shm.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable  Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .