msgctl() -- General Function (libc)

Message control operations
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgctl(id, command, buffer)
int id; int command; struct msqid_ds *buffer;

The function msgctl()  controls the COHERENT's system's messaging facility.
This facility permits processes to pass messages from one another.

Each message queue is controlled by  a structure of type msqid_ds, which is
defined  in header  file <sys/msg.h>.  This  structure points  to the
first and last  messages in the queue, gives the  size of the queue and the
number of messages  in the queue, and names who  can manipulate it and how.
The messages themselves consist of a linked list of structures of type msg,
which  is also  defined  in msg.h.  When  the function  msgget() creates  a
message  queue,  it assigns  to  that queue  an  identification number  and
returns that  number to the calling process.  For  details on this process,
see the Lexicon entry for msgget().

id identifies  the message queue  to be manipulated.  This  value must have
been returned by a call to msgget().

command names  the operation that  you want msgctl()  to perform.  msgctl()
recognizes the following commands:

IPC_STAT  Copy  the  message-queue  structure  identified  by id  into  the
          structure  pointed to  by buffer.  This  command lets  you gather
          information about  a message queue  without actually manipulating
          the queue.

IPC_SET   This  command sets  permissions for  this queue.   It does  so by
          copying  fields  msg_perm.uid,  msg_perm.gid, msg_perm.mode  (low
          nine bits only),  and msg_qbytes from the message-queue structure
          point  to  by buffer  to  structure identified  by  id. Only  the
          superuser root  and the  user who  owns the process  that created
          structure  id  can  execute  this  command.  Note that  only  the
          superuser can  raise the value  of field msg_qbytes,  which gives
          the size of space occupied by the queue, in bytes.

IPC_RMID  Remove  the structure  identified by id,  and destroy  its queue.
          Only the  superuser root and  the user who owns  the process that
          created structure id do this.

If  any of  the following  conditions occur, msgctl()  returns -1  and sets
error to the value in parentheses:

-> id is not a valid message-queue identifier (EINVAL).

-> command is not a valid command (EINVAL).

-> command  equals IPC_STAT,  but the  owner of  the calling  process lacks
   permission to execute this command (EACCES).

-> command equals IPC_RMID or IPC_SET, but the owner of the calling process
   lacks permission to execute the command (EPERM).

-> A process  owned by someone  other than the superuser  root attempted to
   increase field msg_qbytes (EPERM).

-> buffer points to an illegal address (EFAULT).

If all went well, msgctl() returns zero.

Example

For an example of this function, see the Lexicon entry for msgget().

Files

/usr/include/sys/ipc.h
/usr/include/sys/msg.h

See Also

libc,
msgget(),
msgrcv(),
msgsnd()

Notes

For  information on  other methods of  interprocess communication,  see the
Lexicon entries for semctl(), shmctl(), and libsocket.

© 2012 by Stephen A. Ness