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NAME    [Toc]    [Back]

       msgctl - message control operations

SYNOPSIS    [Toc]    [Back]

       # include <sys/types.h>
       # include <sys/ipc.h>
       # include <sys/msg.h>

       int msgctl ( int msqid, int  cmd, struct msqid_ds *buf )

DESCRIPTION    [Toc]    [Back]

       The  function  performs	the  control operation specified by cmd on the
       message queue with identifier msqid.  Legal values for cmd are:

       IPC_STAT    Copy info from the message queue data  structure  into  the
		   structure  pointed  to  by  buf.   The  user must have read
		   access privileges on the message queue.

       IPC_SET	   Write the values of some members of the msqid_ds  structure
		   pointed  to	by  buf  to  the message queue data structure,
		   updating also its  msg_ctime  member.   Considered  members
		   from  the  user  supplied struct msqid_ds pointed to by buf

			msg_perm.mode  /* only lowest 9-bits */

		   The calling process effective user-ID  must	be  one  among
		   super-user,	creator  or  owner of the message queue.  Only
		   the super-user can raise the msg_qbytes  value  beyond  the
		   system parameter MSGMNB.

       IPC_RMID    Remove  immediately	the  message queue and its data structures
 awakening all waiting	reader	and  writer  processes
		   (with an error return and errno set to EIDRM).  The calling
		   process effective user-ID must  be  one  among  super-user,
		   creator or owner of the message queue.

RETURN VALUE    [Toc]    [Back]

       If  successful,	the  return  value  will be 0, otherwise -1 with errno
       indicating the error.

ERRORS    [Toc]    [Back]

       For a failing return, errno will be set to one among the following values:

       EACCES	  The  argument  cmd  is  equal  to  IPC_STAT  but the calling
		  process has no read access permissions on the message  queue

       EFAULT	  The  argument  cmd  has  value  IPC_SET  or IPC_STAT but the
		  address pointed to by buf isn't accessible.

       EIDRM	  The message queue was removed.

       EINVAL	  Invalid value for cmd or msqid.

       EPERM	  The argument cmd has value IPC_SET or IPC_RMID but the calling
 process effective user-ID has insufficient privileges to
		  execute the command.	Note this is also the case  of	a  non
		  super-user  process  trying to increase the msg_qbytes value
		  beyond the value specified by the system parameter MSGMNB.

NOTES    [Toc]    [Back]

       The IPC_INFO, MSG_STAT and MSG_INFO  control  calls  are  used  by  the
       ipcs(8)	program to provide information on allocated resources.	In the
       future these can be modified as needed or moved to a proc  file	system

       Various	fields	in  a  struct msqid_ds were shorts under Linux 2.2 and
       have become longs under Linux 2.4. To take advantage of this, a	recompilation
  under glibc-2.1.91 or later should suffice.  (The kernel distinguishes
 old and new calls by a IPC_64 flag in cmd.)

CONFORMING TO    [Toc]    [Back]

       SVr4, SVID.  SVID does not document the EIDRM error condition.

SEE ALSO    [Toc]    [Back]

       ipc(5), msgget(2), msgsnd(2), msgrcv(2)

Linux 2.4.1			  2001-02-18			     MSGCTL(2)
[ Back ]
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