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MSGCTL(2)		   Linux Programmer's Manual		     MSGCTL(2)

       msgctl -	System V message control operations

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

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

       msgctl()	 performs  the	control	operation specified by cmd on the Sys-
       tem V message queue with	identifier msqid.

       The msqid_ds data structure is defined in _sys/msg.h_ as	follows:

	   struct msqid_ds {
	       struct ipc_perm msg_perm;     /*	Ownership and permissions */
	       time_t	       msg_stime;    /*	Time of	last msgsnd(2) */
	       time_t	       msg_rtime;    /*	Time of	last msgrcv(2) */
	       time_t	       msg_ctime;    /*	Time of	last change */
	       unsigned	long   __msg_cbytes; /*	Current	number of bytes	in
						queue (nonstandard) */
	       msgqnum_t       msg_qnum;     /*	Current	number of messages
						in queue */
	       msglen_t	       msg_qbytes;   /*	Maximum	number of bytes
						allowed	in queue */
	       pid_t	       msg_lspid;    /*	PID of last msgsnd(2) */
	       pid_t	       msg_lrpid;    /*	PID of last msgrcv(2) */

       The ipc_perm structure is defined as follows  (the  highlighted	fields
       are settable using IPC_SET):

	   struct ipc_perm {
	       key_t	      __key;	   /* Key supplied to msgget(2)	*/
	       uid_t	      uid;	   /* Effective	UID of owner */
	       gid_t	      gid;	   /* Effective	GID of owner */
	       uid_t	      cuid;	   /* Effective	UID of creator */
	       gid_t	      cgid;	   /* Effective	GID of creator */
	       unsigned	short mode;	   /* Permissions */
	       unsigned	short __seq;	   /* Sequence number */

       Valid values for	cmd are:

	      Copy  information	from the kernel	data structure associated with
	      msqid into the msqid_ds structure	pointed	to by buf.  The	caller
	      must have	read permission	on the message queue.

	      Write  the  values  of  some  members  of	the msqid_ds structure
	      pointed to by buf	to the kernel data structure  associated  with
	      this  message  queue,  updating  also its	msg_ctime member.  The
	      following	members	of  the	 structure  are	 updated:  msg_qbytes,
	      msg_perm.uid,  msg_perm.gid,  and	 (the least significant	9 bits
	      of) msg_perm.mode.  The effective	UID  of	 the  calling  process
	      must  match  the owner (msg_perm.uid) or creator (msg_perm.cuid)
	      of the message queue, or the caller must be privileged.	Appro-
	      priate privilege (Linux: the CAP_SYS_RESOURCE capability)	is re-
	      quired to	raise the msg_qbytes value beyond the system parameter

	      Immediately  remove  the	message	 queue,	 awakening all waiting
	      reader and writer	processes (with	an error return	and errno  set
	      to EIDRM).  The calling process must have	appropriate privileges
	      or its effective user ID must be either that of the  creator  or
	      owner  of	 the message queue.  The third argument	to msgctl() is
	      ignored in this case.

       IPC_INFO	(Linux-specific)
	      Return information about system-wide message  queue  limits  and
	      parameters  in  the structure pointed to by buf.	This structure
	      is of type msginfo  (thus,  a  cast  is  required),  defined  in
	      _sys/msg.h_ if the _GNU_SOURCE feature test macro	is defined:

		  struct msginfo {
		      int msgpool; /* Size in kibibytes	of buffer pool
				      used to hold message data;
				      unused within kernel */
		      int msgmap;  /* Maximum number of	entries	in message
				      map; unused within kernel	*/
		      int msgmax;  /* Maximum number of	bytes that can be
				      written in a single message */
		      int msgmnb;  /* Maximum number of	bytes that can be
				      written to queue;	used to	initialize
				      msg_qbytes during	queue creation
				      (msgget(2)) */
		      int msgmni;  /* Maximum number of	message	queues */
		      int msgssz;  /* Message segment size;
				      unused within kernel */
		      int msgtql;  /* Maximum number of	messages on all	queues
				      in system; unused	within kernel */
		      unsigned short int msgseg;
				   /* Maximum number of	segments;
				      unused within kernel */

	      The msgmni, msgmax, and msgmnb settings can be changed via /proc
	      files of the same	name; see proc(5) for details.

       MSG_INFO	(Linux-specific)
	      Return a msginfo structure containing the	 same  information  as
	      for IPC_INFO, except that	the following fields are returned with
	      information about	system resources consumed by  message  queues:
	      the msgpool field	returns	the number of message queues that cur-
	      rently exist on the system; the msgmap field returns  the	 total
	      number  of  messages in all queues on the	system;	and the	msgtql
	      field returns the	total number of	bytes in all messages  in  all
	      queues on	the system.

       MSG_STAT	(Linux-specific)
	      Return a msqid_ds	structure as for IPC_STAT.  However, the msqid
	      argument is not a	queue identifier, but instead  an  index  into
	      the kernel's internal array that maintains information about all
	      message queues on	the system.

       On success, IPC_STAT, IPC_SET, and IPC_RMID  return  0.	 A  successful
       IPC_INFO	 or  MSG_INFO  operation returns the index of the highest used
       entry in	the kernel's internal array recording  information  about  all
       message	queues.	  (This	information can	be used	with repeated MSG_STAT
       operations to obtain information	about all queues on  the  system.)   A
       successful MSG_STAT operation returns the identifier of the queue whose
       index was given in msqid.

       On error, -1 is returned	with errno indicating the error.

       On failure, errno is set	to one of the following:

       EACCES The argument cmd is equal	to IPC_STAT or MSG_STAT, but the call-
	      ing  process  does not have read permission on the message queue
	      msqid, and does not have the CAP_IPC_OWNER capability.

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

       EIDRM  The message queue	was removed.

       EINVAL Invalid  value  for cmd or msqid.	 Or: for a MSG_STAT operation,
	      the index	value specified	in msqid referred  to  an  array  slot
	      that is currently	unused.

       EPERM  The  argument cmd	has the	value IPC_SET or IPC_RMID, but the ef-
	      fective user ID of the calling process is	not  the  creator  (as
	      found  in	msg_perm.cuid) or the owner (as	found in msg_perm.uid)
	      of the message queue, and	the caller is not  privileged  (Linux:
	      does not have the	CAP_SYS_ADMIN capability).

       EPERM  An  attempt (IPC_SET) was	made to	increase msg_qbytes beyond the
	      system parameter	MSGMNB,	 but  the  caller  is  not  privileged
	      (Linux: does not have the	CAP_SYS_RESOURCE capability).

       SVr4, POSIX.1-2001.

       The  inclusion of _sys/types.h_ and _sys/ipc.h_ isn't required on Linux
       or by any version of POSIX.  However, some old implementations required
       the inclusion of	these header files, and	the SVID also documented their
       inclusion.  Applications	intended to be portable	to  such  old  systems
       may need	to include these header	files.

       The  IPC_INFO, MSG_STAT and MSG_INFO operations are used	by the ipcs(1)
       program to provide information on allocated resources.  In  the	future
       these may modified or moved to a	/proc filesystem interface.

       Various	fields	in the struct msqid_ds were typed as short under Linux
       2.2 and have become long	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 an IPC_64 flag in cmd.)

       msgget(2),  msgrcv(2),  msgsnd(2),   capabilities(7),   mq_overview(7),

       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at

Linux				  2014-02-24			     MSGCTL(2)


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