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

NAME
       msgget -	get a System V message queue identifier

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

       int msgget(key_t	key, int msgflg);

DESCRIPTION
       The  msgget() system call returns the System V message queue identifier
       associated with the value of the	key argument.  A new message queue  is
       created	if  key	has the	value IPC_PRIVATE or key isn't IPC_PRIVATE, no
       message queue with the given key	key exists, and	IPC_CREAT is specified
       in msgflg.

       If msgflg specifies both	IPC_CREAT and IPC_EXCL and a message queue al-
       ready exists for	key, then msgget() fails with  errno  set  to  EEXIST.
       (This  is  analogous  to	the effect of the combination O_CREAT |	O_EXCL
       for open(2).)

       Upon creation, the least	significant bits of the	argument msgflg	define
       the  permissions	 of the	message	queue.	These permission bits have the
       same format and semantics as the	permissions specified for the mode ar-
       gument of open(2).  (The	execute	permissions are	not used.)

       If  a  new message queue	is created, then its associated	data structure
       msqid_ds	(see msgctl(2))	is initialized as follows:

	      msg_perm.cuid and	msg_perm.uid are set to	the effective user  ID
	      of the calling process.

	      msg_perm.cgid and	msg_perm.gid are set to	the effective group ID
	      of the calling process.

	      The least	significant 9 bits of msg_perm.mode  are  set  to  the
	      least significant	9 bits of msgflg.

	      msg_qnum,	msg_lspid, msg_lrpid, msg_stime, and msg_rtime are set
	      to 0.

	      msg_ctime	is set to the current time.

	      msg_qbytes is set	to the system limit MSGMNB.

       If the message queue already exists the permissions are verified, and a
       check is	made to	see if it is marked for	destruction.

RETURN VALUE
       If successful, the return value will be the message queue identifier (a
       nonnegative integer), otherwise -1 with errno indicating	the error.

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

       EACCES A	message	queue exists for key, but the calling process does not
	      have  permission	to  access  the	 queue,	 and does not have the
	      CAP_IPC_OWNER capability.

       EEXIST IPC_CREAT	and IPC_EXCL were specified in msgflg, but  a  message
	      queue already exists for key.

       ENOENT No  message  queue  exists  for  key  and	msgflg did not specify
	      IPC_CREAT.

       ENOMEM A	message	queue has to be	created	but the	system does  not  have
	      enough memory for	the new	data structure.

       ENOSPC A	 message  queue	has to be created but the system limit for the
	      maximum number of	message	queues (MSGMNI)	would be exceeded.

CONFORMING TO
       SVr4, POSIX.1-2001.

NOTES
       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.

       IPC_PRIVATE isn't a flag	field but a key_t type.	 If this special value
       is  used	for key, the system call ignores everything but	the least sig-
       nificant	9 bits of msgflg and creates a new message queue (on success).

       The following is	a system limit on message queue	resources affecting  a
       msgget()	call:

       MSGMNI System-wide limit	on the number of message queues: policy	depen-
	      dent (on	Linux,	this  limit  can  be  read  and	 modified  via
	      /proc/sys/kernel/msgmni).

   Linux notes
       Until version 2.3.20, Linux would return	EIDRM for a msgget() on	a mes-
       sage queue scheduled for	deletion.

BUGS
       The name	choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
       clearly show its	function.

SEE ALSO
       msgctl(2),  msgrcv(2),  msgsnd(2),  ftok(3),  capabilities(7), mq_over-
       view(7),	svipc(7)

COLOPHON
       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
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-04-30			     MSGGET(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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