msgget

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



NAME
       msgget - get a 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  function returns the 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 asserted in  msgflg  (i.e.,
       msgflg&IPC_CREAT	 is  nonzero).	 The presence in msgflg of the fields
       IPC_CREAT and IPC_EXCL plays the same role, with respect to the	exis-
       tence  of  the message queue, as the presence of O_CREAT and O_EXCL in
       the mode argument of the open(2) system call: i.e. the msgget function
       fails  if  msgflg  asserts  both	 IPC_CREAT and IPC_EXCL and a message
       queue already exists for key.

       Upon creation, the lower 9 bits of  the	argument  msgflg  define  the
       access  permissions  of the message queue.  These permission bits have
       the same format and semantics as the access permissions	parameter  in
       open(2)	or  creat(2)  system calls.  (The execute permissions are not
       used.)

       If a new message queue is created, the  system  call  initializes  the
       system message queue data structure msqid_ds 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  lowest order 9 bits of msg_perm.mode are set to the lowest
	      order 9 bit 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 access permissions are veri-
       fied, 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 has
		  no access permissions to the queue.

       EEXIST	  A  message  queue  exists  for key and msgflg was asserting
		  both IPC_CREAT and IPC_EXCL.

       ENOENT	  No message queue exists for key and msgflg wasn’t asserting
		  IPC_CREAT.

       ENOMEM	  A  message  queue  has to be created but the system has not
		  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.

NOTES
       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 low-
       est order 9 bits of msgflg and creates a new message  queue  (on	 suc-
       cess).

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

       MSGMNI	  System wide maximum number of message queues: policy depen-
		  dent.

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

CONFORMING TO
       SVr4, SVID.  Until version 2.3.20  Linux	 would	return	EIDRM  for  a
       msgget on a message queue scheduled for deletion.

SEE ALSO
       ftok(3), ipc(5), msgctl(2), msgsnd(2), msgrcv(2)



Linux 0.99.13			  1993-11-01			    MSGGET(2)