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NAME    [Toc]    [Back]

       msgget - get a message queue identifier

SYNOPSIS    [Toc]    [Back]

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

       int msgget ( key_t key, int msgflg )

DESCRIPTION    [Toc]    [Back]

       The  function  returns  the  message queue identifier associated to the
       value of the key argument.  A new message queue is created if  key  has
       value  IPC_PRIVATE  or key isn't IPC_PRIVATE, no existing message queue
       is associated to  key,  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  existence
  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

       Furthermore, while creating, 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 verified,
 and a check is made to see if it is marked for destruction.

RETURN VALUE    [Toc]    [Back]

       If successful, the return value will be the message queue identifier (a
       nonnegative integer), 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	  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

       EIDRM	  The message queue is marked for removal.

       ENOENT	  No message queue exists for key and msgflg wasn't  asserting

       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

NOTES    [Toc]    [Back]

       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 lowest
       order 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 maximum number of message queues: policy dependent.

BUGS    [Toc]    [Back]

       Use of IPC_PRIVATE does not actually prohibit other processes from getting
 access to the allocated message queue.

       As  for the files, there is currently no intrinsic way for a process to
       ensure exclusive access to a message queue.  Asserting  both  IPC_CREAT
       and  IPC_EXCL  in  msgflg  only ensures (on success) that a new message
       queue will be created, it doesn't imply exclusive access to the message

CONFORMING TO    [Toc]    [Back]

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

SEE ALSO    [Toc]    [Back]

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

Linux 0.99.13			  1993-11-01			     MSGGET(2)
[ Back ]
 Similar pages
Name OS Title
msgget NetBSD get message queue identifier
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