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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_PRI-
       VATE  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  already  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 per-
       missions specified for the mode argument 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  inte-
       ger), 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 A message queue exists for key and msgflg specified both IPC_CREAT and IPC_EXCL.

       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 significant 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 (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 message 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_overview(7), svipc(7)

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux					    2012-05-31					MSGGET(2)
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