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

       msgget - get a System V message queue identifier

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

       int msgget(key_t key, int msgflg);

       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
       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 permissions  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.

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

       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

       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.

       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.

       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  maximum number of message queues: policy dependent
	      (on  Linux,  this	 limit	can   be   read	  and	modified   via

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

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

       msgctl(2),  msgrcv(2),  msgsnd(2),  ftok(3),  capabilities(7), mq_over‐
       view(7), svipc(7)

       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2012-05-31			     MSGGET(2)

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