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 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 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.)

       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

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

ERRORS

       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 IPC_CREAT and IPC_EXCL.

       EIDRM	  The message queue is marked for removal.

       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  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

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

       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 queue.

CONFORMING TO

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

SEE ALSO

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

Linux 0.99.13							    1993-11-01								 MSGGET(2)