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RedHat 9 (Linux i386) - man page for msgget (redhat section 2)

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 argu-
       ment.  A new message queue is created if key has value IPC_PRIVATE or key  isn't  IPC_PRI-
       VATE,  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 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  inte-
       ger), 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 permis-
		  sions 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)


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