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msgrcv(3)

NAME

     msgrcv -- receive a message from a message queue

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

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

     int
     msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);

DESCRIPTION

     The msgrcv() function receives a message from the message queue specified
     in msqid, and places it into the structure pointed to by msgp.  This
     structure should consist of the following members:

	 long mtype;	/* message type */
	 char mtext[1]; /* body of message */

     mtype is an integer greater than 0 that can be used for selecting mes-
     sages, mtext is an array of bytes, with a size up to that of the system
     limit (MSGMAX).

     The value of msgtyp has one of the following meanings:

     o	 The msgtyp argument is greater than 0.  The first message of type
	 msgtyp will be received.

     o	 The msgtyp argument is equal to 0.  The first message on the queue
	 will be received.

     o	 The msgtyp argument is less than 0.  The first message of the lowest
	 message type that is less than or equal to the absolute value of
	 msgtyp will be received.

     The msgsz argument specifies the maximum length of the requested message.
     If the received message has a length greater than msgsz it will be
     silently truncated if the MSG_NOERROR flag is set in msgflg, otherwise an
     error will be returned.

     If no matching message is present on the message queue specified by
     msqid, the behavior of msgrcv() depends on whether the IPC_NOWAIT flag is
     set in msgflg or not.  If IPC_NOWAIT is set, msgrcv() will immediately
     return a value of -1, and set errno to EAGAIN.  If IPC_NOWAIT is not set,
     the calling process will be blocked until:

     o	 A message of the requested type becomes available on the message
	 queue.

     o	 The message queue is removed, in which case -1 will be returned, and
	 errno set to EINVAL.

     o	 A signal is received and caught.  -1 is returned, and errno set to
	 EINTR.
     o	 msg_lrtime is set to the current time.

     o	 msg_qnum is decremented by 1.

RETURN VALUES

     Upon successful completion, msgrcv() returns the number of bytes received
     into the mtext field of the structure pointed to by msgp.	Otherwise, -1
     is returned, and errno set to indicate the error.

ERRORS

     The msgrcv() function will fail if:

     [EINVAL]		The msqid argument is not a valid message queue iden-
			tifier.

			The message queue was removed while msgrcv() was wait-
			ing for a message of the requested type to become
			available on it.

			The msgsz argument is less than 0.

     [E2BIG]		A matching message was received, but its size was
			greater than msgsz and the MSG_NOERROR flag was not
			set in msgflg.

     [EACCES]		The calling process does not have read access to the
			message queue.

     [EFAULT]		The msgp argument points to an invalid address.

     [EINTR]		The system call was interrupted by the delivery of a
			signal.

     [EAGAIN]		There is no message of the requested type available on
			the message queue, and IPC_NOWAIT is set in msgflg.

SEE ALSO

     msgctl(3), msgget(3), msgsnd(3)

HISTORY

     Message queues appeared in the first release of AT&T System V UNIX.

FreeBSD 5.4		       November 24, 1997		   FreeBSD 5.4

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