msgrcv man page on BSDi

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MSGRCV(2)		    BSD Programmer's Manual		     MSGRCV(2)

NAME
     msgrcv - receive a message from a message queue

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 */

     The mtype field is an integer greater than 0 that can be used for select-
     ing messages, 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 is greater than 0. The first message of type msgtyp will
	 be received.

     o	 The msgtyp is equal to 0. The first message on the queue will be re-
	 ceived.

     o	 The msgtyp 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 function argument msgsz 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 re-
     turn 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 EIDRM.

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

     If a message is successfully received, the data structure associated with
     msqid is updated as follows:

     o	 msg_cbytes is decremented by the size of the message.

     o	 msg_lrpid is set to the pid of the caller.

     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
     msgrcv() will fail if:

     [EINVAL]	   The msqid is not a valid message queue identifier

		   The function argument msgsz is less than 0.

     [EIDRM]	   The message queue was removed while msgrcv() was waiting
		   for a message of the requested type to become available on
		   it.

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

     [EACCESS]	   The calling process does not have read access to the mes-
		   sage queue.

     [EFAULT]	   msgp points to an invalid address.

     [EINTR]	   The system call was interrupted by the delivery of a sig-
		   nal.

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

SEE ALSO
     msgsnd(2),	 msgctl(2),  msgget2

4th Berkeley Distribution	 July 3, 1996				     2
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