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NetBSD 6.1.5 - man page for msgrcv (netbsd section 2)

MSGRCV(2)			     BSD System Calls Manual				MSGRCV(2)

NAME
     msgrcv -- receive a message from a message queue

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/msg.h>

     ssize_t
     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 user-defined structure pointed to by msgp.  This structure must contain a
     first field of type long that will indicate the user-defined type of the message.	The
     remaining fields will contain the contents of the message.  The following is an example of
     what this user-defined structure might look like:

     struct mymsg {
	 long mtype;	/* message type */
	 char mtext[1]; /* body of message */
     };

     mtype is an integer greater than 0 that can be used to select messages.  mtext is an array
     of bytes, with size up to the system limit MSGMAX.

     The value of msgtyp has one of the following meanings:

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

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

     o	 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.

     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 behaviour of
     msgrcv() depends on whether the IPC_NOWAIT flag is set in msgflg or not.  If IPC_NOWAIT is
     set, then msgrcv() will immediately return a value of -1 and set errno to EAGAIN.	If
     IPC_NOWAIT is not set, the calling process will block 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 is set to EINTR.

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

     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:

     [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.

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

     [EFAULT]		msgp points to an invalid address.

     [EIDRM]		The message queue identifier msqid is removed from the system.

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

     [EINVAL]		msqid is not a valid message queue identifier

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

			msgsz is greater than SSIZE_MAX.

     [ENOMSG]		The queue does not contain a message of the desired type and IPC_NOWAIT
			is set.

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

STANDARDS
     The msgrcv system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5'').

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

BSD					 January 26, 2009				      BSD


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