msgrcv(2)							   System Calls 							 msgrcv(2)

NAME

       msgrcv - message receive operation

SYNOPSIS

       #include <sys/msg.h>

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

DESCRIPTION

       The  msgrcv()  function reads a message from the queue associated with the message queue identifier specified by msqid and places it in the
       user-defined buffer pointed to by msgp.

       The msgp argument points to a user-defined buffer that must contain first a field of type long int that will specify the type of  the  mes-
       sage,  and  then  a  data portion that will hold the data bytes of the message. The structure below is an example of what this user-defined
       buffer might look like:

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

       The mtype member is the received message's type as specified by the sending process.

       The mtext member is the text of the message.

       The  msgsz argument specifies the size in bytes of mtext. The received message is truncated to msgsz bytes if it is larger than	msgsz  and
       (msgflg&MSG_NOERROR)  is  non-zero.  The  truncated part of the message is lost and no indication of the truncation is given to the calling
       process.

       The msgtyp argument specifies the type of message requested as follows:

	 o  If msgtyp is 0, the first message on the queue is received.

	 o  If msgtyp is greater than 0, the first message of type msgtyp is received.

	 o  If msgtyp is less than 0, the first message of the lowest type that is less than or equal to the absolute value of msgtyp is received.

       The msgflg argument specifies which of the following actions is to be taken if a message of the desired type is not on the queue:

	 o  If (msgflg&IPC_NOWAIT) is non-zero, the calling process will return immediately with a return value of -1 and errno set to ENOMSG.

	 o  If (msgflg&IPC_NOWAIT) is 0, the calling process will suspend execution until one of the following occurs:

	      o  A message of the desired type is placed on the queue.

	      o  The message queue identifier msqid is removed from the system (see msgctl(2)); when this occurs, errno is set equal to EIDRM  and
		 -1 is returned.

	      o  The  calling  process	receives  a  signal  that  is to be caught; in this case a message is not received and the calling process
		 resumes execution in the manner prescribed in sigaction(2).

       Upon successful completion, the following actions are taken with respect to the data structure associated with msqid (see intro(2)):

	 o  msg_qnum is decremented by 1.

	 o  msg_lrpid is set equal to the process ID of the calling process.

	 o
	    msg_rtime is set equal to the current time.

RETURN VALUES

       Upon successful completion, msgrcv() returns a value equal to the number of bytes actually placed into the buffer mtext. Otherwise,  -1	is
       returned, no message is received, and errno is set to indicate the error.

ERRORS

       The msgrcv() function will fail if:

       E2BIG	       The value of mtext is greater than msgsz and (msgflg&MSG_NOERROR) is 0.

       EACCES	       Operation permission is denied to the calling process.  See intro(2).

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

       EINTR	       The msgrcv() function was interrupted by a signal.

       EINVAL	       The msqid argument is not a valid message queue identifier.

       ENOMSG	       The queue does not contain a message of the desired type and (msgflg&IPC_NOWAIT) is non-zero.

       The msgrcv() function may fail if:

	EFAULT	       The msgp argument points to an illegal address.

USAGE

       The value passed as the msgp argument should be converted to type void *.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       intro(2), msgctl(2), msgget(2), msgsnd(2), sigaction(2), attributes(5), standards(5)

SunOS 5.10							    19 May 1999 							 msgrcv(2)