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Message passing is one method used by XINU threads for interprocess communication. It allows threads to send individual messages to other threads by using the system calls <code>send</code>, <code>receive</code>, and <code>recvclr</code>. Each thread has memory allocated for a single message in its thread control block specifically for messages sent and received using this method. This form of message passing should not be confused with the [[mailbox]] messaging queue system also used for interprocess communication.

Upon creation, each thread is allocated memory in its thread control block for two fields which apply to this message passing system: a 4 byte (<code>int</code> type) message box to contain a single message sent to this thread, and a one byte flag (<code>bool</code> type) to signal if there is an unreceived message waiting in that thread's message box.

Threads use the functions <code>send(tid_typ tid, int msg)</code>, <code>receive()</code>, and <code>recvclr()</code> to utilize this system of message passing.

<code>send(tid_typ tid, int msg)</code> delivers the message passed in as the parameter <code>msg</code> to the message box in the thread control block of the thread with a thread id of <code>tid</code>, also passed in as a parameter. <code>send</code> will always yield the processor by calling reschedule if the receiving thread was in a state of waiting to receive a message (<code>THRRECV</code>).

<code>receive()</code> returns the message waiting in the message box of the thread control block of the thread which called <code>receive</code>. If there is no message waiting for the thread then the thread will go into a state of waiting to receive a message (<code>THRRECV</code>) until a message is passed to the thread. 

<code>recvclr()</code> is a non-blocking version of <code>receive()</code>. If there is a message waiting in the message box of the thread control block of the thread which called <code>receive</code> it returns the message. If there is no message waiting for the thread, then it will simply return <code>OK</code>, signifying that there was no message waiting for the thread. Notice that this does not block the thread that called <code>receive</code> and will always immediately return either the message or <code>OK</code>.