p4 provides facilities for sending and receiving typed messages by p4_send and p4_recv. These are ``blocking'' operations in the sense that when the p4_send returns, the buffer may be reused, and when the p4_recv returns, the message is in the buffer. The ``probe'' operation (p4_msgs_available) allows one to test for the arrival of a message before committing to wait for it. Optional versions of p4_send specify synchronous operation (p4_sendr does not return until the message has been received by the execution of a p4_recv at the destination), heterogeneous communication (p4_sendx has a data type argument that prescribes translation into standard data format with xdr), or user-managed buffers (p4_sendb requires that a buffer be preallocated by p4_msg_alloc to avoid the overhead of copying data from the user's memory into a formatted p4 buffer).