A simple tracing facility is available for determining what messages are received by and sent from the Prolog servant. When message tracing is on, messages sent or received cause a trace message to be written to the current output stream. It will normally be redirected to a file by create_servant/3 or QP_ipc_create_servant(). The UNIX command tail -f may be helpful in looking at the trace messages. Each trace message indicates what the corresponding interprocess message was. The precise form of the trace information depends on whether the Prolog servant is serving a C program or another Prolog program.

Message tracing can be turned on and off by having the servant process call msg_trace/2 which is described below. A master that is a Prolog process can use call_servant/1 to cause the servant to call msg_trace/2. It can also call msg_trace/2 directly to control tracing of its own messages.

To make a servant that serves a C master trace its message, it must either have had tracing turned on before its saved state was created, or it must provide an external routine that can be invoked by the C master to turn on tracing (see Example 4 in ipc-rpc-cpp-exa).