5.4 Format of Debugging Messages

We shall now look at the exact format of the message output by the system at a port. All trace messages are output to the standard error stream, using the print_message/2 predicate; see ref-msg. This allows you to trace programs while they are performing file I/O. The basic format is as follows:

N S    23     F6 Call: T foo(hello,there,_123) ?

N is only used at Exit ports and indicates whether the invocation could backtrack and find alternative solutions. Unintended nondeterminacy is a source of inefficiency, and this annotation can help spot such efficiency bugs. It is printed as ‘?’, indicating that foo/3 could backtrack and find alternative solutions, or ‘ ’ otherwise.

S is a spypoint indicator. If there is a plain spypoint on foo/3, then it is printed as ‘+’. In case of conditional and generic spypoints it takes the form ‘*’ and ‘#’, respectively. Finally, it is printed as ‘ ’, if there is no spypoint on the predicate being traced.

The first number is the unique invocation identifier. It is increasing regardless of whether or not debugging messages are output for the invocations (provided that the debugger is switched on). This number can be used to cross correlate the trace messages for the various ports, since it is unique for every invocation. It will also give an indication of the number of procedure calls made since the start of the execution. The invocation counter starts again for every fresh execution of a command, and it is also reset when retries (see later) are performed.

Just before the second number is an optional frame marker, printed as ‘@’ if present. This marks the location of the current frame, the meaning of which is explained in the next section.

The second number is the current depth; i.e. the number of direct ancestors this goal has, for which a procedure box has been built by the debugger.

The next word specifies the particular port (Call, Exit, Redo, Fail, or Exception).

T is a subterm trace. This is used in conjunction with the ‘^’ command (set subterm), described below. If a subterm has been selected, then T is printed as the sequence of commands used to select the subterm. Normally, however, T is printed as ‘ ’, indicating that no subterm has been selected.

The goal is then printed so that you can inspect its current instantiation state.

The final ‘?’ is the prompt indicating that you should type in one of the commands allowed (see Debug Commands). If this particular port is unleashed, then you will not get this prompt since you have specified that you do not wish to interact at this point.

At Exception ports, the trace message is preceded by a message about the pending exception, formatted as if it would arrive uncaught at the top level.

Note that calls that are compiled inline are not traced.

Block and Unblock ports are exceptions to the above debugger message format. A message

       S -  -  Block: p(_133)

indicates that the debugger has encountered a blocked goal, i.e. one which is temporarily suspended due to insufficiently instantiated arguments (see ref-sem-sec). By default, no interaction takes place at this point, and the debugger simply proceeds to the next goal in the execution stream. The suspended goal will be eligible for execution once the blocking condition ceases to exist, at which time a message

       S -  -  Unblock: p(_133)

is printed. Although Block and Unblock ports are unleashed by default in trace mode, you can make the debugger interact at these ports by using conditional spypoints.