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38 Debugger

Two classes of errors are handled by the Lisp debugger. These are synchronous errors caused by something erring in program code and asynchronous errors caused by some external context of execution (clock interrupts, control-c interrupts). Asynchronous errors can often be postponed if they are delivered at an inconvenient time.

Synchronous errors are frequently handled by directly invoking the debugger. However, there are several places where the strategy of jumping into the debugger is not used. In those situations the compiler emits a stylized breakpoint; a breakpoint instruction (usually an INT3) followed by several bytes of argument data. This will cause a trip through the operating system and ultimately the invocation of the C-level SIGTRAP handler which, in turn, interprets the argument bytes following the breakpoint and dispatches to the correct handler. There is a switch statement in “sigtrap_handler” which gives the whole story on what types of errors rely on this mechanism. The most commonly invoked handler is probably “interrupt_internal_error” as it fields such common exceptions as the use of unbound symbols. To familiarize with the context these traps are created in, one can disassemble just about any function and look at the bottom of the disassembly for blocks of error handling code. There will often be “BREAK 10” opcodes followed by several “BYTE” opcodes with the meaning of the arguments in neatly decoded form off in the right-hand column.

The other types of synchronous errors are those errors delivered by the operating system such as FPU traps and SIGSEGVs. The invocation of those signals should be funneled through a C-level trampoline which makes a callback into Lisp passing all of the signal handler arguments. That code is pretty straight forward and the “interrupt_handle_now” function is pretty much where all of the runtime logic is localized.

Handling asynchronous errors and deferred asynchronous errors is a bit more involved...

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