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Signal handlers are written in C. In the majority of cases, the C handler calls a Lisp function.

Some signals are used by the Runtime. Precisely which depends on the OS in use, but typically this includes

• Whatever you get when writing into unmapped memory (SIGSEGV, or SIGBUS) - used by the GC for triggering a collection. This does not apply to GENCGC; instead, a memory fault error there indicates to the runtime that we have written to an object in an old generation. [ is that right? ]
• Whatever you get when executing a "breakpoint" or "trap" instruction (SIGTRAP or SIGILL, maybe) - this is used for breakpoints, tracing, and some kinds of internal error - see sigtrap_handler?
• (On some systems) whatever you get whenever trying an unaligned access (see pseudo-atomic for details)
• Also writing to pages that have been mprotected, which is used (at least on x86) for control stack exhaustion detection and handling

Usually the handlers for these will continue on and call a user-level Lisp handler if they can't work out anything grungier to do.

Pseudo-atomic sections can be used to defer signal handling. This doesn't actually disable the signal from being raised, and the C handler still gets called. It just puts off calling the related Lisp routine until exit from the PA section.

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