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> lua_sethook() is advertised as being signal-safe. I'm prepared to trust 
> Roberto's judgement that it is, although it seems to me that Posix 
> would prefer that some of those fields be declared as volatile 
> sig_atomic_t. However, I'm sure that Roberto has thought this through, 
> and certainly I haven't noticed any problems with it.

Thanks for the compliment. However, as I said before, it is mostly
impossible to really "think through" a piece of multithreading code.

Rethinking about the subject, I am afraid lua_sethook is not
"theoretically" safe even for signals (as it should be).

The problem is the non-atomicity of pointer accesses. There is a small
risk that, while luaD_callhook is reading L->hook, lua_sethook changes
it, so that luaD_callhook reads an invalid pointer.

I cannot see any simple way to correct this, without imposing some
restrictions on lua_sethook.

-- Roberto