Re: Help: Unwinding the C++ stack...throw, longjmp & threads

[Josh Stern <jstern at citilink dot com>]

Joe writes:
>Josh writes:
>> If this extension would allow exceptions to be thrown from
>> Unix signal handlers, that would be a very cool feature
>> for even non-threaded programs.

>It wouldn't, since we don't support that in C++ either.  The problem
>is that signals can happen anywhere.  Consider
>
>   char * buf = 0;
>   try {
>        buf = malloc(buf_size);
>   }
>   catch (SIGNAL) {
>        if (buf)
>                free(buf);
>   }
>
>This code has a race and a leak, and there's no way to write it correctly,
>because the signal could come after malloc has allocated memory but before
>buf is assigned.

I understand the example, and also (I think) Jason's comment that 
asynchronous exceptions are very hard to  get right.  Just to
pursue this a little bit however...

I would diagnose the problem above as an inadequate implementation
of malloc() (relative to use in a program that could throw from
a signal handler).  An acceptable malloc for such a program would
have to catch the SIGNAL exception at a point where it still
had access to the internal data structures that get adjusted
during the memory allocation.  Offhand, I don't see why it would
so difficult to write such a malloc.  


- Josh