Re: Exception Handling and _NS_COUNT_HACK

[Richard Frith-Macdonald]

On 27 Oct 2008, at 19:02, Samantha Rahn wrote:

> I am finding that Objective-C exceptions are not being caught when my
> GNUstep-built DLL is run as an in-memory COM server within a protected
> memory space, such as (a) Excel and (b) GDB.
>
> I have tracked the problem down to signal handlers not being called.
> Could this be the reason that exceptions are not being handled?
>
> For example, consider the following program that generates a SIGSEGV  
> crash.
>
> --
> #include <signal.h>
> #include <stdio.h>
>
> void recover(int);
>
> int main(int argc, const char *argv[])
> {
> signal(SIGSEGV, recover);
>
> int *intPtr = 0;
> printf("Crash %d", *intPtr);
>
> return 0;
> }
>
> void recover(int signal)
> {
> int value = signal;
>
> printf("Signal generated: %d\n", value);
> }
> --
>
> When running from the command line, the signal handler function,
> recover(), is called as expected:
>
> > Tool
>   Signal generated: 11
> >
>
> However, the problem shows up when the program is run within GDB: the
> signal handler is not called. Any ideas about this and its possible
> ramification for proper exception handling when running the DLL as an
> in-process COM server?

A couple of points ...

1. when running under gdb, gdb catches any signal ... so you then have  
to continue the program specially, telling gdb to pass the signal back  
to the program.  If you just continue the program normally from the  
gdb command line then the signal is never delivered to the  
program(since it was delivered to gdb).

2. the signal handler hack is a workaround for a bug in the gcc  
compiler's stack frame handling ... I've only ever seen the stack  
frame bug on 64bit systems and as far as I know the compiler will  
correctly detect the end of the stack if you are running on a 32bit  
system, in which case no signal is raised when handling an exception  
anyway.
In other words, the signal handler should probably *not* be being  
called anyway.


So ... most likely if the exception handling is not working properly  
for you, the fault lies elsewhere.