Re: Pb when calling a nested function in the inferior

[Michael Snyder <msnyder at cygnus dot com>]

Joel Brobecker wrote:
> 
> Hi,
> 
> We have encountered a pb with gdb when one calls from gdb a nested
> function which accesses some "semi-global" variables. Here,
> "semi-global" means global to the nested function, but local to the
> englobing function. I would appreciate some advice on what can be done
> to correct the problem (if the problem can be solved, or helped, that
> is). Needless to say, if something can be done, I will make the
> necessary changes, and contribute them back.
> 
> Here is an example to illustrate the problem:

I've personally never used (or even seen) this
nested function syntax, so I don't know anything
useful about it.		-- Michael

> 
> <<
> int
> main (void)
> {
>    int first;
>    int result;
> 
>    int
>    get_value (void)
>    {
>       return first;
>    }
> 
>    first = 1;
>    result = get_value ();  /*  This is line 14  */
> }
> >>
> 
> %gcc -g -o hello hello.c
> %gdb hello
> (gdb) break hello.c:14
> (gdb) run
> (gdb) print first
> $1 = 1
> (gdb) p get_value ()
> $2 = -1017254775      <<<---  This value is incorrect (should be 1)
> 
> I could reproduce this problem on several plateforms. I looked at it
> more closely on Linux where I used the lastest gdb from CVS and here are
> my conclusions: From the assembly code generated for hello.c, I can see
> that get_value() expects the caller to place the value of variable
> "first" into %ecx before the call. It does not seem that gdb is doing
> this, so when the call is made, get_value reads a random value in %ecx,
> thus leading to the strange value (in an equivalent program written this
> time in Ada, it leads to a SIGSEGV).
> 
> FWIW, once you get past the line that invokes get_value() using "next",
> the "p get_value()" command starts working fine, since the inferior did
> setup the context which has not been destroyed since.
> 
> The question is: Is there a way for gdb to know that indeed get_value ()
> needs some special context to be setup before being called.  If yes,
> then can it find out what special context is needed?
> 
> Thanks,
> --
> Joel