main(), registers and gdb

[Greg Watson]

On Jan 10, 2007, at 4:38 PM, Daniel Jacobowitz wrote:

> On Wed, Jan 10, 2007 at 04:32:48PM -0700, Greg Watson wrote:
>> If the following code is compiled with 'gcc -g -O0 -o test test.c',
>> the address of argc is passed into func() in the ecx register. Since
>> ecx is not preserved after the call to printf(), the address of argc
>> is corrupted on return from func(). Normally this would not be a
>> problem, since argc is never used in the code.
>
> This is a typical problem.  There is not much that can be done about
> it, although I remember once hearing a proposal that GCC should
> forcibly extend the live ranges of local variables (or at least
> arguments) at -O0 to improve debugging.  That seems sensible to me.

That would be nice. Although this seems like a trivial issue, it can  
potentially effect debugging all MPI programs since they always start  
with 'MPI_Init(&argc, &argv)'.

>
>> However, when run under gdb, commands that view the stack frame
>> produce strange results, and some commands (e.g. -var-update)
>> actually crash the debugger.
>
> A crash is always a bug.

I believe it's gdb bug #2188.

>
>>     Breakpoint 1, main (argc=1, argv=0xbffcef14) at test.c:14
>>     12              func(&argc);
>>     (gdb) n
>>     in func
>>     14              printf("hello\n");
>>     (gdb) where
>>     #0  main (argc=Cannot access memory at address 0x4
>>     ) at test.c:16
>
> And honestly, I have no idea how that happened.  Does it happen
> with a current GDB?  I suspect from the error message that this
> one is not too recent.

It's gdb 6.5, so reasonably recent.

Greg