gdb and python

Hi
I have just started using a python enabled gdb with Qt code and eclipse.
At times gdb(7.4) crashes when the variables window is displayed. I think
this must be something to do with gsb not knowing what type a variable is.

Has anyone else experienced this and is there a fix?

Thanks

Graham
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Breakpoint on first instruction in program

I am using the following assembly language program (doNothingProg.s) for 
instruction purposes:

         .text
         .globl  main
         .type   main,  <at> function
main:
         pushq   %rbp        # save caller's frame pointer
         movq    %rsp, %rbp  # establish our frame pointer
         movl    $0, %eax    # return 0 to caller
         movq    %rbp, %rsp  # restore stack pointer
         popq    %rbp        # restore caller's frame pointer
         ret                 # back to caller

I want to set a breakpoint at the first instruction (pushq %rbp) so 
students can see how the stack frame is created.

I am running Ubuntu 12.04.

Under a previous version ( GNU gdb 6.8-debian) I could do this.
Under the current version (GNU gdb (Ubuntu/Linaro 7.4-2012.04-0ubuntu2) 
7.4-2012.04) when I set the breakpoint on that line then run the 
program, it actually breaks at the third instruction (movl $0, %eax).

I understand that the first two instructions are the prologue to the 
actual "work" of this function, and since gdb is intended for higher 
level languages, but it is possible to make a mistake in the prologue in 
assembly language.

Is there any way to get gdb to honor my request for a breakpoint on the 

Re: Breakpoint on first instruction in program

> I am using the following assembly language program (doNothingProg.s) for 
> instruction purposes:
> 
>          .text
>          .globl  main
>          .type   main,  <at> function
> main:
>          pushq   %rbp        # save caller's frame pointer
>          movq    %rsp, %rbp  # establish our frame pointer
>          movl    $0, %eax    # return 0 to caller
>          movq    %rbp, %rsp  # restore stack pointer
>          popq    %rbp        # restore caller's frame pointer
>          ret                 # back to caller
> 
> I want to set a breakpoint at the first instruction (pushq %rbp) so 
> students can see how the stack frame is created.

break *&main

                                -- Adam

Re: Breakpoint on first instruction in program

Re: Breakpoint on first instruction in program

break *&main

> that is the gdb command?

Yes.  "break main" will break at the instruction past the prologue,
but "break *&main" just uses &main, which is the address of main, as
the breakpoint address.  I've tested this, but I can't say for certain
it will work with every version of gdb.

                                -- Adam

Re: Breakpoint on first instruction in program

Re: Breakpoint on first instruction in program

> thanks for the answer, and after that how do i see how the stack frame is
> created.?

"stepi" steps through each instruction.  "info reg" shows the
registers, so you can use this after each step to watch how they
change.  "info reg ebp esp" will just show you those two registers
specifically.  If ESP is, say, BFFFF830, I use this command to dump 8
words at that address:

x/8xw 0xbffff380

That's all I know.  If there's some clever way to define a macro
command to step an instruction and display the top of the stack all in
one command, someone else will have to help with that.

                                -- Adam

Re: Breakpoint on first instruction in program

On 6/20/2012 1:39 PM, Adam Beneschan wrote:
>   
>> I am using the following assembly language program (doNothingProg.s) for
>> instruction purposes:
>>
>>           .text
>>           .globl  main
>>           .type   main,  <at> function
>> main:
>>           pushq   %rbp        # save caller's frame pointer
>>           movq    %rsp, %rbp  # establish our frame pointer
>>           movl    $0, %eax    # return 0 to caller
>>           movq    %rbp, %rsp  # restore stack pointer
>>           popq    %rbp        # restore caller's frame pointer
>>           ret                 # back to caller
>>
>> I want to set a breakpoint at the first instruction (pushq %rbp) so
>> students can see how the stack frame is created.
> break *&main
>
>                                  -- Adam
Thank you for the response Adam.

Actually, break *main worked for me. (Or just br *main). I'm not in 
Linux right now, but I will double check next time I log in.

I found this by using info gdb and some looking around. As usual, the 
answer is in the documentation, as I often told my students. :-[

--Bob

Re: Breakpoint on first instruction in program

Re: Breakpoint on first instruction in program