How I can recognize global variable in GDB from GAS-source?

Question

Sorry for my bad English.

My workflow:

I write simple program for gnu asm (GAS) test_c.s:

    .intel_syntax noprefix
    .globl my_string
    .data
my_string:
    .ascii "Hello, world!\0"    
    .text
    .globl main
main:
    push rbp
    mov rbp, rsp
    sub rsp, 32

    lea rcx, my_string
    call printf

    add rsp, 32
    pop rbp
    ret

Compile asm-source with debug symbols:
```
gcc -g test_c.s
```

Debug a.exe in GDB:

gdb a -q
Reading symbols from C:\a.exe...done.
(gdb) start
Temporary breakpoint 1 at 0x4014e4: file test_c.s, line 14.
Starting program: C:\a.exe
[New Thread 3948.0x45e4]

Temporary breakpoint 1, main () at test_c.s:14
14              sub     rsp, 32
(gdb) whatis my_string
type = <data variable, no debug info>  <-------------------- why?
(gdb) info variables
All defined variables:
...
Non-debugging symbols:
0x0000000000403000  __data_start__
0x0000000000403000  __mingw_winmain_nShowCmd
0x0000000000403010  my_string  <-------------------- why?
....

Why 'my_string' is 'no debug info'-variable? How can I recognize, that 'my_string' is user defined variable? Some gcc-flags or gas-directives?

P.S.: The file test_c.s listed above is generated by gcc from simple c application test_c.c:

    #include<stdio.h>

    char my_string[] = "Hello, world!";

    int main(void)
    {
        printf(my_string);
    }

    gcc test_c.c -S -masm=intel

I try to debug this C-application and get expected result:

    gcc -g test_c.c
    gdb a -q
    Reading symbols from C:\a.exe...done.
    (gdb) start
    Temporary breakpoint 1 at 0x4014ed: file test_c.c, line 7.
    Starting program: C:\a.exe
    [New Thread 11616.0x1688]

    Temporary breakpoint 1, main () at test_c.c:7
    7           printf(my_string);
    (gdb) whatis my_string
    type = char [18] <-------------------- OK
    (gdb) info variables
    ...
    File test_c.c:
    char my_string[18]; <-------------------- OK
    ...

The problem is that I need for debug information related to the GAS-source, not C

P.S.S.: MinGW-builds x64 v.4.8.1

Jester · Accepted Answer · 2014-02-21 12:55:11Z

0

The reason is simple: you should have generated the asm file from the c file with debugging enabled, that is gcc test_c.c -S -masm=intel -g, to have the compiler emit the required information. If you do that, you will notice a section named .debug_info in your asm source, which, unfortunately, isn't user friendly.

answered Feb 21, 2014 at 12:55

Jester

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6 Comments

Dem0n13 Over a year ago

Is there a workaround? Can I add some minimal debug-information about the type of the my_string for GDB?

Jester Over a year ago

Of course you can add the same kind of debug info that the compiler does, but it's tedious and you have to know the format (which is dwarf2 in this case I think).

nostromo Over a year ago

@Jester Is there a way to (from a regular .s source code GAS), only using as and ld (in other words, not using gcc) to generate debugging symbols (and also being able to list the source code) for gdb?

Jester Over a year ago

You have to insert the debugging info into the assembly source by hand. From there, yes, as and ld will suffice.

Jester Over a year ago

DWARF Debugging Standard Website Also this recent question has an example.

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