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

Question

Sorry for my bad English.

My workflow:

1. 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
   

2. Compile asm-source with debug symbols:

   gcc -g test_c.s
   

3. 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

Answer 1

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.