Debugging Souce Code with Assembly

If we compile C program with -g flag, it tells GCC to include debugging symbols inside the resulting binary (a.out by default). These symbols live in a special section of the ELF file (like .debug_info, .debug_line, .debug_str, etc.) and contain metadata that maps machine instructions back to your original source code.

This allows us to see corresponding source along with assembly while debugging with GDB.

Let’s take this program

// firstprog.c
#include<stdio.h>

int main()
{
	int i;
	for (i=0; i<10; i++) {
		printf("Hello, World!\n");
	}
}

sanketh@sanketh-81de:~/assembly/gdb$ gcc -g firstprog.c
sanketh@sanketh-81de:~/assembly/gdb$ readelf -S a.out | grep debug
  [28] .debug_aranges    PROGBITS         0000000000000000  0000303b
  [29] .debug_info       PROGBITS         0000000000000000  0000306b
  [30] .debug_abbrev     PROGBITS         0000000000000000  00003105
  [31] .debug_line       PROGBITS         0000000000000000  00003159
  [32] .debug_str        PROGBITS         0000000000000000  000031bc
  [33] .debug_line_str   PROGBITS         0000000000000000  00003295

We can see the source code using list command

(-q flag just tells gdb not to print version and some info on startup)

$ gdb -q ./a.out
Reading symbols from ./a.out...
(gdb) list
1	#include<stdio.h>
2
3	int main()
4	{
5		int i;
6		for (i=0; i<10; i++) {
7			printf("Hello, World!\n");
8		}
9	}

You can even control the detail level: • -g1: minimal info (function names, line numbers) • -g2: normal (default) — enough for GDB • -g3: adds macro

We can also

gcc -g -O0 -Wall -o prog prog.c

-0O means no optimizations, which will prevent reordering of instructions. Because optimization (-O2, -O3) can reorder or remove variables, making debugging confusing.

GDB Layouts

GDB has a Text User Interface (TUI) mode that lets you see source code, assembly, and registers in split or full-screen views.

It’s very useful when stepping line-by-line or instruction-by-instruction.

You enter TUI mode using:

(gdb) layout [mode]

Available Layouts

GDB Navigation & Stepping Commands

1. Source-Level (C) Navigation

These commands work on C source lines when the binary is compiled with -g.

2. Assembly-Level Navigation

These work on individual machine instructions, not source lines.

Ideal for reverse engineering or checking compiler output.

3. Breakpoints & Execution Flow

4. Viewing Context While Navigating