Gdb

GDB Notes — Basics, Navigation & Memory Inspection These are concise notes on how to use GDB (GNU Debugger) effectively for analyzing ELF binaries and debugging at both C source and assembly levels. 1. Starting GDB Basic invocation gdb ./a.out gdb -q ./program # Quiet mode (no banner) With arguments gdb --args ./program arg1 arg2 From inside GDB (gdb) run arg1 arg2 2. Compiling for Debugging Compile with the -g flag to include debug symbols: ...

GDB Print Command Reference Guide Basic Print Command print variable_name p variable_name # Short form Print with Format Specifiers Use /format after print to specify output format: Hexadecimal print/x variable # Hex (lowercase) print/x value # Example: 0x5 p/x 255 # Output: 0xff Decimal print/d variable # Signed decimal print/u variable # Unsigned decimal p/d 0xff # Output: 255 Octal print/o variable # Octal format p/o 64 # Output: 0100 Binary print/t variable # Binary (t = "two") p/t 5 # Output: 101 Character print/c variable # As ASCII character p/c 65 # Output: 'A' Floating Point print/f variable # Floating point p/f 3.14159 Address/Pointer print/a variable # As address p/a 0x555555555189 # Shows as address String print/s pointer # Interpret as C string p/s argv[1] Working with Pointers print pointer # Shows address print *pointer # Dereference (shows value) print &variable # Shows address of variable # Example print argv # Address of argv array print *argv # First element (argv[0]) print argv[1] # Second element print *argv[1] # First char of argv[1] Array and String Operations # Print entire array print buffer_one print buffer_two # Print specific elements print buffer_one[0] print buffer_one[3] # Print array slice (if supported) print buffer_one@8 # Print 8 elements starting at buffer_one # View string with length x/8c buffer_one # First 8 chars x/s buffer_one # Until null terminator Type Casting # Cast to different types print (int *)buffer_one # Treat as int pointer print *(int *)buffer_one # Dereference as int print (unsigned char)value # Cast to unsigned char # Example: View buffer as integers print *(int *)&buffer_one print *(long *)&buffer_two Expressions and Calculations # Arithmetic print value + 10 print sizeof(buffer_one) print strlen(buffer_one) # Address calculations print &buffer_one - &buffer_two print (long)&value - (long)&buffer_one # Pointer arithmetic print argv[0] print *(argv + 1) Display Commands (Auto-Print) Set up variables to display automatically after each step: ...

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. ...

Examining Memory with x Command The x (examine) command is more powerful for raw memory: x/[count][format][size] address Size Modifiers b = byte (1 byte) h = halfword (2 bytes) w = word (4 bytes) g = giant word (8 bytes) Format Modifiers x = hexadecimal d = decimal u = unsigned decimal o = octal t = binary c = character s = string i = instruction (disassembly) count: how many units to display ...