Notes

GDB

The GNU Debugger (GDB) is a popular, free, and open-source debugger for C and C++ programs, which is widely used in Unix-like systems. It allows developers to run, stop, and modify the execution of programs, monitor and modify the values of variables, and call functions independently of the program’s normal behavior. GDB offers extensive facilities for tracing and altering the execution of computer programs, making it a powerful tool for debugging and issue resolution

$gdb
GNU gdb (GDB) 7.6.1
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "mingw32".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
(gdb)

Basic

Steps

1. Create C program / programs

2. Compile the C program with debugging information

Compile the C program with the -g flag to include debugging information in the executable file. This flag tells the compiler to add references to the source code in the executable, which allows GDB to map instructions in the compiled binary executable to their corresponding line, variable, or function in the source code.

   gcc -g -o hello hello.c

3. Start GDB

   gdb hello

Run the program

run the core

   (gdb) run

Set a breakpoint

Set a breakpoint at the line where you want to pause the program execution.

   (gdb) break main

Inspect the program state

Once the program execution is paused at the breakpoint, you can inspect the program state.

   (gdb) print var

Continue the execution

   (gdb) continue

Backtrace

It backtracks the execution flow in the stack

   (gdb) bt

Show frame

Shows the value in the frame(resistor).

   (gdb) f 0 # reading the frame 0

4. Exit

(gdb) quit

Other useful commands

layout asm - Displays the assembly code of the program. layout next - Displays the next line of code to be executed.

gcc -g -o hello hello.c here -g flag adds debugging information to the executable file, which allows GDB to map instructions in the compiled binary executable to their corresponding line, variable, or function in the source code.

gcc -O0 -g -o hello hello.c here -O0 flag disables optimization, which can make debugging easier by preserving the original structure of the code.

gcc -o hello hello.c -s here -s flag strips the debugging information from the executable file.

Executes the next line of code in the current function, stepping over any function calls.

(gdb) next

Executes the next line of assembly

(gdb) nexti

refresh - Refreshes the display of the program state in GDB.

x - Examines the contents of memory at a specified address or variable. i - Displays information about the program state, such as the current function, stack frames, and variables. x/i $pc - Displays the assembly instruction at the current program counter (PC) address.

info registers - Displays the values of the CPU registers.

Meaning of the commands

mov %edx,(%rax) - This instruction moves the value in the EDX register into the memory location pointed to by the RAX register. Just like *rax = edx in C.

callq 0x4004b0 <puts@plt> - This instruction calls the function located at the address 0x4004b0, which is the address of the puts function in the Procedure Linkage Table (PLT). The @plt suffix indicates that this is a call to a function that is dynamically linked at runtime.

For more reference visit the manual.