Debug with GDB

Load an Executable

Using GDB to debug requires it recognizes a program’s debug symbols. By compiling with -g option, GDB will understand what source code looks like after loading an executable file:

$ gcc -g -Wall -Werror foo.c # compile with -g option
$ gdb ./a.out  # load all symbols of a.out into GDB

Text User Interface

Text User Interface (TUI) allows developers to visualize source code and to debug like using the Integrated Development Environment (IDE) to trace problems. For a beginner, entering the TUI mode is more understandable than the command line mode. The following key bindings are the most common usages for interacting with TUI.

  1. Ctrl x + a - Enter or leave the TUI mode

  2. Ctrl x + o - Switch the active window

  3. Ctrl x + 1 - Display one window (e.g., source code + GDB shell)

  4. Ctrl x + 2 - Display two windows (e.g., source code + GDB shell + assembly)

  5. Ctrl l - Refresh window

Basic Commands

Start/Stop a program

  1. start - Run an executable file and stop at the beginning

  2. run / r - Run an executable file until finish or stop at a breakpoint

  3. step / s - Run a program step by step with entering a function

  4. next / n - Run a program step by step without entering a function

  5. continue / c - Run a program until finish or stop at a breakpoint

  6. finish - Step out of the current function

Set Breakpoints

  1. b line - Set a breakpoint at the given line in the current file

  2. b file: line - Set a breakpoint at the given line in a given file

  3. b … if cond - Set a breakpoint when the condition is true

  4. clear line - Delete a breakpoint at the given line in the current file

  5. clear file: line - Delete a breakpoint at giving a line in a given file

  6. info breakpoints - Display breakpoints status

  7. enable breakpoints - Enable breakpoints

  8. disable breakpoints - Disable breakpoints

  9. watch cond - Set a watchpoint for inspecting a value

Display Stack

  1. backtrace / bt - Display current stack

  2. frame / f framenum - Select a frame and inspect its status

  3. where - Display the current stack and the line

Print Variables

  1. print / p var - Print value of the given variable

  2. ptype var - Print type info of the given variable

  3. info args - Print function arguments

  4. info locals - Print all local variables

Reverse Run

  1. record - Start recording each instruction step

  2. record stop - Stop recording

  3. rn - Reverse next

  4. rs - Reverse step

int main(int argc, char *argv[]) {
    int out = 0;
    for (int i = 0; i < 10; ++i) {
        out = i * i;
    }
    return out;
}

(gdb) b main
(gdb) r
Starting program: /home/ubuntu/a.out

Breakpoint 1, main (argc=21845, argv=0x0) at test.cc:2
2       {
(gdb) record
...
(gdb) n
(gdb) p out
$1 = 1
(gdb) rn
(gdb) rn
(gdb) p out
$2 = 0

Define a Function

GDB provides an original way for developers to define a customized function. The following snippet shows how to define a function to display the information of the current stack.

(gdb) define sf
Type commands for definition of "sf".
End with a line saying just "end".
>where
>info args
>info locals
>end

Display Memory Contents

int main() {
    char arr[100] = "1234567890abcdefghijklmnopqrstuvwxyz";
    return 0;
}

(gdb) " x/[format] [address expression]
(gdb) " x/[len][format] [address expression]
(gdb) x/s arr
0x7fffffffe620:     "1234567890abcdefghijklmnopqrstuvwxyz"
(gdb) x/10c arr
(gdb) x/5c arr
0x7fffffffe620:     49 '1'  50 '2'  51 '3'  52 '4'  53 '5'
(gdb) x/5b arr
0x7fffffffe620:     0x31    0x32    0x33    0x34    0x35