GNU Debugger (GDB)

Source Attribution

These notes are synthesized from the GNU Debugger (GDB) Tutorial hosted by Tutorialspoint.

What is GNU Debugger (GDB)?

GDB is a portable debugger that runs on many Unix-like systems. It allows you to see what is going on “inside” another program while it executes—or what that program was doing at the moment it crashed.

Key Capabilities

• Crash Analysis: Determine exactly which statement or expression caused a “core dump” (crash).
• Variable Inspection: Examine and modify values of variables or results of expressions at any point during execution.
• Execution Control: Run programs up to a specific point, or step through them line by line to observe state changes.
• Call Stack Analysis: Trace the history of function calls (backtrace) to see how the program reached its current state.

Limitations

• Compilation: GDB cannot debug programs that fail to compile.

• Memory Leaks: While GDB can help find bugs that lead to leaks, it is not a dedicated leak detection tool (use Valgrind for this).

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Installation & Prerequisites

1. Prerequisites

• Compiler: An ANSI-compliant C compiler (like gcc).
• Space: ~115 MB to build from source; ~20 MB for a standard installation.

2. Installation Command (Ubuntu/Debian)

sudo apt-get update
sudo apt-get install libc6-dbg gdb valgrind

3. Verify Installation

gdb --version
# or
gdb -help

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The Debugging Symbol Table

To debug a program effectively, GDB needs a Symbol Table. This maps the machine code in the binary back to your source code (variable names, function names, and line numbers).

Compiling for Debugging

Standard compilation (gcc hello.c -o hello) strips away this information to save space. To include the symbol table, you must use the -g flag:

gcc -g hello.cc -o hello

• Note: Debug builds are larger and slower because they contain this extra metadata.
• Versioning: If you change your code, you must re-compile to generate a new, matching symbol table.

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Core GDB Command Reference

Starting and Stopping

Command	Shorthand	Description
gdb a.out	-	Start GDB with the specified executable.
run	r	Start the program (optionally add arguments: r arg1).
quit	q	Exit GDB.
help	h	List all help topics or a specific command (h break).

Execution Control (Stepping)

Command	Shorthand	Description
list	l	Show 10 lines of source code.
next	n	Run the next line; skips over function calls.
step	s	Run the next line; steps into function calls.
finish	f	Finish the current function and return to the caller.
continue	c	Resume execution until the next breakpoint.

Breakpoints & Watchpoints

Breakpoints stop execution at a location. Watchpoints stop execution when a data condition changes.

• break 45: Stop at line 45.
• break my_func: Stop at the entry of my_func.
• watch x == 3: Stop whenever the condition x == 3 becomes true or false.
• info break: List all current breakpoints.
• delete N: Delete breakpoint number N.

Inspecting & Modifying Variables

• print var: Display the current value of var.
• display var: Automatically print var every time the program stops.
• set var = 10: Change the value of a variable during runtime to test different scenarios.
• call func(): Execute a function manually from the prompt.

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Navigating the Call Stack

The stack shows the path of function calls that led to the current point. A Frame represents one specific function call on that stack.

• bt (Backtrace): Shows the entire call stack.
• up / down: Move focus between the calling and called functions to inspect local variables in different scopes.

Example Backtrace Output:

#0  c_func() at source.c:10
#1  0x08048456 in b_func() at source.c:20
#2  0x080484af in a_func() at source.c:30
#3  0x080483fe in main() at source.c:40

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Signal Handling

Signals (like SIGSEGV for segmentation faults or SIGINT for interrupts) can be managed within GDB.

• handle [signal] [action]: Tell GDB how to react (e.g., handle SIGUSR1 nostop to prevent GDB from pausing when that signal is received).

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GDB Cheat Sheet