Debugging C Program using gdb

Debugging a C Program Using GDB

Step 1: Compile the C Program with Debugging Option -g

Compile your C program using the -g option.
This allows the compiler to include debugging information.

Example:

$ cc -g test.c

(test.c is the sample program to debug)

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Step 2: Launch GDB

Start the C debugger (gdb) as shown below:

$ gdb a.out

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Step 3: Set Up a Breakpoint Inside the C Program

Syntax:

break [file_name]:line_number

Example:

break test.c:3

This sets a breakpoint at line number 3.

Breakpoints are placed where you suspect errors.
During execution, the debugger stops at the breakpoint and allows you to debug.

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Step 4: Execute the C Program in GDB

Run the program using:

run [args]

(args are optional command-line arguments)

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Step 5: Print Variable Values Inside GDB

Syntax:

print {variable}

Example:

print i

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Step 6: Continue, Step Over, and Step Into

When the program stops at a breakpoint, you can:

Continue Execution

c   or   continue

Runs until the next breakpoint.

Step Over

n   or   next

Executes the next line as a single instruction.

Step Into

s   or   step

Similar to next, but enters functions and executes them line by line.

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GDB Command Shortcuts

l      → list source code
p      → print variable
c      → continue execution
s      → step into
ENTER  → repeat previous command

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Miscellaneous GDB Commands

List Source Code

l

Shows source code in debug mode.

l line-number

Shows a specific line.

l function

Shows a specific function.

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Backtrace (Call Stack)

bt

Prints backtrace of all stack frames (or innermost COUNT frames).

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Help

help TOPICNAME

Shows help for a specific topic.

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Quit GDB

quit

Exits the debugger.

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Demo Video

https://www.youtube.com/watch?v=sCtY--xRUyI

1️⃣ Simple C Program to Debug (test.c)

This program intentionally contains a logical bug.

#include <stdio.h>

int add(int a, int b) {
    int result = a - b;   // BUG: should be a + b
    return result;
}

int main() {
    int x = 10;
    int y = 5;

    int sum = add(x, y);

    printf("Sum = %d\n", sum);

    return 0;
}

Expected output:

Sum = 15

Actual output (buggy):

Sum = 5

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2️⃣ Compile with Debug Option

cc -g test.c

This creates:

a.out

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3️⃣ Launch GDB

gdb a.out

You will see:

(gdb)

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4️⃣ List Source Code

(gdb) l

Shows the program lines.

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5️⃣ Set Breakpoint at main()

Option A — by function name

(gdb) break main

Option B — by line number

(gdb) break test.c:10

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6️⃣ Run the Program

(gdb) run

Execution stops at the breakpoint:

Breakpoint 1, main () at test.c:10

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7️⃣ Step Through the Program

Execute next line

(gdb) n

Keep pressing:

n
n

Watch variables:

(gdb) print x
(gdb) print y

Output:

$1 = 10
$2 = 5

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8️⃣ Step Into the Function Call

When cursor reaches:

int sum = add(x, y);

Type:

(gdb) s

Now you enter:

add(int a, int b)

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9️⃣ Inspect Function Variables

(gdb) print a
(gdb) print b

Output:

10
5

Step to next line:

(gdb) n

Now check:

(gdb) print result

Output:

5

🚨 Now you see the problem:

result = a - b

Instead of:

result = a + b

Bug identified!

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🔟 Continue Execution

(gdb) c

Program finishes.

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1️⃣1️⃣ Fix the Bug

Edit program:

int result = a + b;

Recompile:

cc -g test.c

Run again:

./a.out

Correct output:

Sum = 15

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🧠 Quick Debugging Command Summary

Command	Meaning
gdb a.out	Start debugger
l	List source
break main	Set breakpoint
run	Run program
n	Next line
s	Step into function
p var	Print variable
c	Continue
bt	Backtrace
quit	Exit