gdb basics

Learn to debug programs with gdb.

gdb basics

gdb (GNU Debugger) runs C programs step by step and lets you inspect their internal state. To use it, compile with -g: gcc -g program.c -o program.

Then start with gdb ./program. Key commands: run starts the program, quit leaves gdb, list shows source. Even if the program crashes, gdb stays active so you can see where it stopped. This is much faster than adding printf; you can inspect variables on the spot, pause and resume execution at will.

A debuggable program

For gdb to be useful, the program should have several functions. Each function is an anchor point where you can pause in gdb. A simple example: int sum_range(int lo, int hi) sums the integers in a range.

Implementation: int total = 0; for (int i = lo; i <= hi; i++) total += i; return total;. main calls this function and prints the result. In gdb you can break sum_range to pause at the function entry and inspect variables one by one. This structure is the basis for testing each function independently in real projects.

Exercise Write a debuggable program

Write int sum_range(int lo, int hi) that sums from lo to hi. In main, print the result with printf("%d\n", sum_range(1, 10));.

Show example solution

#include <stdio.h>

int sum_range(int lo, int hi) {
    int total = 0;
    for (int i = lo; i <= hi; i++) {
        total += i;
    }
    return total;
}

int main(void) {
    printf("%d\n", sum_range(1, 10));
    return 0;
}

Test yourself

How do you compile a program for gdb debugging?
- gcc -s program.c -o program
- gcc -O2 program.c -o program
- gcc -g program.c -o program
- gcc -c program.c
Show answer

gcc -g program.c -o program — The -g flag adds debug info; gdb uses it.

Complete the sum_range call

printf("%d
", sum_range(1, ____));

Show answer

printf("%d
", sum_range(1, 10));

Match the gdb commands

run

Start program
quit

Leave gdb
list

Show source
-g

Debug info

Next lesson: Breakpoints

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