Debugging with gdb

A debugger runs your program one line at a time and lets you look at every variable on the way. StepVioz's visualizer is a debugger that also draws memory. On your own computer, the standard C debugger is gdb (on macOS, lldb works almost the same way).

A bug to find

#include <stdio.h>

double average(int a[], int n) {
    int total = 0;
    for (int i = 0; i < n; i++) {
        total += a[i];
    }
    return total / n;
}

int main(void) {
    int marks[] = {70, 85, 90, 64};
    printf("average %.2f\n", average(marks, 4));
    return 0;
}

The marks add up to 309, and 309 / 4 is 77.25, but the program prints 77.00. Step through it in the visualizer, and watch total and n in average.

The same search in gdb

First compile with -g (keep the names and line numbers) and -O0 (no optimizing, so the lines run in order):

gcc -g -O0 average.c -o average
gdb ./average

(gdb) break average          stop when average is called
(gdb) run
Breakpoint 1, average (a=0x7ffd..., n=4) at average.c:4
(gdb) next                   run one line
(gdb) next
(gdb) print total
$1 = 309
(gdb) print total / n
$2 = 77

total / n divides two ints, so C throws the fraction away before the result becomes a double. The fix is to make one side a double:

#include <stdio.h>

double average(int a[], int n) {
    int total = 0;
    for (int i = 0; i < n; i++) {
        total += a[i];
    }
    return (double)total / n;
}

int main(void) {
    int marks[] = {70, 85, 90, 64};
    printf("average %.2f\n", average(marks, 4));
    return 0;
}

The commands you need

When a program crashes with Segmentation fault, run it inside gdb: it stops at the crash, and backtrace shows which line of which function was running.

Finding memory bugs

• AddressSanitizer: compile with -fsanitize=address -g (gcc or clang on Linux and macOS). Reading outside an array or using freed memory stops the program with the line number.
• Valgrind (Linux): valgrind --leak-check=full ./app reports leaks: blocks from malloc that were never freed.

The method is the same whatever the tool: reproduce the bug, guess where it is, look at the values there, and change one thing at a time.

Mark lesson complete