IntermediatePractice in the editor

C Errors and Debugging

Every programmer writes bugs. What makes a good programmer is finding them quickly. C gives three kinds of feedback: the compiler's errors and warnings, problems while the program runs, and wrong results.

Compiler errors and warnings

An error stops the build: the program cannot be made until it is fixed. A warning lets the build continue but points at code that is very likely wrong. Always compile with -Wall and treat warnings as errors to fix. Read the first message first: later ones are often caused by the first.

A message has the form file:line:column: error: text. In our editor, clicking it jumps to the line, and a hint in plain words follows it.

Message	Usual cause
`expected ';'`	a semicolon is missing at the end of the line above
`use of undeclared identifier`	a typo in a name, or a variable declared later or in another block
`call to undeclared function`	a missing `#include` or prototype
`format specifies type 'int *'`	`&` is missing in `scanf`
`variable is uninitialized`	the variable is used before it gets a value

Runtime errors

Some mistakes only show when the program runs: dividing by zero, an array index outside the array, using a NULL or freed pointer, or recursion that never stops. C itself does not check most of them, so the program may crash or quietly produce garbage. Turn on Safe mode in the editor's Options: it stops the program at such a mistake and names the function.

ExampleTry it in the editor

#include <stdio.h>

int main(void) {
    int a[3] = {1, 2, 3};
    int sum = 0;
    for (int i = 0; i < 3; i++) {   // try changing < to <= with Safe mode on
        sum += a[i];
    }
    printf("sum = %d\n", sum);
    return 0;
}

Output

sum = 6

printf debugging

When the result is wrong, print the values that lead to it. Add printf lines that show variables inside a loop, run, compare with what you expected, and narrow down where the first wrong value appears. Remove the extra lines when you are done, or keep them behind #if DEBUG.

assert

assert(condition) from assert.h stops the program with a message when a condition that must always be true is false. It documents your assumptions and catches broken ones early:

ExampleTry it in the editor

#include <assert.h>
#include <stdio.h>

double average(const int a[], int n) {
    assert(n > 0);            // dividing by zero would be a bug
    int sum = 0;
    for (int i = 0; i < n; i++) sum += a[i];
    return (double)sum / n;
}

int main(void) {
    int marks[] = {70, 80, 90};
    printf("%.1f\n", average(marks, 3));
    return 0;
}

Output

80.0

Classic beginner mistakes

= instead of == in a condition.
A semicolon right after if (...) or for (...), which makes the body empty.
Forgetting break in a switch.
Loops that run one time too many (<= instead of <).
Integer division where a decimal result was wanted.
Strings: forgetting room for '\0', comparing with == instead of strcmp.

Practice exercises

Put a semicolon right after for (int i = 0; i < 3; i++) in the example above, run it and explain the result.
Add assert(n > 0) to a function of your own and call it with 0.
With Safe mode on, read a[3] from the example array and read the message.