Bugs C will not catch

Introduction

A C compiler trusts you. When a program breaks a rule, such as reading past an array, the C standard says the behaviour is undefined: anything may happen. Often the program seems to work, then fails later, on another computer, or in an exam.

StepVioz checks for these bugs while it runs your program, and stops at the exact line. Here is the list to remember:

In many languages a mistake produces a clear error. In C it may produce a wrong number, a crash an hour later, or nothing at all until the program is moved to another computer. Knowing these bugs by name is one of the most valuable things this course can give you.

How it works

Try them

Each line of this program has one of the bugs. Run it, read the message, fix that line, and run again until it reaches the end:

#include <stdio.h>

int main(void) {
    int count;
    int total = 0;
    int values[3] = {4, 5, 6};
    for (int i = 0; i <= 3; i++) {
        total += values[i];
    }
    count = count + 1;
    printf("%d %d\n", total, count);
    return 0;
}

Syntax

gcc -Wall -Wextra -g program.c -o program
gcc -fsanitize=address -g program.c -o program

Example

Here is the program from above with its two bugs fixed: count is given a first value, and the loop stops before it goes past the end.

#include <stdio.h>

int main(void) {
    int count = 0;
    int total = 0;
    int values[3] = {4, 5, 6};
    for (int i = 0; i < 3; i++) {
        total += values[i];
    }
    count = count + 1;
    printf("%d %d\n", total, count);
    return 0;
}

Line by line

1. int count = 0; gives the variable a value before it is used.
2. values has three elements, at indexes 0, 1 and 2.
3. The loop runs while i < 3, so it never goes past the end.
4. count = count + 1 now reads a real value.
5. printf shows the total of 15 and the count of 1.

Expected result

15 1

In the real world

Memory-safety bugs of these kinds make up a large share of serious security problems in big C and C++ programs. That is why compilers offer sanitizers, and why careful teams run them on every change.

Common mistakes

• Assuming that a program is correct because it printed the right answer once.
• Ignoring compiler warnings.
• Changing code at random until the symptom goes away.

Best practices

• Compile with -Wall -Wextra, and fix every warning.
• Use a sanitizer while developing.
• Reduce a bug to the smallest program that still shows it.

Summary

C trusts the programmer, so mistakes such as overruns, uninitialised values and use after free may fail silently. Warnings, sanitizers and careful habits catch these bugs. StepVioz checks for them as your program runs.

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