Learn Debugging with Diagnostic Output | Preventing and Debugging Failures

Assertions are useful for catching logic errors by halting the program when a condition fails, but they do not show you how your program reached that state. Using diagnostic output with printf() statements lets you observe your program's behavior in real time. This approach helps you follow the flow of execution, track variable values, and quickly spot where things go wrong. In C, where advanced runtime debugging tools are often limited or unavailable, adding printf() statements is a practical way to make debugging faster and more effective.

main.c


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#include <stdio.h>

int divide(int numerator, int denominator) {
    printf("Entering divide with numerator = %d, denominator = %d\n", numerator, denominator);
    if (denominator == 0) {
        printf("Error: Attempted division by zero!\n");
        return 0;
    }
    int result = numerator / denominator;
    printf("Division result = %d\n", result);
    return result;
}

int main() {
    printf("Program started\n");
    int a = 20;
    int b = 4;
    printf("Before calling divide: a = %d, b = %d\n", a, b);
    int res1 = divide(a, b);
    printf("After first divide: result = %d\n", res1);

    b = 0;
    printf("Before calling divide with zero denominator: a = %d, b = %d\n", a, b);
    int res2 = divide(a, b);
    printf("After second divide: result = %d\n", res2);
    printf("Program finished\n");
    return 0;
}

Using printf() statements to output messages at key points in your code is one of the simplest and most effective debugging methods in C. By printing the values of variables and noting when specific parts of your program are executed, you can clearly see the path your program takes. This approach makes it much easier to pinpoint where errors occur, understand unexpected behavior, and verify that your logic is working as intended. Because C has limited built-in debugging tools, adding printf() calls gives you immediate, actionable insights into your program's flow and state.

main.c


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#include <stdio.h>

void process_data(int value) {
    printf("Processing value: %d\n", value);
    if (value < 0) {
        printf("Warning: Negative value encountered: %d\n", value);
    }
    // Simulate some processing
    int result = value * 2;
    printf("Result after processing: %d\n", result);
}

int main() {
    printf("Starting data processing loop\n");
    int data[] = {5, -3, 12, 0, -7};
    int size = sizeof(data) / sizeof(data[0]);
    for (int i = 0; i < size; i++) {
        printf("\nIteration %d:\n", i+1);
        process_data(data[i]);
    }
    printf("Data processing completed\n");
    return 0;
}

Printing key variables during execution lets you identify incorrect values or unexpected states without needing a debugger. By displaying the contents of important variables at specific points in your program, you can see exactly where things start to go wrong. This method helps you pinpoint logic errors or faulty assumptions, making it easier to find and fix bugs, even when advanced debugging tools are unavailable.

main.c


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

void compute(int x) {
    printf("compute called with x = %d\n", x);
    assert(x != 0 && "Error: x must not be zero");
    int result = 100 / x;
    printf("Result of 100 / x = %d\n", result);
}

int main() {
    printf("Program started\n");
    int a = 5;
    int b = 0;
    printf("Calling compute with a = %d\n", a);
    compute(a);
    printf("Calling compute with b = %d (should trigger assertion)\n", b);
    compute(b);
    printf("Program finished\n");
    return 0;
}

Combining diagnostic output with assertions gives you a powerful approach to debugging in C. Assertions immediately catch errors when a condition fails, stopping the program before a problem spreads. By also adding diagnostic output using printf() statements, you can track the values and steps that led up to the failure. This way, you not only catch errors early with assertions but also see the sequence of events and variable changes that caused the issue. Together, these techniques make it much easier to understand, locate, and fix bugs in your code.

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Section 3. Chapter 2

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main.c


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#include <stdio.h>

int divide(int numerator, int denominator) {
    printf("Entering divide with numerator = %d, denominator = %d\n", numerator, denominator);
    if (denominator == 0) {
        printf("Error: Attempted division by zero!\n");
        return 0;
    }
    int result = numerator / denominator;
    printf("Division result = %d\n", result);
    return result;
}

int main() {
    printf("Program started\n");
    int a = 20;
    int b = 4;
    printf("Before calling divide: a = %d, b = %d\n", a, b);
    int res1 = divide(a, b);
    printf("After first divide: result = %d\n", res1);

    b = 0;
    printf("Before calling divide with zero denominator: a = %d, b = %d\n", a, b);
    int res2 = divide(a, b);
    printf("After second divide: result = %d\n", res2);
    printf("Program finished\n");
    return 0;
}

main.c


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#include <stdio.h>

void process_data(int value) {
    printf("Processing value: %d\n", value);
    if (value < 0) {
        printf("Warning: Negative value encountered: %d\n", value);
    }
    // Simulate some processing
    int result = value * 2;
    printf("Result after processing: %d\n", result);
}

int main() {
    printf("Starting data processing loop\n");
    int data[] = {5, -3, 12, 0, -7};
    int size = sizeof(data) / sizeof(data[0]);
    for (int i = 0; i < size; i++) {
        printf("\nIteration %d:\n", i+1);
        process_data(data[i]);
    }
    printf("Data processing completed\n");
    return 0;
}

main.c


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

void compute(int x) {
    printf("compute called with x = %d\n", x);
    assert(x != 0 && "Error: x must not be zero");
    int result = 100 / x;
    printf("Result of 100 / x = %d\n", result);
}

int main() {
    printf("Program started\n");
    int a = 5;
    int b = 0;
    printf("Calling compute with a = %d\n", a);
    compute(a);
    printf("Calling compute with b = %d (should trigger assertion)\n", b);
    compute(b);
    printf("Program finished\n");
    return 0;
}

Everything was clear?

Thanks for your feedback!

Section 3. Chapter 2