C Debugging
Debugging
Debugging is the process of finding and fixing errors (bugs) in your program.
Bugs are mistakes that make your program crash, behave incorrectly, or give the wrong output.
Before you start debugging, make sure your code is clean and organized:
- Use proper indentation to keep the structure clear.
- Give your variables clear, meaningful names that describe what they store.
Clean code is easier to read - and easier to debug!
In the sections below, we'll introduce several common debugging techniques.
1. Print Debugging
Use printf() to print values at different points
in your code to see what's going wrong:
int x = 10;
int y = 0;
printf("Before division\n"); // Debug output
int z = x / y; // Crashes (division by zero)
printf("After division\n"); //
Never runs
If you don't see "After division", the program crashed at x / y.
2. Check Variable Values
Print variables to see if they are what you expect:
int x = 10;
int y = 5;
int result = x - y;
printf("Result:
%d\n", result); // Result: 5
Expected 15? That means the logic is wrong: try using
x + y instead.
3. Use a Debugger Tool
IDEs like Visual Studio, Code::Blocks, and VS Code have built-in debuggers that work with C. These tools let you:
- Pause your program using breakpoints
- Step through your code line by line
- Watch variables and see unexpected values as they change
Tip: Start with printf() debugging. Once you're comfortable, explore your IDE's debugger to get even better insights into your code.
4. Learn from Error Messages
The C compiler and runtime errors often tell you what went wrong and where. For example:
error: expected ';' before 'printf'
Easy solution: Fix the missing semicolon!
Debugging with Safe Checks
Some bugs, like dividing by zero (used in the first example of this page), are known to cause crashes.
If you know an operation could fail, you can check for it in advance and avoid the crash:
Example
int main() {
int x = 10;
int y = 0;
printf("Before division\n");
if (y != 0) { // // Check that y is not zero before dividing
int z = x / y;
printf("Result: %d\n", z);
} else {
printf("Error: Division by zero!\n"); // // Print error message instead of crashing
}
printf("After division\n");
return 0;
}
Try it Yourself »
Now, instead of crashing, the program prints an error message and continues running. This is an important part of writing safe, debuggable code.
Another Example - Out-of-Bounds Array Access
Accessing elements outside the array is another common mistake.
In the example below, we use printf() to check the index value before using it:
Example
int main() {
int numbers[3] = {10, 20, 30};
int index = 5;
printf("Index = %d\n", index);
if (index >= 0 && index < 3) { // Make sure the index is within the valid range (0 to 2)
printf("Value = %d\n", numbers[index]);
} else {
printf("Error: Index out of bounds!\n");
}
return 0;
}
Try it Yourself »
Summary
- Use
printf()to print values and trace your code. - Read error messages carefully - they often tell you what went wrong and where.
- Add checks (like
if (y != 0)) to avoid crashes before they happen. - Use an IDE's debugger when you're ready for deeper debugging.
- Debugging helps you understand your program better and fix problems faster.
