Writing Code that Doesn't Break

In a perfect world, users always type what we expect, and memory is infinite. In the real world, things fail. A great C programmer spends 20% of their time writing logic and 80% handling potential errors.

1. The errno Variable

When a standard C function (like opening a file or allocating memory) fails, it sets a global hidden variable called errno to a specific error number.

• perror("Message"): Prints your custom message followed by a human-readable description of what went wrong (e.g., "File not found" or "Permission denied").
• strerror(errno): Returns a string describing the error code so you can log it.

2. Common Runtime Crashes

• Segmentation Fault: You tried to touch memory you don't own. (Usually a bad pointer).
• Stack Overflow: Your recursion went too deep and ran out of stack space.
• Bus Error: You tried to access memory in a way the hardware doesn't allow (misalignment).
• Arithmetic Exception: Usually a "Division by Zero" error.

3. Logic Errors (The Silent Killers)

The program doesn't crash, but it produces the wrong output.

• Off-by-one: Your loop runs 11 times instead of 10.
• Assignment instead of Equality: Writing if (x = 5) instead of if (x == 5). The first one makes x equal to 5 and is ALWAYS true!

Debugging Best Practices

1. Read the Compiler Warnings: Use the -Wall (Warn All) flag with GCC. If the compiler is complaining, don't ignore it!
2. Print Debugging: The classic "Insert printf everywhere" method. printf("Reached line 42, val = %d\n", val);
3. Use a Debugger (GDB): Learn to use a real debugger. It allows you to pause time, step through code line-by-line, and see exactly what is happening in RAM.

Use the assert(condition) macro from <assert.h>. It will instantly crash your program during development if a condition that should be true is actually false. It's like a tripwire for bugs!