Types of errors and debugging

Types of errors

1. Syntax errors: violation of the language rules, such as a missing ; or misspelled keyword. Detected by the compiler; the program does not compile.
2. Logical errors: the program runs but gives wrong output because the logic is wrong, such as using + instead of *. No error message is shown.
3. Runtime errors: occur during execution, such as division by zero, invalid memory access or opening a missing file. The program stops abnormally.
4. Linker errors: a function used is not found when linking, such as a misspelled prntf giving "undefined reference".
5. Semantic errors: statements that follow the grammar but have no valid meaning, such as a + b = c; (a value cannot be assigned to an expression). The compiler reports them.

Debugging a C program

• Read the compiler's error and warning messages (compile with gcc -Wall) and fix the first error first, as one error can cause many.
• Trace the program by hand with sample data (dry run) and compare with the expected result.
• Insert printf() statements to display variable values at key points.
• Use a debugger (such as gdb or the IDE debugger) to set breakpoints, step through statements and watch variables.
• Divide the program into functions and test each separately.
• Test with normal, boundary and invalid data.