What is undefined behavior in C?

Undefined behavior (UB) means the C standard gives no rule about what the program will do. It may work, crash, or give a wrong result, and it can change with the compiler or the options.

Common examples:

• Reading an uninitialized variable.
• Accessing an array outside its limits.
• Dereferencing a NULL or dangling pointer.
• Signed integer overflow, like INT_MAX + 1.
• Modifying a variable two times in one expression, like i = i++ + ++i.
• Dividing by zero.

The danger is that the program may look fine on your computer. Compile with -Wall -Wextra and use -fsanitize=undefined to catch many of them.

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