How to avoid undefined behavior

Know common causes

Undefined behavior includes out-of-bounds access, use-after-free, null dereference, signed overflow, invalid shifts, and mismatched variadic format arguments.

• Initialize objects before reading them
• Keep indexes and pointer arithmetic within bounds
• Match every printf/scanf specifier to its argument
• Never use memory after its lifetime ends

Understand the consequence

Undefined behavior is not a predictable error value. A program may appear to work, crash, leak data, or change behavior under optimization or on another compiler.

Turn on diagnostics

Warnings catch suspicious source patterns; sanitizers add runtime checks during testing. They improve detection but cannot prove a program safe.

cc -std=c17 -Wall -Wextra -Wpedantic -Wconversion -g \
  -fsanitize=address,undefined program.c -o program
./program

Design for safety

Track buffer capacities, validate indexes and arithmetic before use, check library return values, and make ownership and lifetimes explicit. Test boundary cases such as empty input and maximum lengths.