When you write some code and put it on a spacecraft headed into the far reaches of space, you need to it work, no matter what. Mistakes can mean loss of mission or even loss of life. In 2006, Gerard Holzmann of the NASA/JPL Laboratory for Reliable Software wrote a paper called The Power of 10: Rules for Developing Safety-Critical Code. The rules focus on testability, readability, and predictability:
Avoid complex flow constructs, such as goto and recursion. All loops must have fixed bounds. This prevents runaway code. Avoid heap memory allocation. Restrict functions to a single printed page. Use a minimum of two runtime assertions per function. Restrict the scope of data to the smallest possible. Check the return value of all non-void functions, or cast to void to indicate the return value is useless. Use the preprocessor sparingly. Limit pointer use to a single dereference, and do not use function pointers. Compile with all possible warnings active; all warnings should then be addressed before release of the software.