C and pointers Chapter 18 runtime environment 18.4 summary

18.4 summary
     The best way to improve program efficiency is to choose a better method for it . The next best way to improve the speed of program execution is to evaluate the performance of the program , Look at the place where the program takes the most time . Focusing optimization measures on this part of the program will produce the best results .
     Tips ：
     Learning the runtime environment of a machine is both beneficial and dangerous ： It's useful because the knowledge you get allows you to do something that you can't do in other ways ; It is dangerous because if there is anything in the program that depends on this knowledge , It may damage the portability of the program . In this era , Computers are developing rapidly , Many machines are out of date before they are put on the shelf . therefore , The possibility of program switching from one machine to another is very realistic , So we very much hope that the code has good portability .
18.5 Summary of warnings
    2. Cannot link programs produced by different compilers .
18.6 Summary of programming tips
    1. Use stdarg Implement variable parameter list .
    3. Using certain environment specific techniques will make the program non portable .