CSC161 2010F, Class 23: Building Libraries

Overview:
* Why We Need Code Libraries.
* Simple Code Libraries in C.
* Lab.

Admin:
* Cookies and napkins
* For tomorrow: Read Unit Testing 101 for Non-Programmers.
  * Links on outline and Web site (I hope)
* Remember: You can vote on campus today!
* Grading has now dropped to the bottom of my todo list.  Sorry.

Libraries:
* Idea: We often use the same code in multiple programs.
  * If you design code well, it *should* be usable in multiple programs.a
* How do we get the same code used in multiple programs?
  * Cut and paste.
    * Easy.
    * But ...
      * Can be a lot of effort to cut and paste.
      * Might want to encapsulate: If the code relies on certain "global"
        variables, different functions may have overlapping globals.
  * Write a program to do the cutting and pasting for you.
    * Same problems as above
* Typical solution: Build a "library" - a collection of encapsulated
  code that you can join with your own code.

In C:
* Separate files for separate basic components
* Can compile together with
  cc -o program file1.c file2.c file3.c
* This wastes time recompiling if you only change one file
* So most programmers build OBJECT files (mostly-compiled code) first.
        cc -c file1.c
  creates file1.o
        make file1.o
  cc -o program file1.o file2.o file3.o

Make helps!
* You don't need rules for the .o files, Make knows them already
* You do need a rule for building the program.  Rules look like this

TARGET: DEPENDENCIES
        INSTRUCTIONS

program: file1.o file2.o file3.o
        cc -o program file1.o file2.o file3.o