Compiling Multiple Files: Some Theory

• We need to discuss what the compiler does to turn your code into an executable.
• We logically divide the compiler into at least three components.
  1. The preprocessor : prepares the files for compilation.
  2. The compiler proper: turns the source code into executable code
  3. The linker: combines (or links) multiple executable code files into a single executable.
• The preprocessor
  • The preprocessor handles the preprocessor directives.
  • #include <iostream>
    • It searches the standard header file locations and literally inserts this code into the file.
    • This can be recursive as header files include other header files.
    • Look at /usr/include/c++/9/cmath (on mirkwood)
    • Look at /usr/include/math.h
  • There are other preprocessor directives
    • #pragma
    • #define
    • #ifndef - #end
  • g++ -E trinaryTest.cpp | more
  • This may or may not be stored in a file.
  • 
• The compiler (proper)
  • Takes the code produced by the preprocessor and creates object code.
  • This may or may not be stored in a file.
  • This may or may not be in the machine language of the target machine.
  • But you can assume it is for simplicity.
  • 
• The linker
  • Combines the object files.
  • Along with system libraries
  • To produce a single executable.
  • 
• This is how a traditional program is compiled.
• For our program we have a few more steps
  • I have written a program called trinaryTest.cpp.
  • It uses the code from Trinary.h/cpp
• To build this
  • First compile Trinary.cpp to produce an object file Trinary.o
  • To do this
    • g++ -c -o Trinary.o Trinary.cpp
  • Next compile trinaryTest.cpp
    • g++ -o trinaryTest TrinaryTest.cpp Trinary.o
  • 
• You need to remember to do this every time you change any of these files.
• Make can simplify this
  • Make implements the idea of a dependency.
  • Some are built in or can be deduced.
    • Trinary.o depends on Trinary.cpp
    • trinaryTest depends trinaryTest.cpp
  • Others need to be coded.
    • Trinary.o also depends on Trinary.h
    • trinaryTest depends on Trinary.o
  • To code these we add the following lines to Makefile
    • trinaryTest: Trinary.o
    • Trinary.o: Trinary.h
  • Make can then look at file times and deduce what needs to be recompiled.