Compiler and Linker

• many C programs are hundreds of thousands of lines of code
• compiler and linker can be used to split program up so that it can be edited and recompiled in small, quick pieces
• compiler produces an object file which is then linked to other object files to create an executable

                -------------                   ----------
  ask.c  --->   | compiler  | --->  ask.o  ---> | linker | --> guessask
                -------------                   ----------
                                                 /|\
                                                  |
                -------------                     |
  guess.c  ---> | compiler  | ---> guess.o -------
                -------------                     |
                                                  |
                                                 \|/
                -------------                   ----------
  canned.c ---> | compiler  | ---> canned.o --> | linker | --> guesscanned
                -------------                   ----------

• the 'make' tool can be used to simplify compiling process, a Makefile is a specification of what to recompile and link when a single file is changed, so that the entire program does not have to be recompiled since that would be very time-consuming
• for example if we change canned.c, we only have to recompile canned.c and then relink it with guess.o to create guesscanned, we do not have to recompile guess.c since it hasn't changed
• you will use 'make' extensively in your assignments and tutorials, although I have set up the Makefiles for you

Preprocessor

• First phase of the compiler
• Only does textual substitution, that is it rewrites the program to a different version of the program and passes that different version to the other phases in the compiler
• all prepreprocessor directives have hash symbol '#' in the first column
• how to define a macro

  /* general form of macro definition */
  #define  name   text_to_substitute_for_name 
  /* examples of macro definitions */
  #define MAX_VALUE 3
  #define begin {
  #define end }
  
  /* code using example macros */
  begin                      /* preprocessed to { */
    int array[MAX_VALUE];    /* preprocessed to int array[3] */
  end                        /* preprocessed to } */

• an include directive reads all the text in the specified file into the program

  /* general form of include directive */
  #include  file_name
  
  /* examples */
  #include  "guess.h"   /* read guess.h */
  #include  <stdio.h>   /* read stdio.h from standard directory */

• by convention, most include files end in '.h' and are referred to as header files
• macro definitions and function prototypes are usually put into a header file