Header file includes

• When C programs are compiled, compiler first runs separate program called preprocessor
  • preprocessor operates on C program files provided and looks at preprocessor directives included
  • preprocessor directive - line in C program beginning with #
  • features include header file inclusion, macro expansion, conditional compilation
• #include <header> - searches for file in angled brackets and replaces include statement with file
  • when angled brackets are provided, included file should be part of compiler standard library - search will be done in directory that typically stores such files (e.g. /usr/include)
  • double quotes - search path expanded to include current source file directory
    • compiler can also be provided include path for files with -I option

Macro Expansion

• Macros are defined with #define preprocessor directive
• object-like and function-like macros
• object-like: #define identifier value - identifier must not have any white space, value may
  • preprocessor will search and replace each occurence of identifier with corresponding value
• common case of define directive is for defining constants, e.g. #define MAX 10
• note that the expansion of one macro can effect a different macro

e.g:

#include <stdio.h>
#define FIRST SECOND
#define SECOND third
#define third int

FIRST main(void){
  printf("Hello\\n");
}

Conditional Compilation

• Useful feature of development is to conditional suppress code segments that are irrelevant or to only compile segments that are relevant
• 6 directives for conditional compilation - #if, #ifdef, #ifndef, #elif, #else, #endif
• condition is evaluated by preprocessor, if condition is true, block immediately following is forwarded to compiler, suppressed if false
• e.g: compiler depend on platform

#ifdef __unix__ /* __unix__ is usually defined by compilers targeting Unix systems */
# include <unistd.h>
#elif defined _WIN32 /* _WIN32 is usually defined by compilers targeting 32 or 64 bit Windows systems */
# include <windows.h>
#endif