C Pre-processor introduction & working

Quick Recap

• Compiler converts textual form of a C program into an executable.
• There are four phases for a C program to become an executable:
  • Preprocessing → Compilation → Assembly → Linking

What is a C preprocessor

• C preprocessor comes under action before the actual compilation process.
• C preprocessor is not a part of the c compiler
• It is a text substitution tool
• All preprocessor commands begin with a hash symbol(#)

Preprocessor commands example

• #define
• #include → inserts other header files
• #undef
• #ifdef
• #ifndef
• #if
• #else
• #elif

Two common #include formats

• In C programming there are two common formats for #includes:
  • #include < headerfile.h > // The angles brackets say to look in the standard system directories
  • #include "myfile.h" // The quotation marks say to look in the current directory.
• Disk drive full path is allowed, but discouraged since it is not portable:
  • #include <C:\Program files\Harry\bhau\somefile.h > // Too specific
  • #include <sys/file.h> // Relative and porable path to the standard locations.

Using #define for debugging

• #define directive can be used for debugging
• We can have printing statements that we only only active when debugging
• We can "protect" them in a "ifdef" block as follows:
  • #define DEBUG
    #ifdef DEBUG // if DEBUG is defined then execute below statements
    // executable code
    #undef DEBUG

Macros using #define

• We can also create macros using #define
• Macors operate much like functions, but because they are expanded in place and are generally faster

                        
                        #define PI 3.14
                        #define SQUARE (x) x*x // made like a function
                        int radius = 5;
                        int area = PI * SQUARE(radius); // area = 3.14 * radius * radius and this will resolve during preprocessing time