Preprocessor

1. PRE-PROCESSOR DIRECTIVES IN C BY JOHN JOSE

2. INTRODUCTION One of the unique feature of C language is preprocessor. The pre processor is a program that processes the source code before it passes through compiler Preprocessor directives are divided into three categories, 1.Macro substitution directives 2.file inclusion directives 3.compiler control directives

3. Macro Substitution: Macro substitution is a process where an identifier in a program replaced by predefined string composed of one or more tokens It is achieved by #define directives

4. Examples #define PI 3.142 #define TRUE 1 #define AND && #define LESSTHAN < #define MESSAGE "welcome to C"

5. Example(program by programmer) #include< stdio.h > #define LESSTHAN < int main() { int a = 30; if(a LESSTHAN 40) printf ("a is Smaller"); return(0); }

6. Program is processed by pre-processor int main() { int a = 30; if(a < 40) printf ("a is Smaller"); return(0); } Program is processed by compiler a is Smaller

7. FILE INCLUSSION An external file containing functions or macro definition can be included as a part of program so that we need not rewrite those function or macro definition. This is achieved by #include directives

8. Example Step1 : Type this Code  In this Code write only function definition as you write in General C Program int add( int a,int b) { return( a+b ); }

9. Step 2 Save Above Code with  [.h ] Extension . Let name of our header file be  myhead  [ myhead.h ] Compile Code if required.

10. Step 3 : Write Main Program #include< stdio.h > # include"myhead.h " void main() { int number1=10,number2=10,number3; number3 = add(number1,number2); printf ("Addition of Two numbers : %d",number3); }

11. Include Our New Header File . Instead of writing < myhead.h> use this terminology “myhead.h” All the Functions defined in the myhead.h header file are now ready for use . Directly  call function add(); [ Provide proper parameter and take care of return type ] Note While running your program precaution to be taken : Both files [  myhead.h and sample.c  ] should be in same folder.

12. ADDITIONAL PREPROCESSOR DIRECTIVES #ELIF DIRECTIVE: #ELIF Enable us to establish an if else sequence for testing multiple conditions. #PRAGMA DIRECTIVES: #PRAGMA Is an implementation oriented directive that allow us to specify various instruction to be given to compiler Syntax : # pragma name #ERROR : #ERROR Directive is used to produce diagonastic messages during debugging Syntax :#ERROR error message.

13. PREPROCESSOR OPERATORS There are two preprocessor operators namely, 1. Stringizing operator (#): it is used in definition of macro functions. This operators allows a formal argument within macro definition to be converted to string Syntax : # define sum( xy ) printf (# xy ”=%f/ n”,xy );

14. 2.TOKEN PASTING OPERATORS The token pasting operator enables us to combine two within a macro definition to form a single token Syntax : #define combine(string1,string2)s1##s2