Preprocesser in c

before actual compile.......some .....cont.....

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2. Preprocessor Directives In C Muhammed Thanveer Melayi Muhammed [email protected] Muhammed Thanveer Melayi Twitter Profile Linkedin Profile

3. What is Preprocessors The C Preprocessor  is not part of the compiler, but is a separate step in the compilation process. In simplistic terms, a C Preprocessor is just a text substitution tool and they instruct compiler to do required pre-processing before actual compilation All preprocessor commands begin with a pound symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in first column

4.  important preprocessor directives Directive and Description The unconditional directives are: #include - Inserts a particular header from another file #define - Defines a preprocessor macro # undef - Undefines a preprocessor macro The conditional directives are: # ifdef - If this macro is defined # ifndef - If this macro is not defined #if - Test if a compile time condition is true #else - The alternative for #if # elif - #else an #if in one statement # endif - End preprocessor conditional Other directives include: # - Stringization , replaces a macro parameter with a string constant ## - Token merge, creates a single token from two adjacent ones

5. Use of the preprocessor programs easier to develop easier to read easier to modify example # include < stdio.h > #define PI 3.1415 int main () { int radius ; float area; printf ("Enter the radius: "); scanf ("% d",&radius ); area=PI*radius*radius; printf ("Area=%.2f",area ); return ; }

6. # ifdef and # endif in c It only checks identifier has been defied or not . # ifdef < Identifer >     -------------     ------------- #else     -------------     ------------- # endif Example # include< stdio.h > #define ABC 25 #define PQR "Exact Help" int  main(){      int  num = 3;     # ifdef  ABC          printf ("% d",ABC * ABC);     #else           printf ("% s",PQR );     # endif return  0 ; } Output : 625

7. # error directive in c Syntax :  #error it only issue an error message and this error message includes . Example #include< stdio.h > # ifndef  __MATH_H #error First include then compile #else int  main(){     float  a,b =25;     a= sqrt (b);     printf ("% f",a );     return 0; } # endif Output: compiler error --> Error directive :First include then compile

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