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C pre-processor

The C processor is a macro processor which is used to compile the source code of the program (step by step) . It is not a part of the compiler. All pre-processor commands begin with the hash symbol (#). The C pre-processor provides various facilities that are:

  • Inclusion of header
  • Macro expansion
  • Conditional compilation
  • Line control
Preprocessor Syntax/Description
Macro Macro is used to define the constant value and data types. Syntax: #define
Header file inclusion The source code of the file “file_name” is included in the main program at the specified place. Syntax: #include <file_name>
Conditional compilation It is used to set of commands in source program. Syntax: #ifdef, #endif, #if, #else, #ifndef
Other directives #undef is used to defined a macro variable and #Pragma is used to call a function before and after main function. Syntax: #undef, #pragma
  C language, there are various stages of the execution process. The given following diagram will help you:   There are following important pre-processor directories in C language.
Directive Description
#define Substitutes a pre-processor macro.
#include It is used to inserts a particular header from another file.
#undef It is used to undefine a preprocessor macro.
#ifdef It is used to return true if this macro is defined.
#ifdef It is used to return if this macro is defined.
#if It is used to check compile time condition.
#else The alternative for #if.
#elif #if and #else in one statement.
#endif It is used to end pre-processor conditional.
#error It is used to print an error message on stderr.
#pragma Issues special command to the compiler, using a standardized method.
  Example: 1
Example 2
#ifndef MESSAGE
 #define MESSAGE “You Wish”
  C Macros A macro is a name which stands for a fragment of code. Before using macro we must define it with ‘#define’ directive. It is the name of a macro. There are two types of macros.
  1. Object-like macros
  2. Function-like macros 
  1. Object-Like Macros:
The object-like macro is an identifier that is replaced by value. It is widely used to represent numeric constants. Example:
Pi =3.147
  1. Function like Macros:
Function like macros is used as a function call. Let us consider an example.
#define Max(a,b)((a)>(b)?(a):(b))
Here MAX is the macro name. There are some predefined macros :
Predefined Macros Description
_DATE_ It is defined as the current date with “MMM DD YYY” format.
_TIME_ It is defined as current time with “HH:MM:SS” format.
_FILE_ It contains the current filename as a string literal.
_STDC_ It is defined as 1 when compiler compiles with the ANSI standard.
__STDC_VERSION__ It is defined as standard’s version
_GNUC_ It is defined if and only if this is  GNU C. etc
Example of Macros
#include <stdio.h>
printf("File :%s\n",__FILE__);
printf("Date :%s\n",__DATE__);
printf("Time :%s\n",__TIME__);
printf("Line :%d\n",__LINE__);
File :main.c
Date :Apr 24 2019
Time :11:50:39
Line :6
  C #define and #include. C #defined: It defines constant value. It can be any of the basic data types. Syntax:
#define token value
Example: 1
#include <stdio.h>
#define PI 3.14
main() {
#include <stdio.h>
#define number 3.14
#define letter 'A'
#define letter_sequence "C_Tutorial"
#define backslash_char '\?'
void main(){
 printf("value of number : %f \n", number );
 printf("value of letter : %c \n", letter );
 printf("value of letter_sequence : %s \n", letter_sequence);
 printf("value of backslash_char : %c \n", backslash_char);
value of number : 3.140000

value of letter : A

value of letter_sequence : C_Tutorial

value of backslash_char  : ?
  #include: It is used to paste the code of given file into the current file. We can use system-defined and used-defined header files. There are two types of #include directive.
  1. #include< filename>
  2. #include ”filename”
int main(){
 printf("Hello World ");
 return 0;
Hello World
  C #undef: It is used to check undefined the constant or macro. It is defined by #define. Syntax:
#undef token
Example 1:
#include <stdio.h>
#define PI 3.14
#undef PI
main() {
Compile time Error: PI undeclared
Example 2:
#include <stdio.h>
#define number 15 // variable define
int squ=number*number;
#undef number
int main() {
 return 0;
  #ifdef, #else and #endif The ‘#ifdef’ directive is used to check whether the macro is defined or not. If it is defined then, ‘if’ clause statements are included in the source code. Otherwise “else” clause statements are included in the source file for compilation and execution. Example : #ifdef
#define AGE 18
void main(){
#ifdef AGE
printf("You are Eligible for voting\n");
printf("Not eligible\n");
You are Eligible for voting

  C #Pragma: The C #Pragma is used to call a function before and after the main function. It also provides additional information to the compiler. There are several types of pragmas in GCC compiler.
  • AArch64 Pragmas
  • ARM Pragmas
  • M32C Pragmas
  • MeP Pragmas
  • RS/6000 and PowerPC Praglmas
  • S/390 Pragmas:
  • Darwin Pragmas:
  • Solaris Pragmas: etc.
#pragma token
#include <stdio.h>
void function1( );
void function2( );
#pragma startup fun1
#pragma exit fun2
int main(){
 printf ( "\n Now we are in main function" ) ;
 return 0;
void fun1(){
 printf("\nFun1 is called before main function call");
void fun2(){
 printf ( "\nFun2 is called just before end of main function" ) ;
Now we are in main function