C Programming Tutorial
Description
In the C Programming Language, the #ifdef directive allows for conditional compilation. The preprocessor determines if the provided macro exists before including the subsequent code in the compilation process.
The #ifdef preprocessor directive checks if macro is defined by #define. If yes, it executes the code otherwise #else code is executed, if present.
Syntax:
#ifdef MACRO //code #endif
Syntax with #else:
#ifdef MACRO //successful code #else //else code #endif
C #ifdef example
Let's see a simple example to use #ifdef preprocessor directive.
#include <stdio.h> #include <conio.h> #define NOINPUT void main() { int a=0; #ifdef NOINPUT a=2; #else printf("Enter a:"); scanf("%d", &a); #endif printf("Value of a: %d\n", a); getch(); }
Output:
Value of a: 2
But, if you don't define NOINPUT, it will ask user to enter a number.
#include <stdio.h> #include <conio.h> void main() { int a=0; #ifdef NOINPUT a=2; #else printf("Enter a:"); scanf("%d", &a); #endif printf("Value of a: %d\n", a); getch(); }
Output:
Enter a:5
Value of a: 5
Example
The following example shows how to use the #ifdef directive in the C language:
/* Example using #ifdef directive by C programming*/ #include <stdio.h> #define YEARS_OLD 10 int main() { #ifdef YEARS_OLD printf("C programming is over %d years old.\n", YEARS_OLD); #endif printf("C programming is a great resource.\n"); return 0; }
Here is the output of the executable program:
C programming is over 10 years old.
C programming is a great resource.
A common use for the #ifdef directive is to enable the insertion of platform specific source code into a program.
The following is an example of this:
/* Example using #ifdef directive for inserting platform * specific source code by C programming*/ #include <stdio.h> #define UNIX 1 int main() { #ifdef UNIX printf("UNIX specific function calls go here.\n"); #endif printf("C programming is over 10 years old.\n"); return 0; }
The output of this program is:
UNIX specific function calls go here.
C programming is over 10 years old.
In this example, the UNIX source code is enabled. To disable the UNIX source code, change the line #define UNIX 1 to #define UNIX 0.