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.
