C #undef Directive

The #undef directive in C is used to undefine a previously defined macro. It is often employed to remove or reset a macro definition within the scope of a program.

Example

#include <stdio.h>

#define PI 3.14159  // Define PI macro

int main() {
    printf("Value of PI: %f\n", PI);

    #undef PI  // Undefine PI macro

    #ifdef PI
        printf("PI is still defined.\n");
    #else
        printf("PI is undefined.\n");
    #endif

    return 0;
}

Output

Value of PI: 3.141590
PI is undefined.

Explanation

• In the example, we first define a macro PI with a value of 3.14159.
• After using it in the printf statement, we use #undef PI to undefine the PI macro. Subsequently, we check if PI is still defined using #ifdef.
• Since we've undefine the macro, the output shows that PI is undefined.

Use

• • Conditional Compilation: The #undef directive is often used in conjunction with conditional compilation to selectively define or undefine macros based on certain conditions.
• • Macro Redefinition: It allows for the redefinition of macros within different parts of the code, allowing flexibility in macro usage.

Summary

• #undef is used to undefine a previously defined macro.
• It is commonly used for conditional compilation and macro redefinition.
• Undefining a macro makes it undefined and renders it unusable in the remaining code.