C #if

The #if directive is used for conditional compilation. It allows you to include or exclude portions of code from compilation based on specific conditions that you specify. The #if directive checks whether a given expression evaluates to true or false, and if it evaluates to true, the code within the #if block is included in the compilation.

Here’s the basic syntax of the #if directive:

#if expression
   // Code to compile if the expression is true
#endif

• expression is a C expression that is evaluated by the preprocessor. If the expression is nonzero (true), the code within the #if block is compiled. If the expression is zero (false), the code within the #if block is excluded from compilation.

Here’s an example of how the #if directive can be used:

#include <stdio.h>

#define DEBUG 1 // Define DEBUG macro

int main() {
#if DEBUG
    printf("Debug mode is enabled.\n");
#else
    printf("Debug mode is disabled.\n");
#endif

    return 0;
}

In this example:

1. The DEBUG macro is defined with a value of 1 using #define. This is often used to enable debug code when needed.
2. Inside the program, the #if directive checks whether the DEBUG macro evaluates to true (nonzero). Since it is defined as 1, the expression is true, and the code within the #if block is included in compilation.
3. As a result, “Debug mode is enabled.” is printed when the program is executed.

You can use #if to conditionally compile code based on various conditions, including macro definitions, expressions, or predefined macros that the preprocessor recognizes.

Here’s an example that uses predefined macros to check the target platform:

#include <stdio.h>

int main() {
#if defined(_WIN32)
    printf("Running on Windows.\n");
#else
    printf("Running on a non-Windows platform.\n");
#endif

    return 0;
}

In this case, the defined(_WIN32) expression checks if the _WIN32 predefined macro is defined, which is typically the case when compiling for Windows platforms. Depending on the platform, it prints a corresponding message.