#ifndef in C

In C Language, the "#ifndef" directive is another preprocessor directive used for conditional compilation, similar to "#ifdef". #ifndef stands for "if not defined" and is used for conditional compilation.

However, "#ifndef" checks if a particular macro has not been defined.

If the specified macro is not defined, the code between "#ifndef" and "#endif" will be included in the compilation.

Syntax:

#ifndef macro_name
    // Code to be compiled if macro_name is not defined
#endif

ifndef Directive example:

#include <stdio.h>

#ifndef DEBUG_MODE
#define DEBUG_MODE
#endif

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

    printf("Normal execution.\n");

    return 0;
}

we're checking whether "DEBUG_MODE" is not defined using "#ifndef".
If "DEBUG_MODE" is not defined, it will be defined using "#define DEBUG_MODE".
Then, the code inside "#ifndef DEBUG_MODE" and "#else" blocks will include the debug mode message during compilation.

Conditional Checking ifndef:

Example:

#ifndef DEBUG_MODE
#define LOG(msg) printf("DEBUG Mode Disabled: %s\n", msg)
#else
#define LOG(msg) printf("DEBUG Mode Enabled: %s\n", msg)
#endif

int main() {
    LOG("This is a log message.");
    return 0;
}

Here, we are checking `DEBUG_MODE` macro is not defined using the `#ifndef` Directive and the condition met true.

It executing `if` condition `#define LOG(msg) printf("DEBUG Mode Disabled: %s\n", msg)`.

Output:

DEBUG Mode Disabled: This is a log message.

Example:

#include <stdio.h>

#define DEBUG_MODE
#ifndef DEBUG_MODE
#define LOG(msg) printf("DEBUG Mode Disabled: %s\n", msg)
#else
#define LOG(msg) printf("DEBUG Mode Enabled: %s\n", msg)
#endif

int main() {
    LOG("This is a log message.");
    return 0;
}

In this example, we define the `DEBUG_MODE` macro using the `#define` directive before checking `#ifndef`.

so if we check condition `ifndef` DEBUG_MODE it becomes false and code fall to else condition.

it execute else condition `#define LOG(msg) printf("DEBUG Mode Enabled: %s\n", msg)`.

Output:

DEBUG Mode Enabled: This is a log message.

Note:

"#ifndef" is commonly used for conditional compilation to include specific sections of code only if certain macros are not defined.

It's particularly useful for guarding against multiple inclusions of header files.

<-- #ifdef

#undef -->