> ## Documentation Index
> Fetch the complete documentation index at: https://learn.acadlink.app/llms.txt
> Use this file to discover all available pages before exploring further.

# Using #ifdef

> Conditional compilation with #ifdef, #ifndef, #if, #else, and #endif

# Conditional Compilation — `#ifdef`

Conditional compilation directives let you **include or exclude parts of the code** before compilation, based on whether a macro is defined or a condition is true. This is useful for platform-specific code, debug builds, and feature flags.

***

## `#ifdef` — If Defined

Compiles the block only if the macro **has been defined**:

```c theme={null}
#define DEBUG

#ifdef DEBUG
    printf("Debug mode ON\n");
#endif
```

If `DEBUG` is defined, the `printf` is compiled. If not, it's completely removed.

***

## `#ifndef` — If Not Defined

Compiles the block only if the macro **has NOT been defined** (most commonly used as include guards):

```c theme={null}
#ifndef MY_HEADER_H
#define MY_HEADER_H

// header content here

#endif
```

***

## `#if`, `#elif`, `#else`, `#endif`

More powerful: evaluate a constant expression, not just whether a macro exists.

```c theme={null}
#define VERSION 2

#if VERSION == 1
    printf("Running version 1\n");
#elif VERSION == 2
    printf("Running version 2\n");
#else
    printf("Unknown version\n");
#endif
```

***

## Complete Example: Debug vs Release Build

```c theme={null}
#include <stdio.h>

// Comment out the next line to build in release mode
#define DEBUG_MODE

int main() {
    int x = 42;

#ifdef DEBUG_MODE
    printf("[DEBUG] x = %d\n", x);
#endif

    printf("Program running.\n");
    return 0;
}
```

**With `DEBUG_MODE` defined:**

```
[DEBUG] x = 42
Program running.
```

**Without `DEBUG_MODE`:**

```
Program running.
```

***

## Platform-Specific Code

```c theme={null}
#ifdef _WIN32
    printf("Running on Windows\n");
#elif defined(__linux__)
    printf("Running on Linux\n");
#elif defined(__APPLE__)
    printf("Running on macOS\n");
#endif
```

***

## Summary of Conditional Directives

| Directive      | Meaning                           |
| -------------- | --------------------------------- |
| `#ifdef NAME`  | Compile if `NAME` is defined      |
| `#ifndef NAME` | Compile if `NAME` is NOT defined  |
| `#if expr`     | Compile if expression is non-zero |
| `#elif expr`   | Else-if branch                    |
| `#else`        | Else branch                       |
| `#endif`       | End of conditional block          |
| `#undef NAME`  | Undefine a macro                  |