> ## 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.

# Other Preprocessor Commands

> #pragma, #error, #line, and predefined macros in C

# Other Preprocessor Commands

Beyond `#define`, `#include`, and `#ifdef`, the C preprocessor provides several other useful directives.

***

## `#pragma`

`#pragma` provides implementation-specific (compiler-specific) instructions. The most widely-used are:

**`#pragma once`** — modern include guard (prevents a header from being included multiple times):

```c theme={null}
#pragma once

// header declarations here
```

**`#pragma warning`** (MSVC) — suppress or enable specific warnings:

```c theme={null}
#pragma warning(disable : 4996)   // suppress deprecated function warning
```

**`#pragma comment`** (MSVC) — link a library:

```c theme={null}
#pragma comment(lib, "ws2_32.lib")
```

<Note>
  `#pragma` directives are not portable — they vary between compilers. Code using them may not compile on a different compiler.
</Note>

***

## `#error`

Forces a **compile-time error** with a custom message. Useful for catching configuration mistakes:

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

#ifndef MAX_SIZE
    #error "MAX_SIZE must be defined before including this file!"
#endif

int arr[MAX_SIZE];
```

If `MAX_SIZE` is not defined, compilation stops immediately with the specified message.

***

## `#line`

Changes the line number (and optionally the filename) reported by the compiler in error messages:

```c theme={null}
#line 100 "myfile.c"
// Compiler will now report the next line as line 100 in "myfile.c"
```

Mainly used by code-generation tools to keep error messages pointing to the original source.

***

## Predefined Macros

The C standard defines several macros that are automatically available in every program:

| Macro      | Type     | Description                                  |
| ---------- | -------- | -------------------------------------------- |
| `__FILE__` | `char *` | Current source file name                     |
| `__LINE__` | `int`    | Current line number                          |
| `__DATE__` | `char *` | Compilation date                             |
| `__TIME__` | `char *` | Compilation time                             |
| `__func__` | `char *` | Current function name (C99+)                 |
| `__STDC__` | `int`    | 1 if the compiler conforms to the C standard |

**Example:**

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

void myFunction() {
    printf("Function : %s\n", __func__);
    printf("File     : %s\n", __FILE__);
    printf("Line     : %d\n", __LINE__);
    printf("Compiled : %s at %s\n", __DATE__, __TIME__);
}

int main() {
    myFunction();
    return 0;
}
```

**Output (example):**

```
Function : myFunction
File     : main.c
Line     : 5
Compiled : Jun 13 2026 at 14:30:00
```

These are very useful for debugging and logging.

***

## `#undef`

Remove a previously defined macro:

```c theme={null}
#define LIMIT 100
// ... use LIMIT ...
#undef LIMIT
// LIMIT is no longer defined from here
```