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):

#pragma once

// header declarations here

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

#pragma warning(disable : 4996)   // suppress deprecated function warning

#pragma comment (MSVC) — link a library:

#pragma comment(lib, "ws2_32.lib")

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

`#error`

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

#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:

#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:

#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:

#define LIMIT 100
// ... use LIMIT ...
#undef LIMIT
// LIMIT is no longer defined from here

Fundamentals

Functions

Array

String

Structure

Union

Pointer

File Handling

Preprocessor

Other Preprocessor Commands

Other Preprocessor Commands

`#pragma`

`#error`

`#line`

Predefined Macros

`#undef`

​Other Preprocessor Commands

​#pragma

​#error

​#line

​Predefined Macros

​#undef

Other Preprocessor Commands

`#pragma`

`#error`

`#line`

Predefined Macros

`#undef`