Editing C preprocessor (section)

==== Code inclusion ====
To include the content of one file into another, the preprocessor replaces a line that starts with <code>#include</code> with the content of the file specified after the directive. The inclusion may be logical in the sense that the resulting content may not be stored on disk and certainly is not overwritten to the source file. The file being included need not contain any sort of code, as this directive will copy the contents of whatever file is included in-place, but the most typical use of <code>#include</code> is to include a header file (or in some rarer cases, a source file).

In the following example code, the preprocessor replaces the line <code>#include &lt;stdio.h&gt;</code> with the content of the standard library header file named 'stdio.h' in which the [[Function (computer programming)|function]] <code>printf()</code> and other symbols are declared.

<syntaxhighlight lang="cpp">
#include <stdio.h>

int main(void) {
    printf("Hello, World!\n");
    return 0;
}
</syntaxhighlight>

In this case, the file name is enclosed in angle brackets to denote that it is a system file. For a file in the [[codebase]] being [[software build|built]], double-quotes are used instead. The preprocessor may use a different search algorithm to find the file based on this distinction.

For C, a header file is usually named with a <code>.h</code> extension. In C++, the convention for file extension varies with common extensions <code>.h</code> and <code>.hpp</code>. But the preprocessor includes a file regardless of the extension. In fact, sometimes code includes <code>.c</code> or <code>.cpp</code> files.

To prevent including the same file multiple times which often leads to a compiler error, a header file typically contains an [[include guard|{{mono|#include}} guard]] or if supported by the preprocessor [[pragma once|{{mono|#pragma once}}]] to prevent multiple inclusion.