.3 File Extension

Unix Section 3 Manual Page

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What is a 3 file?

A 3 file is a Unix manual page (man page) that describes a library function. You can read a 3 file's contents using the Unix man utility. Each 3 file is named for the function it documents.

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Screenshot of a .3 file in Microsoft Visual Studio Code
3 file open in Microsoft Visual Studio Code

Man pages contain documentation for system-level features of a Unix-based operating system. The pages are typically grouped into eight sections, each of which contains documentation for a subset of system information.

The third man page section describes library functions that can be used in the operating system. In particular, these sections describe C library functions.

For example, the file attr_get.3 contains documentation related to the attr_get and attr_getf library functions. It includes a quick synopsis of how to use the functions, a detailed description of the functions, and additional information.

NOTE: Other sections' Unix man pages use the .1, .2, .4, .5, .6, .7, and .8 extensions. Man pages are also commonly compressed into .GZ files (which may use the .gz.# extension).

How to open a 3 file

In Unix-like OSes, you can open a 3 file by calling the man utility using the name of the file. For example, to open attr_get.3, enter man attr_get(3).

Since 3 files are plain text files, you can also open them in any text editor, such as Vim (Linux, Mac), Microsoft Notepad (Windows), and Apple TextEdit (Mac).

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Programs that open 3 files

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Updated February 24, 2022

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