.3 File Extension
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.
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.
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.
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).