The bifcl program simply takes a .bif file as input and generates C++ header/source files along with a .bro script that all-together provide the declaration and implementation of Bro Built-In-Functions (BIFs), which can then be compiled and shipped as part of a Bro plugin.

A BIF allows one to write arbitrary C++ code and access it via a function call inside a Bro script. In this way, they can also be used to access parts of Bro's internal C++ API that aren't already exposed via their own BIFs.