Build System Overview

This document provides an overview on how the build system works. It is targeted at people wanting to learn about internals of the build system. It is not meant for persons who casually interact with the build system. That being said, knowledge empowers, so consider reading on.

The build system is composed of many different components working in harmony to build the source tree. We begin with a graphic overview.

digraph build_components {
   "configure" -> "config.status" -> "build backend" -> "build output"

Phase 1: Configuration

Phase 1 centers around the configure script, which is a bash shell script. The file is generated from a file called which is written in M4 and processed using Autoconf 2.13 to create the final configure script. You don’t have to worry about how you obtain a configure file: the build system does this for you.

The primary job of configure is to determine characteristics of the system and compiler, apply options passed into it, and validate everything looks OK to build. The primary output of the configure script is an executable file in the object directory called config.status. configure also produces some additional files (like However, the most important file in terms of architecture is config.status.

The existence of a config.status file may be familiar to those who have worked with Autoconf before. However, Mozilla’s config.status is different from almost any other config.status you’ve ever seen: it’s written in Python! Instead of having our configure script produce a shell script, we have it generating Python.

Now is as good a time as any to mention that Python is prevalent in our build system. If we need to write code for the build system, we do it in Python. That’s just how we roll. For more, see Python and the Build System.

config.status contains 2 parts: data structures representing the output of configure and a command-line interface for preparing/configuring/generating an appropriate build backend. (A build backend is merely a tool used to build the tree - like GNU Make or Tup). These data structures essentially describe the current state of the system and what the existing build configuration looks like. For example, it defines which compiler to use, how to invoke it, which application features are enabled, etc. You are encouraged to open up config.status to have a look for yourself!

Once we have emitted a config.status file, we pass into the realm of phase 2.

Phase 2: Build Backend Preparation and the Build Definition

Once configure has determined what the current build configuration is, we need to apply this to the source tree so we can actually build.

What essentially happens is the automatically-produced config.status Python script is executed as soon as configure has generated it. config.status is charged with the task of tell a tool how to build the tree. To do this, config.status must first scan the build system definition.

The build system definition consists of various files in the tree. There is roughly one file per directory or per set of related directories. Each files defines how its part of the build config works. For example it says I want these C++ files compiled or look for additional information in these directories. config.status starts with the file from the root directory and then descends into referenced files by following DIRS variables or similar.

As the files are read, data structures describing the overall build system definition are emitted. These data structures are then fed into a build backend, which then performs actions, such as writing out files to be read by a build tool. e.g. a make backend will write a Makefile.

When config.status runs, you’ll see the following output:

Reticulating splines...
Finished reading 1096 files into 1276 descriptors in 2.40s
Backend executed in 2.39s
2188 total backend files. 0 created; 1 updated; 2187 unchanged
Total wall time: 5.03s; CPU time: 3.79s; Efficiency: 75%

What this is saying is that a total of 1096 files were read. Altogether, 1276 data structures describing the build configuration were derived from them. It took 2.40s wall time to just read these files and produce the data structures. The 1276 data structures were fed into the build backend which then determined it had to manage 2188 files derived from those data structures. Most of them already existed and didn’t need changed. However, 1 was updated as a result of the new configuration. The whole process took 5.03s. Although, only 3.79s was in CPU time. That likely means we spent roughly 25% of the time waiting on I/O.

For more on how files work, see Files.

Phase 3: Invocation of the Build Backend

When most people think of the build system, they think of phase 3. This is where we take all the code in the tree and produce Firefox or whatever application you are creating. Phase 3 effectively takes whatever was generated by phase 2 and runs it. Since the dawn of Mozilla, this has been make consuming Makefiles. However, with the transition to files, you may soon see non-Make build backends, such as Tup or Visual Studio.

When building the tree, most of the time is spent in phase 3. This is when header files are installed, C++ files are compiled, files are preprocessed, etc.