apenwarr-redo/Documentation/index.md

redo: a recursive, general-purpose build system

redo is a competitor to the long-lived, but sadly imperfect, make program. There are many such competitors, because many people over the years have been dissatisfied with make's limitations. However, of all the replacements I've seen, only redo captures the essential simplicity and flexibility of make, while avoiding its flaws. To my great surprise, it manages to do this while being simultaneously simpler than make, more flexible than make, and more powerful than make.

Although I wrote redo and I would love to take credit for it, the magical simplicity and flexibility comes because I copied verbatim a design by Daniel J. Bernstein (creator of qmail and djbdns, among many other useful things). He posted some very terse notes on his web site at one point (there is no date) with the unassuming title, "Rebuilding target files when source files have changed." Those notes are enough information to understand how the system is supposed to work; unfortunately there's no code to go with it. I get the impression that the hypothetical "djb redo" is incomplete and Bernstein doesn't yet consider it ready for the real world.

I was led to that particular page by random chance from a link on The djb way, by Wayne Marshall.

After I found out about djb redo, I searched the Internet for any sign that other people had discovered what I had: a hidden, unimplemented gem of brilliant code design. I found only one interesting link: Alan Grosskurth, whose Master's thesis at the University of Waterloo was about top-down software rebuilding, that is, djb redo. He wrote his own (admittedly slow) implementation in about 250 lines of shell script.

If you've ever thought about rewriting GNU make from scratch, the idea of doing it in 250 lines of shell script probably didn't occur to you. redo is so simple that it's actually possible. For testing, I actually wrote an even more minimal version, which always rebuilds everything instead of checking dependencies, in 210 lines of shell (about 4 kbytes).

The design is simply that good.

My implementation of redo is called redo for the same reason that there are 75 different versions of make that are all called make. It's somehow easier that way. Hopefully it will turn out to be compatible with the other implementations, should there be any.

My extremely minimal implementation, called do, is in the minimal/ directory of this repository.

(Want to discuss redo? See the bottom of this file for information about our mailing list.)

What's so special about redo?

The theory behind redo is almost magical: it can do everything make can do, only the implementation is vastly simpler, the syntax is cleaner, and you can do even more flexible things without resorting to ugly hacks. Also, you get all the speed of non-recursive make (only check dependencies once per run) combined with all the cleanliness of recursive make (you don't have code from one module stomping on code from another module).

(Disclaimer: my current implementation is not as fast as make for some things, because it's written in python. Eventually I'll rewrite it an C and it'll be very, very fast.)

The easiest way to show it is with an example.

Create a file called default.o.do:

redo-ifchange $2.c
gcc -MD -MF $2.d -c -o $3 $2.c
read DEPS <$2.d
redo-ifchange ${DEPS#*:}

Create a file called myprog.do:

DEPS="a.o b.o"
redo-ifchange $DEPS
gcc -o $3 $DEPS

Of course, you'll also have to create a.c and b.c, the C language source files that you want to build to create your application.

In a.c:

#include <stdio.h>
#include "b.h"

int main() { printf(bstr); }

In b.h:

extern char *bstr;

In b.c:

char *bstr = "hello, world!\n";

Now you simply run:

$ redo myprog

And it says:

redo  myprog
redo    a.o
redo    b.o

Now try this:

$ touch b.h
$ redo myprog

Sure enough, it says:

redo  myprog
redo    a.o

Did you catch the shell incantation in default.o.do where it generates the autodependencies? The filename default.o.do means "run this script to generate a .o file unless there's a more specific whatever.o.do script that applies."

The key thing to understand about redo is that declaring a dependency is just another shell command. The redo-ifchange command means, "build each of my arguments. If any of them or their dependencies ever change, then I need to run the current script over again."

Dependencies are tracked in a persistent .redo database so that redo can check them later. If a file needs to be rebuilt, it re-executes the whatever.do script and regenerates the dependencies. If a file doesn't need to be rebuilt, redo can calculate that just using its persistent .redo database, without re-running the script. And it can do that check just once right at the start of your project build.

But best of all, as you can see in default.o.do, you can declare a dependency after building the program. In C, you get your best dependency information by trying to actually build, since that's how you find out which headers you need. redo is based on the following simple insight: you don't actually care what the dependencies are before you build the target; if the target doesn't exist, you obviously need to build it. Then, the build script itself can provide the dependency information however it wants; unlike in make, you don't need a special dependency syntax at all. You can even declare some of your dependencies after building, which makes C-style autodependencies much simpler.

(GNU make supports putting some of your dependencies in include files, and auto-reloading those include files if they change. But this is very confusing - the program flow through a Makefile is hard to trace already, and even harder if it restarts randomly from the beginning when a file changes. With redo, you can just read the script from top to bottom. A redo-ifchange call is like calling a function, which you can also read from top to bottom.)

What projects use redo?

Here are a few open source examples:

• Liberation Circuit is a straightforward example of a C++ binary (a game) compiled with redo.

• WvStreams uses a more complex setup producing several binaries, libraries, and scripts. It shows how to produce output files in a different directory than the source files.

• WvBuild can cross-compile several dependencies, like openssl and zlib, and then builds WvStreams using those same libraries. It's a good example of redo/make interop and complex dependencies.

• There's an experimental variant of Buildroot that uses redo in order to clean up its dependency logic.

• You can also find some limited examples in the t/111-example/ subdir of the redo project itself.

If you switch your program's build process to use redo, please let us know and we can link to it here.

(Please don't use the code in the t/ directory as serious examples of how to use redo. Many of the tests are doing things in deliberately psychotic ways in order to stress redo's code and find bugs.)

How does this redo compare to other redo implementations?

djb never released his version, so other people have implemented their own variants based on his published specification.

This version, sometimes called apenwarr/redo, is probably the most advanced one, including support for parallel builds, advanced build logs, and helpful debugging features. It's currently written in python for easier experimentation, but the plan is to eventually migrate it to plain C. (Some people like to call this version "python-redo", but I don't like that name. We shouldn't have to rename it just because we port the code to C.)

Here are some other redo variants (thanks to Nils Dagsson Moskopp for many of these links):

• Alan Grosskurth's redo thesis and related sh implementation. (Arguably, this paper is the one that got all the rest of us started.)

• Nils Dagsson Moskopp's redo-sh is a completely self-sufficient sh-based implementation.

• apenwarr's minimal/do is included with this copy of redo. It's also sh-based, but intended to be simple and failsafe, so it doesn't understand how to "redo" targets more than once.

• Christian Neukirchen's redo-c, a C implementation.

• Jonathan de Boyne Pollard's fork of Alan Grosskurth's redo (another sh-based implementation).

• Jonathan de Boyne Pollard's redo rewritten in C++

• Gyepi Sam's redux in Go

• jekor's redo in Haskell

• Shanti Bouchez-Mongardé (mildred)'s fork of apenwarr's redo in python

• Tharre's redo in C

• catenate's credo, a (very rearchitected) variant written for the Inferno Shell.

The redo design is so simple and elegant that many individuals have been inspired to (and able to) write their own version of it. In the honoured tradition of Unix's make, they (almost) all just use the same name, redo. Unfortunately, many of these implementations are unmaintained, slightly incompatible with the "standard" redo semantics, and/or have few or no automated tests.

At the time of this writing, none of them except apenwarr/redo (ie. this project) support parallel builds (redo -j). For large projects, parallel builds are usually essential.