Ninja
=====

Introduction
------------

Ninja is yet another build system.  It takes as input a file
describing interdependencies of files (typically source code and
output executables) and orchestrates building them, _quickly_.

Ninja joins a sea of other build systems.  Its distinguishing goal is
to be fast.  It is born from my work on the Chromium browser project,
which has well over 20,000 source files and whose other build systems
(including one built from custom non-recursive Makefiles) take tens of
seconds to start compiling after changing one file.

Here are some of the design goals of Ninja:

* very fast (i.e., instant) incremental builds, even for very large
  projects.

* very little implicit policy; "explicit is better than implicit".

* get dependencies correct, and in particular situations that are
  difficult to get right with Makefiles (e.g. outputs need an implicit
  dependency on the command line used to generate them; C source code
  need to be able to benefit from gcc's `-M` flags for header
  dependencies).

* when convenience and speed are in conflict, prefer speed.

Some explicit _non-goals_:

* convenient syntax for writing build files by hand.  _You should
  generate your ninja files using another program_.  This is how we
  can sidestep many policy decisions.

* built-in rules. _Out of the box, ninja has no rules for
  e.g. compiling C code._

* build-time customization of the build. _Options belong in
  the program that generates the ninja files_.

* build-time decision-making ability such as conditionals or search
  paths. _Making decisions is slow.  Ninja strikes its target without
  any hesitation._

To restate, Ninja manages to be simpler and faster than other build
systems by being much more stupid.  It has no built-in knowledge of
how to build C source, or link libraries, or install binaries.

You instead decide this policy when you create your project's `.ninja`
files.  Customization and configuration are out of scope; instead you
should provide customization in the system that generates your
`.ninja` files, like how autoconf provides `./configure`.

Conceptual overview
~~~~~~~~~~~~~~~~~~~
Ninja evaluates a graph of dependencies between files, and runs
whichever commands are necessary to make your build target up to date.
If you are familiar with Make, Ninja is very similar.

A build file (default name: `build.ninja`) provides a list of _rules_
-- short names for longer commands, like how to run the compiler --
along with a list of _build_ statements saying how to build files
using the rules -- which rule to apply to which inputs to produce
which ouputs.

Conceptually, `build` statements describe the dependency graph of your
project, while `rule` statements describe how to generate the files
along a given edge of the graph.

Semantic differences from make
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

* A rule may point at a path for extra implicit dependency
  information.  This makes it easy to get header dependencies correct
  for C/C++ code.

* A build edge may have multiple outputs.

* Outputs implicitly depend on the command line that was used to generate
  them, which means that changing e.g. compilation flags will cause
  the outputs to rebuild.

* Output directories are always implicitly created before running the
  command that relies on them.

User interface differences from make
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

* Rules can provide shorter descriptions of the command being run, so
  you can print e.g. `CC foo.o` instead of a long command line while
  building.

* Command output is always buffered.  This means commands running in
  parallel don't interleave their output, and when a command fails we
  can put its failure output next to the full command line that
  produced the failure.


Getting started
---------------

The included `bootstrap.sh` should hopefully produce a working `ninja`
binary, by first blindly compiling all non-test files together then
re-building ninja using itself.

Usage is currently just

----------------
ninja target
----------------

where `target` is a known output described by `build.ninja` in the
current directory.


Creating .ninja files
~~~~~~~~~~~~~~~~~~~~~
Here's a basic `.ninja` file that demonstrates most of the syntax.

---------------------------------
cflags = -Wall

rule cc
  command = gcc $cflags -c $in -o $out

build foo.o: cc foo.c
---------------------------------

Variables
~~~~~~~~~
Despite the non-goal of being convenient to write by hand, to keep
build files readable (debuggable), Ninja supports declaring shorter
reusable names for strings.  A declaration like the following

----------------
cflags = -g
----------------

can be used on the right side of an equals sign, deferencing it with
a dollar sign, like this:

----------------
rule cc
  command = gcc $cflags -c $in -o $out
----------------

Variables can also be referenced using curly braces like `${in}`.

Variables might better be called "bindings", in that a given variable
cannot be changed, only shadowed.


Rules
~~~~~

Rules begin with a line consisting of the `rule` keyword and a name
for the rule.  Then follows an indented set of `variable = value` lines.

The basic example above declares a new rule named `cc`, along with the
command to run.  (In the context of a rule, the `command` variable is
special and defines the command to run.  A full list of special
variables is provided in <<ref_rule,the reference>>.)

Within the context of a rule, two additional special variables are
available: `$in` expands to the list of input files (`foo.c`) and
`$out` to the output file (`foo.o`) for the command.


Build statements
~~~~~~~~~~~~~~~~

Build statements begin with the `build` keyword, and have the format
+build _outputs_: _rulename_ _inputs_+.  Such a declaration says that
all of the output files are derived from the input files.  When the output
files are missing or when the inputs change, ninja will run the rule
to regenerate the outputs.

The basic example above describes how to build `foo.o`, using the `cc`
rule.

In the scope of a `build` block (including in the evaluation of its
associated `rule`), the variable `$in` is the list of inputs and the
variable `$out` is the list of outputs.

A build statement may be followed by an indented set of `key = value`
pairs, much like a rule.  These variables will shadow any variables
when evaluating the variables in the command.  For example:

----------------
cflags = -Wall -Werror
rule cc
  command = gcc $cflags -c $in -o $out

# If left unspecified, builds get the outer $cflags.
build foo.o: cc foo.c

# But you can can shadow variables like cflags for a particular build.
build special.o: cc.special.c
  cflags = -Wall
----------------

For more discussion of how scoping works, consult <<ref_scope,the
reference>>.

If you need anything more complicated information passed from the
build statement to the rule (for example, if the rule needs the file
extension of the first input), pass that through as an extra variable,
like how `cflags` is passed above.


Ninja file reference
--------------------
A file is a series of declarations.  A declaration can be one of:

1. A rule declaration, which begins with +rule _rulename_+.

2. A build edge, which looks like +build _output1_ _output2_:
   _rulename_ _input1_ _input2_+. +
   Order-only dependencies may be tacked on the end with +_|
   _dependency1_ _dependency2_+.

3. Variable declarations, which look like +_variable_ = _value_+.

4. References to more files, which look like +subninja _path_+ or
   +include _path_+.  The difference between these is explained below
   <<ref_scope,in the discussion about scoping>>.

Comments begin with `#` and extend to the end of the line.

Newlines are significant, but they can be escaped by putting a `\`
before them.

Other whitespace is only significant if it's at the beginning of a
line.  If a line is intended more than the previous one, it's
considered part of its parent's scope; if it is indented less than the
previous one, it closes the previous scope.

Rule variables
~~~~~~~~~~~~~~
[[ref_rule]]

A `rule` block contains a list of `key = value` declarations that
affect the processing of the rule.  Here is a full list of special
keys.

`command` (_required_):: the command line to run.

`depfile`:: path to an optional `Makefile` that contains _implicit
  dependencies_.  Implicit dependencies are inputs to a build that are
  not given on the command line; the best example is how `gcc` has the
  `-M` family of flags to output the list of headers a given `.c` file
  depends on.
+
----
rule cc
  depfile = $out.d
  command = gcc -MMD -MF $out.d [other gcc flags here]
----

`description`:: a short description of the command, used to pretty-print
  the command as it's running.  The `-v` flag controls whether to print
  the full command or its description; if a command fails, the full command
  line will always be printed before the command's output.

Additionally, the special `$in` and `$out` variables expand to the
space-separated list of files provided to the `build` line referencing
this `rule`.

Build dependencies
~~~~~~~~~~~~~~~~~~
There are three types of build dependencies which are subtly different.

1. Explicit dependencies, as listed in a build line.  These are
   available as the `$in` variable in the rule.  Changes in these files
   cause the output to be rebuilt; if these file are missing and
   ninja doesn't know how to build them, the build is aborted.

2. Implicit dependencies, as picked up from a `depfile` attribute on
   a rule.  Changes in these files cause the output to be rebuilt; if
   they are missing, they are just skipped.

3. Order-only dependencies, expressed with the syntax `| dep1 dep2` on
   the end of a build line.  When these are missing, the output is not
   rebuilt until they are built, but once they are available further
   changes to the files do not affect the output.  Order-only
   dependencies can be useful for bootstrapping implicit dependencies:
   for example, to generate a header file before starting a subsequent
   compilation step.

Evaluation and scoping
~~~~~~~~~~~~~~~~~~~~~~
[[ref_scope]]

Top-level variable declarations are scoped to the file they occur in.

The `subninja` keyword, used to include another `.ninja` file,
introduces a new scope.  The included `subninja` file may use the
variables from the parent file, and shadow their values for the file's
scope, but it won't affect values of the variables in the parent.

To include another `.ninja` file in the current scope, much like a C
`#include` statement, use `include` instead of `subninja`.

Variable declarations indented in a `build` block are scoped to the
`build` block.  This scope is inherited by the `rule`.  The full
lookup order for a variable referenced in a rule is:

1. Rule-level variables (i.e. `$in`, `$command`).

2. Build-level variables from the `build` that references this rule.

3. File-level variables from the file that the `build` line was in.

4. Variables from the file that included that file using the
   `subninja` keyword.
