3 Evan Martin <martine@danga.com>
9 Ninja is yet another build system. It takes as input the
10 interdependencies of files (typically source code and output
11 executables) and orchestrates building them, _quickly_.
13 Ninja joins a sea of other build systems. Its distinguishing goal is
14 to be fast. It is born from
15 http://neugierig.org/software/chromium/notes/2011/02/ninja.html[my
16 work on the Chromium browser project], which has over 30,000 source
17 files and whose other build systems (including one built from custom
18 non-recursive Makefiles) can take ten seconds to start building after
19 changing one file. Ninja is under a second.
21 Philosophical overview
22 ~~~~~~~~~~~~~~~~~~~~~~
24 Where other build systems are high-level languages, Ninja aims to be
27 Build systems get slow when they need to make decisions. When you are
28 in a edit-compile cycle you want it to be as fast as possible -- you
29 want the build system to do the minimum work necessary to figure out
30 what needs to be built immediately.
32 Ninja contains the barest functionality necessary to describe
33 arbitrary dependency graphs. Its lack of syntax makes it impossible
34 to express complex decisions.
36 Instead, Ninja is intended to be used with a separate program
37 generating its input files. The generator program (like the
38 `./configure` found in autotools projects) can analyze system
39 dependencies and make as many decisions as possible up front so that
40 incremental builds stay fast. Going beyond autotools, even build-time
41 decisions like "which compiler flags should I use?" or "should I
42 build a debug or release-mode binary?" belong in the `.ninja` file
48 Here are the design goals of Ninja:
50 * very fast (i.e., instant) incremental builds, even for very large
53 * very little policy about how code is built. Different projects and
54 higher-level build systems have different opinions about how code
55 should be built; for example, should built objects live alongside
56 the sources or should all build output go into a separate directory?
57 Is there an "package" rule that builds a distributable package of
58 the project? Sidestep these decisions by trying to allow either to
59 be implemented, rather than choosing, even if that results in
62 * get dependencies correct, and in particular situations that are
63 difficult to get right with Makefiles (e.g. outputs need an implicit
64 dependency on the command line used to generate them; to build C
65 source code you need to use gcc's `-M` flags for header
68 * when convenience and speed are in conflict, prefer speed.
70 Some explicit _non-goals_:
72 * convenient syntax for writing build files by hand. _You should
73 generate your ninja files using another program_. This is how we
74 can sidestep many policy decisions.
76 * built-in rules. _Out of the box, Ninja has no rules for
77 e.g. compiling C code._
79 * build-time customization of the build. _Options belong in
80 the program that generates the ninja files_.
82 * build-time decision-making ability such as conditionals or search
83 paths. _Making decisions is slow._
85 To restate, Ninja is faster than other build systems because it is
86 painfully simple. You must tell Ninja exactly what to do when you
87 create your project's `.ninja` files.
92 Ninja is closest in spirit and functionality to make, relying on
93 simple dependencies between file timestamps.
95 But fundamentally, make has a lot of _features_: suffix rules,
96 functions, built-in rules that e.g. search for RCS files when building
97 source. Make's language was designed to be written by humans. Many
98 projects find make alone adequate for their build problems.
100 In contrast, Ninja has almost no features; just those necessary to get
101 builds correct while punting most complexity to generation of the
102 ninja input files. Ninja by itself is unlikely to be useful for most
105 Here are some of the features Ninja adds to make. (These sorts of
106 features can often be implemented using more complicated Makefiles,
107 but they are not part of make itself.)
109 * A Ninja rule may point at a path for extra implicit dependency
110 information. This makes it easy to get header dependencies correct
113 * A build edge may have multiple outputs.
115 * Outputs implicitly depend on the command line that was used to generate
116 them, which means that changing e.g. compilation flags will cause
117 the outputs to rebuild.
119 * Output directories are always implicitly created before running the
120 command that relies on them.
122 * Rules can provide shorter descriptions of the command being run, so
123 you can print e.g. `CC foo.o` instead of a long command line while
126 * Builds are always run in parallel, based by default on the number of
127 CPUs your system has. Underspecified build dependencies will result
130 * Command output is always buffered. This means commands running in
131 parallel don't interleave their output, and when a command fails we
132 can print its failure output next to the full command line that
133 produced the failure.
136 Using Ninja for your project
137 ----------------------------
139 Ninja currently works on Unix-like systems. It's seen the most testing
140 on Linux (and has the best performance there) but it runs fine on Mac
141 OS X and FreeBSD. Ninja has some preliminary Windows support but the
142 full details of the implementation -- like how to get C header
143 interdependencies correct and fast when using MSVC's compiler -- is
146 If your project is small, Ninja's speed impact is likely unnoticeable.
147 Some build timing numbers are included below. (However, even for
148 small projects it sometimes turns out that Ninja's limited syntax
149 forces simpler build rules that result in faster builds.) Another way
150 to say this is that if you're happy with the edit-compile cycle time
151 of your project already then Ninja won't help.
153 There are many other build systems that are more user-friendly or
154 featureful than Ninja itself. For some recommendations: the Ninja
155 author found http://gittup.org/tup/[the tup build system] influential
156 in Ninja's design, and thinks https://github.com/apenwarr/redo[redo]'s
157 design is quite clever.
159 Ninja's benefit comes from using it in conjunction with a smarter
162 http://code.google.com/p/gyp/[gyp]:: The meta-build system used to
163 generate build files for Google Chrome. gyp can generate Ninja files
164 for Linux and Mac and is used by many Chrome developers; support for
165 Windows is in progress. See the
166 http://code.google.com/p/chromium/wiki/NinjaBuild[Chromium Ninja
167 documentation for more details]. gyp is relatively unpopular outside
168 of the Chrome and v8 world.
170 * For Chrome (~30k source files), Ninja reduced no-op builds from
171 around 15 seconds to under one second.
172 * https://plus.google.com/108996039294665965197/posts/SfhrFAhRyyd[A
173 Mozilla developer compares build systems]: "While chromium's full
174 build is 2.15x slower than firefox's, a nop build is 78.2x faster!
175 That is really noticeable during development. No incremental build
176 of firefox can be faster than 57.9s, which means that in practice
177 almost all of them will be over a minute."
179 http://www.cmake.org/[CMake]:: A widely used meta-build system that
180 can generate Ninja files on Linux as of CMake version 2.8.8. (There
181 is some Mac and Windows support -- http://www.reactos.org[ReactOS]
182 uses Ninja on Windows for their buildbots, but those platforms are not
183 yet officially supported by CMake as the full test suite doesn't
186 * For building Blender, one user reported "Single file rebuild is 0.97
187 sec, same on makefiles was 3.7sec."
188 * For building LLVM on Windows, one user reported no-op build times:
189 "ninja: 0.4s / MSBuild: 11s / jom: 53s".
191 others:: Ninja ought to fit perfectly into other meta-build software
192 like http://industriousone.com/premake[premake]. If you do this work,
198 Run `ninja`. By default, it looks for a file named `build.ninja` in
199 the current directory and builds all out-of-date targets. You can
200 specify which targets (files) to build as command line arguments.
202 `ninja -h` prints help output. Many of Ninja's flags intentionally
203 match those of Make; e.g `ninja -C build -j 20` changes into the
204 `build` directory and runs 20 build commands in parallel. (Note that
205 Ninja defaults to running commands in parallel anyway, so typically
206 you don't need to pass `-j`.)
209 Environment variables
210 ~~~~~~~~~~~~~~~~~~~~~
212 Ninja supports one environment variable to control its behavior:
213 `NINJA_STATUS`, the progress status printed before the rule being run.
215 Several placeholders are available:
217 `%s`:: The number of started edges.
218 `%t`:: The total number of edges that must be run to complete the build.
219 `%p`:: The percentage of started edges.
220 `%r`:: The number of currently running edges.
221 `%u`:: The number of remaining edges to start.
222 `%f`:: The number of finished edges.
223 `%o`:: Overall rate of finished edges per second
224 `%c`:: Current rate of finished edges per second (average over builds specified by -j or its default)
225 `%e`:: Elapsed time in seconds. _(Available since Ninja 1.2.)_
226 `%%`:: A plain `%` character.
228 The default progress status is `"[%s/%t] "` (note the trailing space
229 to separate from the build rule). Another example of possible progress status
230 could be `"[%u/%r/%f] "`.
235 The `-t` flag on the Ninja command line runs some tools that we have
236 found useful during Ninja's development. The current tools are:
239 `query`:: dump the inputs and outputs of a given target.
241 `browse`:: browse the dependency graph in a web browser. Clicking a
242 file focuses the view on that file, showing inputs and outputs. This
243 feature requires a Python installation.
245 `graph`:: output a file in the syntax used by `graphviz`, a automatic
246 graph layout tool. Use it like:
249 ninja -t graph mytarget | dot -Tpng -ograph.png
252 In the Ninja source tree, `ninja graph.png`
253 generates an image for Ninja itself. If no target is given generate a
254 graph for all root targets.
256 `targets`:: output a list of targets either by rule or by depth. If used
257 like +ninja -t targets rule _name_+ it prints the list of targets
258 using the given rule to be built. If no rule is given, it prints the source
259 files (the leaves of the graph). If used like
260 +ninja -t targets depth _digit_+ it
261 prints the list of targets in a depth-first manner starting by the root
262 targets (the ones with no outputs). Indentation is used to mark dependencies.
263 If the depth is zero it prints all targets. If no arguments are provided
264 +ninja -t targets depth 1+ is assumed. In this mode targets may be listed
265 several times. If used like this +ninja -t targets all+ it
266 prints all the targets available without indentation and it is faster
267 than the _depth_ mode.
269 `commands`:: given a list of targets, print a list of commands which, if
270 executed in order, may be used to rebuild those targets, assuming that all
271 output files are out of date.
273 `clean`:: remove built files. By default it removes all built files
274 except for those created by the generator. Adding the `-g` flag also
275 removes built files created by the generator (see <<ref_rule,the rule
276 reference for the +generator+ attribute>>). Additional arguments are
277 targets, which removes the given targets and recursively all files
280 If used like +ninja -t clean -r _rules_+ it removes all files built using
283 Files created but not referenced in the graph are not removed. This
284 tool takes in account the +-v+ and the +-n+ options (note that +-n+
287 `compdb`:: given a list of rules, each of which is expected to be a
288 C family language compiler rule whose first input is the name of the
289 source file, prints on standard output a compilation database in the
290 http://clang.llvm.org/docs/JSONCompilationDatabase.html[JSON format] expected
291 by the Clang tooling interface.
292 _Available since Ninja 1.2._
295 Writing your own Ninja files
296 ----------------------------
298 The remainder of this manual is only useful if you are constructing
299 Ninja files yourself: for example, if you're writing a meta-build
300 system or supporting a new language.
305 Ninja evaluates a graph of dependencies between files, and runs
306 whichever commands are necessary to make your build target up to date.
307 If you are familiar with Make, Ninja is very similar.
309 A build file (default name: `build.ninja`) provides a list of _rules_
310 -- short names for longer commands, like how to run the compiler --
311 along with a list of _build_ statements saying how to build files
312 using the rules -- which rule to apply to which inputs to produce
315 Conceptually, `build` statements describe the dependency graph of your
316 project, while `rule` statements describe how to generate the files
317 along a given edge of the graph.
322 Here's a basic `.ninja` file that demonstrates most of the syntax.
323 It will be used as an example for the following sections.
325 ---------------------------------
329 command = gcc $cflags -c $in -o $out
331 build foo.o: cc foo.c
332 ---------------------------------
336 Despite the non-goal of being convenient to write by hand, to keep
337 build files readable (debuggable), Ninja supports declaring shorter
338 reusable names for strings. A declaration like the following
344 can be used on the right side of an equals sign, dereferencing it with
345 a dollar sign, like this:
349 command = gcc $cflags -c $in -o $out
352 Variables can also be referenced using curly braces like `${in}`.
354 Variables might better be called "bindings", in that a given variable
355 cannot be changed, only shadowed. There is more on how shadowing works
356 later in this document.
361 Rules declare a short name for a command line. They begin with a line
362 consisting of the `rule` keyword and a name for the rule. Then
363 follows an indented set of `variable = value` lines.
365 The basic example above declares a new rule named `cc`, along with the
366 command to run. In the context of a rule, the `command` variable
367 defines the command to run, `$in` expands to the list of
368 input files (`foo.c`), and `$out` to the output files (`foo.o`) for the
369 command. A full list of special variables is provided in
370 <<ref_rule,the reference>>.
375 Build statements declare a relationship between input and output
376 files. They begin with the `build` keyword, and have the format
377 +build _outputs_: _rulename_ _inputs_+. Such a declaration says that
378 all of the output files are derived from the input files. When the
379 output files are missing or when the inputs change, Ninja will run the
380 rule to regenerate the outputs.
382 The basic example above describes how to build `foo.o`, using the `cc`
385 In the scope of a `build` block (including in the evaluation of its
386 associated `rule`), the variable `$in` is the list of inputs and the
387 variable `$out` is the list of outputs.
389 A build statement may be followed by an indented set of `key = value`
390 pairs, much like a rule. These variables will shadow any variables
391 when evaluating the variables in the command. For example:
394 cflags = -Wall -Werror
396 command = gcc $cflags -c $in -o $out
398 # If left unspecified, builds get the outer $cflags.
399 build foo.o: cc foo.c
401 # But you can shadow variables like cflags for a particular build.
402 build special.o: cc special.c
405 # The variable was only shadowed for the scope of special.o;
406 # Subsequent build lines get the outer (original) cflags.
407 build bar.o: cc bar.c
411 For more discussion of how scoping works, consult <<ref_scope,the
414 If you need more complicated information passed from the build
415 statement to the rule (for example, if the rule needs "the file
416 extension of the first input"), pass that through as an extra
417 variable, like how `cflags` is passed above.
419 If the top-level Ninja file is specified as an output of any build
420 statement and it is out of date, Ninja will rebuild and reload it
421 before building the targets requested by the user.
426 _Available since Ninja 1.1._
428 Pools allow you to allocate one or more rules or edges a finite number
429 of concurrent jobs which is more tightly restricted than the default
432 This can be useful, for example, to restrict a particular expensive rule
433 (like link steps for huge executables), or to restrict particular build
434 statements which you know perform poorly when run concurrently.
436 Each pool has a `depth` variable which is specified in the build file.
437 The pool is then referred to with the `pool` variable on either a rule
438 or a build statement.
440 No matter what pools you specify, ninja will never run more concurrent jobs
441 than the default parallelism, or the number of jobs specified on the command
445 # No more than 4 links at a time.
449 # No more than 1 heavy object at a time.
450 pool heavy_object_pool
460 # The link_pool is used here. Only 4 links will run concurrently.
461 build foo.exe: link input.obj
463 # A build statement can be exempted from its rule's pool by setting an
464 # empty pool. This effectively puts the build statement back into the default
465 # pool, which has infinite depth.
466 build other.exe: link input.obj
469 # A build statement can specify a pool directly.
470 # Only one of these builds will run at a time.
471 build heavy_object1.obj: cc heavy_obj1.cc
472 pool = heavy_object_pool
473 build heavy_object2.obj: cc heavy_obj2.cc
474 pool = heavy_object_pool
479 Generating Ninja files from code
480 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
482 `misc/ninja_syntax.py` in the Ninja distribution is a tiny Python
483 module to facilitate generating Ninja files. It allows you to make
484 Python calls like `ninja.rule(name='foo', command='bar',
485 depfile='$out.d')` and it will generate the appropriate syntax. Feel
486 free to just inline it into your project's build system if it's
496 The special rule name `phony` can be used to create aliases for other
497 targets. For example:
500 build foo: phony some/file/in/a/faraway/subdir/foo
503 This makes `ninja foo` build the longer path. Semantically, the
504 `phony` rule is equivalent to a plain rule where the `command` does
505 nothing, but phony rules are handled specially in that they aren't
506 printed when run, logged (see below), nor do they contribute to the
507 command count printed as part of the build process.
509 `phony` can also be used to create dummy targets for files which
510 may not exist at build time. If a phony build statement is written
511 without any dependencies, the target will be considered out of date if
512 it does not exist. Without a phony build statement, Ninja will report
513 an error if the file does not exist and is required by the build.
516 Default target statements
517 ~~~~~~~~~~~~~~~~~~~~~~~~~
519 By default, if no targets are specified on the command line, Ninja
520 will build every output that is not named as an input elsewhere.
521 You can override this behavior using a default target statement.
522 A default target statement causes Ninja to build only a given subset
523 of output files if none are specified on the command line.
525 Default target statements begin with the `default` keyword, and have
526 the format +default _targets_+. A default target statement must appear
527 after the build statement that declares the target as an output file.
528 They are cumulative, so multiple statements may be used to extend
529 the list of default targets. For example:
536 This causes Ninja to build the `foo`, `bar` and `baz` targets by
544 For each built file, Ninja keeps a log of the command used to build
545 it. Using this log Ninja can know when an existing output was built
546 with a different command line than the build files specify (i.e., the
547 command line changed) and knows to rebuild the file.
549 The log file is kept in the build root in a file called `.ninja_log`.
550 If you provide a variable named `builddir` in the outermost scope,
551 `.ninja_log` will be kept in that directory instead.
555 Version compatibility
556 ~~~~~~~~~~~~~~~~~~~~~
558 _Available since Ninja 1.2._
560 Ninja version labels follow the standard major.minor.patch format,
561 where the major version is increased on backwards-incompatible
562 syntax/behavioral changes and the minor version is increased on new
563 behaviors. Your `build.ninja` may declare a variable named
564 `ninja_required_version` that asserts the minimum Ninja version
565 required to use the generated file. For example,
568 ninja_required_version = 1.1
571 declares that the build file relies on some feature that was
572 introduced in Ninja 1.1 (perhaps the `pool` syntax), and that
573 Ninja 1.1 or greater must be used to build. Unlike other Ninja
574 variables, this version requirement is checked immediately when
575 the variable is encountered in parsing, so it's best to put it
576 at the top of the build file.
578 Ninja always warns if the major versions of Ninja and the
579 `ninja_required_version` don't match; a major version change hasn't
580 come up yet so it's difficult to predict what behavior might be
584 C/C++ header dependencies
585 ~~~~~~~~~~~~~~~~~~~~~~~~~
587 To get C/C++ header dependencies (or any other build dependency that
588 works in a similar way) correct Ninja has some extra functionality.
590 The problem with headers is that the full list of files that a given
591 source file depends on can only be discovered by the compiler:
592 different preprocessor defines and include paths cause different files
593 to be used. Some compilers can emit this information while building,
594 and Ninja can use that to get its dependencies perfect.
596 Consider: if the file has never been compiled, it must be built anyway,
597 generating the header dependencies as a side effect. If any file is
598 later modified (even in a way that changes which headers it depends
599 on) the modification will cause a rebuild as well, keeping the
600 dependencies up to date.
602 When loading these special dependencies, Ninja implicitly adds extra
603 build edges such that it is not an error if the listed dependency is
604 missing. This allows you to delete a header file and rebuild without
605 the build aborting due to a missing input.
610 `gcc` (and other compilers like `clang`) support emitting dependency
611 information in the syntax of a Makefile. (Any command that can write
612 dependencies in this form can be used, not just `gcc`.)
614 To bring this information into Ninja requires cooperation. On the
615 Ninja side, the `depfile` attribute on the `build` must point to a
616 path where this data is written. (Ninja only supports the limited
617 subset of the Makefile syntax emitted by compilers.) Then the command
618 must know to write dependencies into the `depfile` path.
619 Use it like in the following example:
624 command = gcc -MMD -MF $out.d [other gcc flags here]
627 The `-MMD` flag to `gcc` tells it to output header dependencies, and
628 the `-MF` flag tells it where to write them.
633 _(Available since Ninja 1.3.)_
635 It turns out that for large projects (and particularly on Windows,
636 where the file system is slow) loading these dependency files on
639 Ninja 1.3 can instead process dependencies just after they're generated
640 and save a compacted form of the same information in a Ninja-internal
643 Ninja supports this processing in two forms.
645 1. `deps = gcc` specifies that the tool outputs `gcc`-style dependencies
646 in the form of Makefiles. Adding this to the above example will
647 cause Ninja to process the `depfile` immediately after the
648 compilation finishes, then delete the `.d` file (which is only used
651 2. `deps = msvc` specifies that the tool outputs header dependencies
652 in the form produced by Visual Studio's compiler's
653 http://msdn.microsoft.com/en-us/library/hdkef6tk(v=vs.90).aspx[`/showIncludes`
654 flag]. Briefly, this means the tool outputs specially-formatted lines
655 to its stdout. No `depfile` attribute is necessary.
661 A file is a series of declarations. A declaration can be one of:
663 1. A rule declaration, which begins with +rule _rulename_+, and
664 then has a series of indented lines defining variables.
666 2. A build edge, which looks like +build _output1_ _output2_:
667 _rulename_ _input1_ _input2_+. +
668 Implicit dependencies may be tacked on the end with +|
669 _dependency1_ _dependency2_+. +
670 Order-only dependencies may be tacked on the end with +||
671 _dependency1_ _dependency2_+. (See <<ref_dependencies,the reference on
674 3. Variable declarations, which look like +_variable_ = _value_+.
676 4. Default target statements, which look like +default _target1_ _target2_+.
678 5. References to more files, which look like +subninja _path_+ or
679 +include _path_+. The difference between these is explained below
680 <<ref_scope,in the discussion about scoping>>.
685 Ninja is mostly encoding agnostic, as long as the bytes Ninja cares
686 about (like slashes in paths) are ASCII. This means e.g. UTF-8 or
687 ISO-8859-1 input files ought to work. (To simplify some code, tabs
688 and carriage returns are currently disallowed; this could be fixed if
689 it really mattered to you.)
691 Comments begin with `#` and extend to the end of the line.
693 Newlines are significant. Statements like `build foo bar` are a set
694 of space-separated tokens that end at the newline. Newlines and
695 spaces within a token must be escaped.
697 There is only one escape character, `$`, and it has the following
701 `$` followed by a newline:: escape the newline (continue the current line
702 across a line break).
704 `$` followed by text:: a variable reference.
706 `${varname}`:: alternate syntax for `$varname`.
708 `$` followed by space:: a space. (This is only necessary in lists of
709 paths, where a space would otherwise separate filenames. See below.)
711 `$:` :: a colon. (This is only necessary in `build` lines, where a colon
712 would otherwise terminate the list of outputs.)
714 `$$`:: a literal `$`.
716 A `build` or `default` statement is first parsed as a space-separated
717 list of filenames and then each name is expanded. This means that
718 spaces within a variable will result in spaces in the expanded
723 build $spaced/baz other$ file: ...
724 # The above build line has two outputs: "foo bar/baz" and "other file".
727 In a `name = value` statement, whitespace at the beginning of a value
728 is always stripped. Whitespace at the beginning of a line after a
729 line continuation is also stripped.
732 two_words_with_one_space = foo $
734 one_word_with_no_space = foo$
738 Other whitespace is only significant if it's at the beginning of a
739 line. If a line is indented more than the previous one, it's
740 considered part of its parent's scope; if it is indented less than the
741 previous one, it closes the previous scope.
747 Two variables are significant when declared in the outermost file scope.
749 `builddir`:: a directory for some Ninja output files. See <<ref_log,the
750 discussion of the build log>>. (You can also store other build output
753 `ninja_required_version`:: the minimum version of Ninja required to process
754 the build correctly. See <<ref_versioning,the discussion of versioning>>.
761 A `rule` block contains a list of `key = value` declarations that
762 affect the processing of the rule. Here is a full list of special
765 `command` (_required_):: the command line to run. This string (after
766 $variables are expanded) is passed directly to `sh -c` without
767 interpretation by Ninja. Each `rule` may have only one `command`
768 declaration. To specify multiple commands use `&&` (or similar) to
769 concatenate operations.
771 `depfile`:: path to an optional `Makefile` that contains extra
772 _implicit dependencies_ (see <<ref_dependencies,the reference on
773 dependency types>>). This is explicitly to support C/C++ header
774 dependencies; see <<ref_headers,the full discussion>>.
776 `deps`:: _(Available since Ninja 1.3.)_ if present, must be one of
777 `gcc` or `msvc` to specify special dependency processing. See
778 <<ref_headers,the full discussion>>. The generated database is
779 stored as `.ninja_deps` in the `builddir`, see <<ref_toplevel,the
780 discussion of `builddir`>>.
782 `description`:: a short description of the command, used to pretty-print
783 the command as it's running. The `-v` flag controls whether to print
784 the full command or its description; if a command fails, the full command
785 line will always be printed before the command's output.
787 `generator`:: if present, specifies that this rule is used to
788 re-invoke the generator program. Files built using `generator`
789 rules are treated specially in two ways: firstly, they will not be
790 rebuilt if the command line changes; and secondly, they are not
793 `in`:: the shell-quoted space-separated list of files provided as
794 inputs to the build line referencing this `rule`. (`$in` is provided
795 solely for convenience; if you need some subset or variant of this
796 list of files, just construct a new variable with that list and use
799 `in_newline`:: the same as `$in` except that multiple inputs are
800 separated by newlines rather than spaces. (For use with
801 `$rspfile_content`; this works around a bug in the MSVC linker where
802 it uses a fixed-size buffer for processing input.)
804 `out`:: the shell-quoted space-separated list of files provided as
805 outputs to the build line referencing this `rule`.
807 `restat`:: if present, causes Ninja to re-stat the command's outputs
808 after execution of the command. Each output whose modification time
809 the command did not change will be treated as though it had never
810 needed to be built. This may cause the output's reverse
811 dependencies to be removed from the list of pending build actions.
813 `rspfile`, `rspfile_content`:: if present (both), Ninja will use a
814 response file for the given command, i.e. write the selected string
815 (`rspfile_content`) to the given file (`rspfile`) before calling the
816 command and delete the file after successful execution of the
819 This is particularly useful on Windows OS, where the maximal length of
820 a command line is limited and response files must be used instead.
822 Use it like in the following example:
826 command = link.exe /OUT$out [usual link flags here] @$out.rsp
828 rspfile_content = $in
830 build myapp.exe: link a.obj b.obj [possibly many other .obj files]
837 There are three types of build dependencies which are subtly different.
839 1. _Explicit dependencies_, as listed in a build line. These are
840 available as the `$in` variable in the rule. Changes in these files
841 cause the output to be rebuilt; if these file are missing and
842 Ninja doesn't know how to build them, the build is aborted.
844 This is the standard form of dependency to be used for e.g. the
845 source file of a compile command.
847 2. _Implicit dependencies_, either as picked up from
848 a `depfile` attribute on a rule or from the syntax +| _dep1_
849 _dep2_+ on the end of a build line. The semantics are identical to
850 explicit dependencies, the only difference is that implicit dependencies
851 don't show up in the `$in` variable.
853 This is for expressing dependencies that don't show up on the
854 command line of the command; for example, for a rule that runs a
855 script, the script itself should be an implicit dependency, as
856 changes to the script should cause the output to rebuild.
858 Note that dependencies as loaded through depfiles have slightly different
859 semantics, as described in the <<ref_rule,rule reference>>.
861 3. _Order-only dependencies_, expressed with the syntax +|| _dep1_
862 _dep2_+ on the end of a build line. When these are out of date, the
863 output is not rebuilt until they are built, but changes in order-only
864 dependencies alone do not cause the output to be rebuilt.
866 Order-only dependencies can be useful for bootstrapping dependencies
867 that are only discovered during build time: for example, to generate a
868 header file before starting a subsequent compilation step. (Once the
869 header is used in compilation, a generated dependency file will then
870 express the implicit dependency.)
875 Variables are expanded in paths (in a `build` or `default` statement)
876 and on the right side of a `name = value` statement.
878 When a `name = value` statement is evaluated, its right-hand side is
879 expanded immediately (according to the below scoping rules), and
880 from then on `$name` expands to the static string as the result of the
881 expansion. It is never the case that you'll need to "double-escape" a
882 value to prevent it from getting expanded twice.
884 All variables are expanded immediately as they're encountered in parsing,
885 with one important exception: variables in `rule` blocks are expanded
886 when the rule is _used_, not when it is declared. In the following
887 example, the `demo` rule prints "this is a demo of bar".
891 command = echo "this is a demo of $foo'
898 Evaluation and scoping
899 ~~~~~~~~~~~~~~~~~~~~~~
901 Top-level variable declarations are scoped to the file they occur in.
903 The `subninja` keyword, used to include another `.ninja` file,
904 introduces a new scope. The included `subninja` file may use the
905 variables from the parent file, and shadow their values for the file's
906 scope, but it won't affect values of the variables in the parent.
908 To include another `.ninja` file in the current scope, much like a C
909 `#include` statement, use `include` instead of `subninja`.
911 Variable declarations indented in a `build` block are scoped to the
912 `build` block. The full lookup order for a variable expanded in a
913 `build` block (or the `rule` is uses) is:
915 1. Special built-in variables (`$in`, `$out`).
917 2. Build-level variables from the `build` block.
919 3. Rule-level variables from the `rule` block (i.e. `$command`).
920 (Note from the above discussion on expansion that these are
921 expanded "late", and may make use of in-scope bindings like `$in`.)
923 4. File-level variables from the file that the `build` line was in.
925 5. Variables from the file that included that file using the