How Ninja works

This page provides an overview of how Ninja works in Fuchsia.

General overview

The Fuchsia build system uses Ninja to launch build commands in parallel. The following steps describe Ninja's behavior:

  1. Load the Ninja build plan, from a top-level file which can itself include several other .ninja files.

    In the Fuchsia build, these are created by the GN build tool. This operation builds a dependency graph in memory.

  2. Load the Ninja build log and deps log if they exist.

    This operation adds dependency edges that were discovered during the previous successful Ninja build invocation. This allows fast incremental builds, but at some cost to correctness.

  3. Determine which build outputs (also known as "targets") need to be generated.

    Starting from the targets named on the command-line, recursively walk their dependencies to determine which final and intermediates outputs are stale relative to their inputs, and thus need to be rebuilt. Commands that need to be re-run are correctly ordered in a directed acyclic graph.

  4. Launch required build commands in parallel based on the number of CPUs on the host system (or an explicit -j<count> parameter).

    Another way to control parallelism is to use -l<max_load> to limit the max load value on the system. A command is ready to run when the inputs that ninja knows about have been updated.

Status display

During a build, Ninja launches multiple commands in parallel, and by default, it will buffer their output (stdout and stderr) until their completion.

Ninja also prints a status line (for instance, when fx build is used) that describes the following:

  • The number of commands that have completed.
  • The total number of commands that must be run to complete the build2
  • The number of currently running commands.
  • A description of the last-completed command. Which typically includes a small mnemonic (for example, ACTION or CXX) followed by a list of output targets.
[102/345](36) ACTION path/to/some/build/artifact

The example above means that 102 commands have been completed so far, out of 345, and that there are currently 36 parallel commands launched by Ninja, while path/to/some/build/artifact is the latest build artifact to be generated.

It is possible to customize the content of the status line by setting the NINJA_STATUS environment variable.

If any command generates some output, or if it fails, Ninja will update the status line with that command's description, then print its output or an error message. It will then resume printing the status line after it, for example:

[102/345](24) ACTION path/to/some/build/artifact
<output of the command which generated 'path/to/some/build/artifact'>
[101/345](23) ACTION path/to/another/build/artifact

In practice, this is how most compiler warnings are printed.

As a special exception, if a command is in the special console pool it will be able to print directly to the terminal. This is useful for long-running commands that need to print their own status updates.

Ninja ensures that only one console command can be launched at the same time, and also suspends its own status line updates until its completion. However, note that other non-console commands are still run in parallel in the background, and their outputs buffered. The Fuchsia build uses this feature for all commands that invoke Bazel, since these tend to be long, and Bazel provides its own status updates to the terminal.

Fuchsia-specific status display

As a special Fuchsia-specific improvement, Ninja will display a table of the oldest long-running commands, with their run time, to better understand what's going on during a build. This is only enabled in smart terminals.

Set NINJA_STATUS_MAX_COMMANDS=<count> in your environment to change the number of commands being displayed. fx buildsets its default to 4, which looks like;

[0/28477](260) STAMP host_x64/obj/tools/configc/configc_sdk_meta_generated_file.stamp
  0.4s | STAMP obj/sdk/zircon_sysroot_meta_verify.stamp
  0.4s | CXX obj/BUILD_DIR/fidling/gen/sdk/fidl/
  0.4s | CXX obj/BUILD_DIR/fidling/gen/sdk/fidl/
  0.4s | CXX obj/BUILD_DIR/fidling/gen/sdk/fidl/

For more details, see Fuchsia feature: status of pending commands.

Ninja build dependency graph

The graph that Ninja constructs from the build plan contains only two types of nodes3:

  • Target nodes: A Target node simply corresponds to a file path known to Ninja. That path is always relative to the build directory.

  • Action nodes: An Action node models a single command to run to generate output files, from a given set of input files.

Note the following information:

  • A Target node that is not the output of any Action node is called a source file.

  • A Target node that is not the input of any Action node must be the output of a given Action node, and is called a final output.

  • A Target node that is both the output of an Action as well as the input of another one is called an intermediate target, or intermediate output.

  • Each Action can point to zero or more input Target nodes in the graph.

  • Each Action can have one or more output Target nodes in the graph. An action cannot have zero outputs, otherwise Ninja wouldn't know when to run its command.

  • Action nodes have no names, so there is no way to reference them directly when invoking Ninja. Only file paths, that is, targets.

Ninja build plan

The Ninja build plan is defined by a file at the top of the build directory, which can include a other *.ninja file with include or subninja statements. The following is a summary of its most important features (for full details, see the Ninja manual).

Inside .ninja files, Action nodes are defined through a build statement:

build <outputs>: <rule_name> <inputs>

<outputs> is a list of output paths, <inputs> is a list of input paths, and <rule_name> is the name of a Ninja rule, which acts as a recipe to craft the final command to run. Rules are defined by a special rule statement:

rule <rule_name>
   command = <command expression>

<command expression> can contain the special $in and $out keywords that will be expanded into the list of inputs and outputs of the corresponding build rule.

rule copy_file
  command = cp -f $in $out

build output.txt: copy_file input.txt

The example above is a trivial build plan that tells Ninja that to build output.txt, the command cp -f input.txt output.txt must be run.

Implicit outputs

It is possible for a command to have additional outputs that must not appear in the $out expansion. These can be separated from explicit outputs with the | separator.

rule copy_file
  command = cp -f $in $out && touch $out.stamp

build output.txt | output.txt.stamp: copy_file input.txt

The example above tells Ninja that the command to build output.txt will copy input.txt into it and create an output.txt.stamp file too.

Implicit inputs

Similarly, it is possible to tell Ninja that some inputs should not be expanded from the $in expression, by using the | on the right side of the build statement.

rule cxx_compile
  command = c++ -c $in -o $out

build foo.o: cxx_compile | foo.h

The example above tells Ninja that compiling will use foo.h as an input, even though this file does not appear in the compiler command explicitly.

Order-only inputs

It is possible to tell Ninja that some file paths are runtime dependencies of some outputs, and thus should be built "with" them. This uses the || separator on the right side of the build statement, and must always appear after any potential | separator, if there is one.

rule cxx_binary
  command = c++ -o $out $in -ldl

rule cxx_shared_library
  command = c++ -shared -o $out $in

build cxx_shared_library:

build program: cxx_binary ||

The example above tells Ninja that whenever program needs to be built, then will need to be built as well, but that order is not important. In other words, it is ok to run the command that generates program before the one that generates In this example, this works if the binary only loads the library through dlopen() at runtime.

Reducing rebuilds with the restat optimization

Some commands may not change their output file's timestamp if their content did not change. Ninja can use this to reduce the total number of commands to run during a build invocation.

To support this, a rule definition must set the special restat variable to a non-empty value. This causes Ninja to re-stat the command's outputs after executing it. Each output whose modification time didn't change will be treated as though it had never needed to be built, and Ninja will remove any commands that use it as input from the pending commands list.

# A rule to invoke the script that processes some input
# and generates a manifest as output. `restat` is set to indicate that the
# script will not update $out's timestamp if the file exists and its content
# is already correct.

rule create_manifest
  command = ../../ --input $in --output $out
  restat = 1

build package_manifest.json: create_manifest package_list.txt

build create_archive package_manifest.json

In the example above, if the developer changes package_list.txt in a way that does not change the package_manifest.json output file, then the final will not need to be re-generated. To support this, every time Ninja runs a command, it records for each output file a summary that includes a hash of the command and the timestamp of the most recent input, into a special file at $BUILD_DIR/.ninja_build, called the Ninja build log.

On the next Ninja invocation, the build log timestamp is used instead of the filesystem one (if it is newer), to determine whether the file needs to be regenerated. Hence, in the example above, the newer timestamp for package_list.txt would be associated with package_manifest.json even though its filesystem timestamp would be older. Without this feature, Ninja would try to rebuild the manifest file on every build invocation.

Discovering implicit inputs at build time with depfiles

A command launched by Ninja can generate a special dependency file (abbreviated as depfile) that lists extra implicit inputs, that is, inputs to the command that do not appear in the build plan. This information is read by Ninja, which records it in the binary file named $BUILD_DIR/.ninja_deps, as known as the "Ninja deps log". On the next Ninja invocation, the deps log is loaded automatically and all recorded implicit inputs are added to the dependency graph.

For example, this is useful for C++ compilation commands to list all included headers, even those that are not listed explicitly in the corresponding .ninja file. If such a header is modified by the developer, the next Ninja invocation will see the change and cause the corresponding C++ sources, and any of its dependencies, to be recompiled.

This works by adding a depfile variable declaration to a rule definition as in:

rule cc
    depfile = $out.d
    command = gcc -MD -MF $out.d [other gcc flags here]

Note that by default, depfile are removed by Ninja after they have been ingested into the binary deps log. To inspect which depfile dependencies were recorded, take one of the following actions:

  • Run ninja -C <build_dir> -t deps <target>, where <build_dir> is the build directory, and <target> is the path to an output file, relative to <build_dir>. The -t deps option invokes a Ninja tool that prints the content of the deps log for this output file.

    Note however that the deps log is a binary append-only file, so will accumulate depfile dependencies over several Ninja build invocations, so this may list more implicit dependencies than those generated by the last command.

  • Remove the build artifact, then invoke ninja with the -d keepdepfile option, which forces Ninja to leave all dependency files in the build directory (after copying their content to the binary deps log). This allows manual inspection of its content, for example:

    $ rm $BUILD_DIR/foo.o
    $ ninja -C $BUILD_DIR -d keepdepfile foo.o
    $ cat $BUILD_DIR/foo.o.d

    Note that the exact depfile path depends on the rule definition. As a convention most command just append a .d suffix to the first output path, but this is not enforced by Ninja.

Correctness issues with depfiles

The deps log works extremely well when there are no changes to the build plan, because Ninja will detect on the next incremental build which depfile-listed implicit inputs where changed, and rebuild anything that depends on them.

However, when the build plan changed, entries in the Ninja deps log can become stale, and will add incorrect edges in the dependency graph of the next Ninja invocation. And sometimes, these will break the next build invocation. This happens in particular when dependencies are removed from the build plan, but still recorded in the deps log.

In practice, this results in random incremental build failures that can happen locally, or on our infra builders (both in CQ or CI). And there is unfortunately now way to solve the problem at the moment, as the deps log is part of Ninja's design, and it is impossible to detect "stale" entries (since the exact details of the old build plans are long gone when they are used).

The usual workaround is to perform a clean build. Fuchsia even implements "clean build fences" to work around the most problematic cases.

  1. More specifically, the Ninja dependency graph is very similar to the Bazel action graph, and a Ninja target corresponds to a Bazel File object. 

  2. This number can decrease during the build if Ninja determines that the outputs of some commands are considered up-to-date

  3. For historical reasons, the Ninja source code uses a C++ class named Node to model targets, and a C++ class named Edge to model actions. However since this is frequently a source of great confusion when reading the code, this document will not follow this misleading convention.