# GN Reference

*This page is automatically generated from* `gn help --markdown all`.

## **--args**: Specifies build arguments overrides.

```
  See "gn help buildargs" for an overview of how build arguments work.

  Most operations take a build directory. The build arguments are taken
  from the previous build done in that directory. If a command specifies
  --args, it will override the previous arguments stored in the build
  directory, and use the specified ones.

  The args specified will be saved to the build directory for subsequent
  commands. Specifying --args="" will clear all build arguments.

```

### **Formatting**

```
  The value of the switch is interpreted in GN syntax. For typical usage
  of string arguments, you will need to be careful about escaping of
  quotes.

```

### **Examples**

```
  gn gen out/Default --args="foo=\"bar\""

  gn gen out/Default --args='foo="bar" enable=true blah=7'

  gn check out/Default --args=""
    Clears existing build args from the directory.

  gn desc out/Default --args="some_list=[1, false, \"foo\"]"


```
## **--[no]color**: Forces colored output on or off.

```
  Normally GN will try to detect whether it is outputting to a terminal
  and will enable or disable color accordingly. Use of these switches
  will override the default.

```

### **Examples**

```
  gn gen out/Default --color

  gn gen out/Default --nocolor


```
## **--dotfile**: override the name of the ".gn" file.

```
  Normally GN loads the ".gn"file  from the source root for some basic
  configuration (see "gn help dotfile"). This flag allows you to
  use a different file.

  Note that this interacts with "--root" in a possibly incorrect way.
  It would be nice to test the edge cases and document or fix.


```
## **--markdown**: write the output in the Markdown format.

## **--[no]color**: Forces colored output on or off.

```
  Normally GN will try to detect whether it is outputting to a terminal
  and will enable or disable color accordingly. Use of these switches
  will override the default.

```

### **Examples**

```
  gn gen out/Default --color

  gn gen out/Default --nocolor


```
## **-q**: Quiet mode. Don't print output on success.

```
  This is useful when running as a part of another script.


```
## **--root**: Explicitly specify source root.

```
  Normally GN will look up in the directory tree from the current
  directory to find a ".gn" file. The source root directory specifies
  the meaning of "//" beginning with paths, and the BUILD.gn file
  in that directory will be the first thing loaded.

  Specifying --root allows GN to do builds in a specific directory
  regardless of the current directory.

```

### **Examples**

```
  gn gen //out/Default --root=/home/baracko/src

  gn desc //out/Default --root="C:\Users\BObama\My Documents\foo"


```
## **--time**: Outputs a summary of how long everything took.

```
  Hopefully self-explanatory.

```

### **Examples**

```
  gn gen out/Default --time


```
## **--tracelog**: Writes a Chrome-compatible trace log to the given file.

```
  The trace log will show file loads, executions, scripts, and writes.
  This allows performance analysis of the generation step.

  To view the trace, open Chrome and navigate to "chrome://tracing/",
  then press "Load" and specify the file you passed to this parameter.

```

### **Examples**

```
  gn gen out/Default --tracelog=mytrace.trace


```
## **-v**: Verbose logging.

```
  This will spew logging events to the console for debugging issues.
  Good luck!


```
## **gn args <out_dir> [--list] [--short] [--args]**

```
  See also "gn help buildargs" for a more high-level overview of how
  build arguments work.

```

### **Usage**
```
  gn args <out_dir>
      Open the arguments for the given build directory in an editor
      (as specified by the EDITOR environment variable). If the given
      build directory doesn't exist, it will be created and an empty
      args file will be opened in the editor. You would type something
      like this into that file:
          enable_doom_melon=false
          os="android"

      Note: you can edit the build args manually by editing the file
      "args.gn" in the build directory and then running
      "gn gen <out_dir>".

  gn args <out_dir> --list[=<exact_arg>] [--short]
      Lists all build arguments available in the current configuration,
      or, if an exact_arg is specified for the list flag, just that one
      build argument.

      The output will list the declaration location, default value, and
      comment preceeding the declaration. If --short is specified,
      only the names and values will be printed.

      If the out_dir is specified, the build configuration will be
      taken from that build directory. The reason this is needed is that
      the definition of some arguments is dependent on the build
      configuration, so setting some values might add, remove, or change
      the default values for other arguments. Specifying your exact
      configuration allows the proper arguments to be displayed.

      Instead of specifying the out_dir, you can also use the
      command-line flag to specify the build configuration:
        --args=<exact list of args to use>

```

### **Examples**
```
  gn args out/Debug
    Opens an editor with the args for out/Debug.

  gn args out/Debug --list --short
    Prints all arguments with their default values for the out/Debug
    build.

  gn args out/Debug --list=target_cpu
    Prints information about the "target_cpu" argument for the out/Debug
    build.

  gn args --list --args="os=\"android\" enable_doom_melon=true"
    Prints all arguments with the default values for a build with the
    given arguments set (which may affect the values of other
    arguments).


```
## **gn check <out_dir> [<label_pattern>] [--force]**

```
  "gn check" is the same thing as "gn gen" with the "--check" flag
  except that this command does not write out any build files. It's
  intended to be an easy way to manually trigger include file checking.

  The <label_pattern> can take exact labels or patterns that match more
  than one (although not general regular expressions). If specified,
  only those matching targets will be checked. See
  "gn help label_pattern" for details.

  The .gn file may specify a list of targets to be checked. Only these
  targets will be checked if no label_pattern is specified on the
  command line. Otherwise, the command-line list is used instead. See
  "gn help dotfile".

```

### **Command-specific switches**

```
  --force
      Ignores specifications of "check_includes = false" and checks
      all target's files that match the target label.

```

### **Examples**

```
  gn check out/Debug
      Check everything.

  gn check out/Default //foo:bar
      Check only the files in the //foo:bar target.

  gn check out/Default "//foo/*
      Check only the files in targets in the //foo directory tree.


```
## **gn clean <out_dir>**

```
  Deletes the contents of the output directory except for args.gn and
  creates a Ninja build environment sufficient to regenerate the build.


```
## **gn desc <out_dir> <target label> [<what to show>] [--blame]**

```
  Displays information about a given labeled target for the given build.
  The build parameters will be taken for the build in the given
  <out_dir>.

```

### **Possibilities for <what to show>**
```
  (If unspecified an overall summary will be displayed.)

  sources
      Source files.

  inputs
      Additional input dependencies.

  public
      Public header files.

  check_includes
      Whether "gn check" checks this target for include usage.

  allow_circular_includes_from
      Permit includes from these targets.

  visibility
      Prints which targets can depend on this one.

  testonly
      Whether this target may only be used in tests.

  configs
      Shows configs applied to the given target, sorted in the order
      they're specified. This includes both configs specified in the
      "configs" variable, as well as configs pushed onto this target
      via dependencies specifying "all" or "direct" dependent
      configs.

  deps
      Show immediate or recursive dependencies. See below for flags that
      control deps printing.

  public_configs
  all_dependent_configs
      Shows the labels of configs applied to targets that depend on this
      one (either directly or all of them).

  forward_dependent_configs_from
      Shows the labels of dependencies for which dependent configs will
      be pushed to targets depending on the current one.

  script
  args
  depfile
      Actions only. The script and related values.

  outputs
      Outputs for script and copy target types.

  defines       [--blame]
  include_dirs  [--blame]
  cflags        [--blame]
  cflags_cc     [--blame]
  cflags_cxx    [--blame]
  ldflags       [--blame]
  lib_dirs
  libs
      Shows the given values taken from the target and all configs
      applying. See "--blame" below.

  --blame
      Used with any value specified by a config, this will name
      the config that specified the value. This doesn't currently work
      for libs and lib_dirs because those are inherited and are more
      complicated to figure out the blame (patches welcome).

```

### **Flags that control how deps are printed**

```
  --all
      Collects all recursive dependencies and prints a sorted flat list.
      Also usable with --tree (see below).

  --as=(buildfile|label|output)
      How to print targets.

      buildfile
          Prints the build files where the given target was declared as
          file names.
      label  (default)
          Prints the label of the target.
      output
          Prints the first output file for the target relative to the
          current directory.

  --testonly=(true|false)
      Restrict outputs to targets with the testonly flag set
      accordingly. When unspecified, the target's testonly flags are
      ignored.

  --tree
      Print a dependency tree. By default, duplicates will be elided
      with "..." but when --all and -tree are used together, no
      eliding will be performed.

      The "deps", "public_deps", and "data_deps" will all be
      included in the tree.

      Tree output can not be used with the filtering or output flags:
      --as, --type, --testonly.

  --type=(action|copy|executable|group|shared_library|source_set|
          static_library)
      Restrict outputs to targets matching the given type. If
      unspecified, no filtering will be performed.

```

### **Note**

```
  This command will show the full name of directories and source files,
  but when directories and source paths are written to the build file,
  they will be adjusted to be relative to the build directory. So the
  values for paths displayed by this command won't match (but should
  mean the same thing).

```

### **Examples**

```
  gn desc out/Debug //base:base
      Summarizes the given target.

  gn desc out/Foo :base_unittests deps --tree
      Shows a dependency tree of the "base_unittests" project in
      the current directory.

  gn desc out/Debug //base defines --blame
      Shows defines set for the //base:base target, annotated by where
      each one was set from.


```
## **gn format [--dump-tree] [--in-place] [--stdin] BUILD.gn**

```
  Formats .gn file to a standard format.

```

### **Arguments**
```
  --dry-run
      Does not change or output anything, but sets the process exit code
      based on whether output would be different than what's on disk.
      This is useful for presubmit/lint-type checks.
      - Exit code 0: successful format, matches on disk.
      - Exit code 1: general failure (parse error, etc.)
      - Exit code 2: successful format, but differs from on disk.

  --dump-tree
      For debugging only, dumps the parse tree.

  --in-place
      Instead of writing the formatted file to stdout, replace the input
      file with the formatted output. If no reformatting is required,
      the input file will not be touched, and nothing printed.

  --stdin
      Read input from stdin (and write to stdout). Not compatible with
      --in-place of course.

```

### **Examples**
```
  gn format //some/BUILD.gn
  gn format some\BUILD.gn
  gn format /abspath/some/BUILD.gn
  gn format --stdin


```
## **gn gen**: Generate ninja files.

```
  gn gen <out_dir>

  Generates ninja files from the current tree and puts them in the given
  output directory.

  The output directory can be a source-repo-absolute path name such as:
      //out/foo
  Or it can be a directory relative to the current directory such as:
      out/foo

  See "gn help" for the common command-line switches.


```
## **gn help <anything>**
```
  Yo dawg, I heard you like help on your help so I put help on the help
  in the help.


```
## **gn ls <out_dir> [<label_pattern>] [--all-toolchains] [--as=...]**
```
      [--type=...] [--testonly=...]

  Lists all targets matching the given pattern for the given build
  directory. By default, only targets in the default toolchain will
  be matched unless a toolchain is explicitly supplied.

  If the label pattern is unspecified, list all targets. The label
  pattern is not a general regular expression (see
  "gn help label_pattern"). If you need more complex expressions,
  pipe the result through grep.

```

### **Options**

```
  --as=(buildfile|label|output)
      How to print targets.

      buildfile
          Prints the build files where the given target was declared as
          file names.
      label  (default)
          Prints the label of the target.
      output
          Prints the first output file for the target relative to the
          current directory.

  --all-toolchains
      Matches all toolchains. When set, if the label pattern does not
      specify an explicit toolchain, labels from all toolchains will be
      matched. When unset, only targets in the default toolchain will
      be matched unless an explicit toolchain in the label is set.

  --testonly=(true|false)
      Restrict outputs to targets with the testonly flag set
      accordingly. When unspecified, the target's testonly flags are
      ignored.

  --type=(action|copy|executable|group|shared_library|source_set|
          static_library)
      Restrict outputs to targets matching the given type. If
      unspecified, no filtering will be performed.

```

### **Examples**

```
  gn ls out/Debug
      Lists all targets in the default toolchain.

  gn ls out/Debug "//base/*"
      Lists all targets in the directory base and all subdirectories.

  gn ls out/Debug "//base:*"
      Lists all targets defined in //base/BUILD.gn.

  gn ls out/Debug //base --as=output
      Lists the build output file for //base:base

  gn ls out/Debug --type=executable
      Lists all executables produced by the build.

  gn ls out/Debug "//base/*" --as=output | xargs ninja -C out/Debug
      Builds all targets in //base and all subdirectories.

  gn ls out/Debug //base --all-toolchains
      Lists all variants of the target //base:base (it may be referenced
      in multiple toolchains).


```
## **gn refs <out_dir> (<label_pattern>|<label>|<file>|@<response_file>)* [--all]**
```
        [--all-toolchains] [--as=...] [--testonly=...] [--type=...]

  Finds reverse dependencies (which targets reference something). The
  input is a list containing:

   - Target label: The result will be which targets depend on it.

   - Config label: The result will be which targets list the given
     config in its "configs" or "public_configs" list.

   - Label pattern: The result will be which targets depend on any
     target matching the given pattern. Patterns will not match
     configs. These are not general regular expressions, see
     "gn help label_pattern" for details.

   - File name: The result will be which targets list the given file in
     its "inputs", "sources", "public", or "data". Any input
     that does not contain wildcards and does not match a target or a
     config will be treated as a file.

   - Response file: If the input starts with an "@", it will be
     interpreted as a path to a file containing a list of labels or
     file names, one per line. This allows us to handle long lists
     of inputs without worrying about command line limits.

```

### **Options**

```
  --all
      When used without --tree, will recurse and display all unique
      dependencies of the given targets. For example, if the input is
      a target, this will output all targets that depend directly or
      indirectly on the input. If the input is a file, this will output
      all targets that depend directly or indirectly on that file.

      When used with --tree, turns off eliding to show a complete tree.

  --all-toolchains
      Normally only inputs in the default toolchain will be included.
      This switch will turn on matching all toolchains.

      For example, a file is in a target might be compiled twice:
      once in the default toolchain and once in a secondary one. Without
      this flag, only the default toolchain one will be matched and
      printed (potentially with its recursive dependencies, depending on
      the other options). With this flag, both will be printed
      (potentially with both of their recursive dependencies).

  --as=(buildfile|label|output)
      How to print targets.

      buildfile
          Prints the build files where the given target was declared as
          file names.
      label  (default)
          Prints the label of the target.
      output
          Prints the first output file for the target relative to the
          current directory.

  -q
     Quiet. If nothing matches, don't print any output. Without this
     option, if there are no matches there will be an informational
     message printed which might interfere with scripts processing the
     output.

  --testonly=(true|false)
      Restrict outputs to targets with the testonly flag set
      accordingly. When unspecified, the target's testonly flags are
      ignored.

  --tree
      Outputs a reverse dependency tree from the given target.
      Duplicates will be elided. Combine with --all to see a full
      dependency tree.

      Tree output can not be used with the filtering or output flags:
      --as, --type, --testonly.

  --type=(action|copy|executable|group|shared_library|source_set|
          static_library)
      Restrict outputs to targets matching the given type. If
      unspecified, no filtering will be performed.

```

### **Examples (target input)**

```
  gn refs out/Debug //tools/gn:gn
      Find all targets depending on the given exact target name.

  gn refs out/Debug //base:i18n --as=buildfiles | xargs gvim
      Edit all .gn files containing references to //base:i18n

  gn refs out/Debug //base --all
      List all targets depending directly or indirectly on //base:base.

  gn refs out/Debug "//base/*"
      List all targets depending directly on any target in //base or
      its subdirectories.

  gn refs out/Debug "//base:*"
      List all targets depending directly on any target in
      //base/BUILD.gn.

  gn refs out/Debug //base --tree
      Print a reverse dependency tree of //base:base

```

### **Examples (file input)**

```
  gn refs out/Debug //base/macros.h
      Print target(s) listing //base/macros.h as a source.

  gn refs out/Debug //base/macros.h --tree
      Display a reverse dependency tree to get to the given file. This
      will show how dependencies will reference that file.

  gn refs out/Debug //base/macros.h //base/basictypes.h --all
      Display all unique targets with some dependency path to a target
      containing either of the given files as a source.

  gn refs out/Debug //base/macros.h --testonly=true --type=executable
          --all --as=output
      Display the executable file names of all test executables
      potentially affected by a change to the given file.


```
## **action**: Declare a target that runs a script a single time.

```
  This target type allows you to run a script a single time to produce
  or more output files. If you want to run a script once for each of a
  set of input files, see "gn help action_foreach".

```

### **Inputs**

```
  In an action the "sources" and "inputs" are treated the same:
  they're both input dependencies on script execution with no special
  handling. If you want to pass the sources to your script, you must do
  so explicitly by including them in the "args". Note also that this
  means there is no special handling of paths since GN doesn't know
  which of the args are paths and not. You will want to use
  rebase_path() to convert paths to be relative to the root_build_dir.

  You can dynamically write input dependencies (for incremental rebuilds
  if an input file changes) by writing a depfile when the script is run
  (see "gn help depfile"). This is more flexible than "inputs".

  It is recommended you put inputs to your script in the "sources"
  variable, and stuff like other Python files required to run your
  script in the "inputs" variable.

  The "deps" and "public_deps" for an action will always be
  completed before any part of the action is run so it can depend on
  the output of previous steps. The "data_deps" will be built if the
  action is built, but may not have completed before all steps of the
  action are started. This can give additional parallelism in the build
  for runtime-only dependencies.

```

### **Outputs**

```
  You should specify files created by your script by specifying them in
  the "outputs".

  The script will be executed with the given arguments with the current
  directory being that of the root build directory. If you pass files
  to your script, see "gn help rebase_path" for how to convert
  file names to be relative to the build directory (file names in the
  sources, outputs, and inputs will be all treated as relative to the
  current build file and converted as needed automatically).

```

### **File name handling**

```
  All output files must be inside the output directory of the build.
  You would generally use |$target_out_dir| or |$target_gen_dir| to
  reference the output or generated intermediate file directories,
  respectively.

```

### **Variables**

```
  args, data, data_deps, depfile, deps, outputs*, script*,
  inputs, sources
  * = required

```

### **Example**

```
  action("run_this_guy_once") {
    script = "doprocessing.py"
    sources = [ "my_configuration.txt" ]
    outputs = [ "$target_gen_dir/insightful_output.txt" ]

    # Our script imports this Python file so we want to rebuild if it
    # changes.
    inputs = [ "helper_library.py" ]

    # Note that we have to manually pass the sources to our script if
    # the script needs them as inputs.
    args = [ "--out", rebase_path(target_gen_dir, root_build_dir) ] +
           rebase_path(sources, root_build_dir)
  }


```
## **action_foreach**: Declare a target that runs a script over a set of files.

```
  This target type allows you to run a script once-per-file over a set
  of sources. If you want to run a script once that takes many files as
  input, see "gn help action".

```

### **Inputs**

```
  The script will be run once per file in the "sources" variable. The
  "outputs" variable should specify one or more files with a source
  expansion pattern in it (see "gn help source_expansion"). The output
  file(s) for each script invocation should be unique. Normally you
  use "{{source_name_part}}" in each output file.

  If your script takes additional data as input, such as a shared
  configuration file or a Python module it uses, those files should be
  listed in the "inputs" variable. These files are treated as
  dependencies of each script invocation.

  You can dynamically write input dependencies (for incremental rebuilds
  if an input file changes) by writing a depfile when the script is run
  (see "gn help depfile"). This is more flexible than "inputs".

  The "deps" and "public_deps" for an action will always be
  completed before any part of the action is run so it can depend on
  the output of previous steps. The "data_deps" will be built if the
  action is built, but may not have completed before all steps of the
  action are started. This can give additional parallelism in the build
  for runtime-only dependencies.

```

### **Outputs**

```
  The script will be executed with the given arguments with the current
  directory being that of the root build directory. If you pass files
  to your script, see "gn help rebase_path" for how to convert
  file names to be relative to the build directory (file names in the
  sources, outputs, and inputs will be all treated as relative to the
  current build file and converted as needed automatically).

```

### **File name handling**

```
  All output files must be inside the output directory of the build.
  You would generally use |$target_out_dir| or |$target_gen_dir| to
  reference the output or generated intermediate file directories,
  respectively.

```

### **Variables**

```
  args, data, data_deps, depfile, deps, outputs*, script*,
  inputs, sources*
  * = required

```

### **Example**

```
  # Runs the script over each IDL file. The IDL script will generate
  # both a .cc and a .h file for each input.
  action_foreach("my_idl") {
    script = "idl_processor.py"
    sources = [ "foo.idl", "bar.idl" ]

    # Our script reads this file each time, so we need to list is as a
    # dependency so we can rebuild if it changes.
    inputs = [ "my_configuration.txt" ]

    # Transformation from source file name to output file names.
    outputs = [ "$target_gen_dir/{{source_name_part}}.h",
                "$target_gen_dir/{{source_name_part}}.cc" ]

    # Note that since "args" is opaque to GN, if you specify paths
    # here, you will need to convert it to be relative to the build
    # directory using "rebase_path()".
    args = [
      "{{source}}",
      "-o",
      rebase_path(relative_target_gen_dir, root_build_dir) +
        "/{{source_name_part}}.h" ]
  }



```
## **assert**: Assert an expression is true at generation time.

```
  assert(<condition> [, <error string>])

  If the condition is false, the build will fail with an error. If the
  optional second argument is provided, that string will be printed
  with the error message.

```

### **Examples**:
```
  assert(is_win)
  assert(defined(sources), "Sources must be defined")


```
## **config**: Defines a configuration object.

```
  Configuration objects can be applied to targets and specify sets of
  compiler flags, includes, defines, etc. They provide a way to
  conveniently group sets of this configuration information.

  A config is referenced by its label just like a target.

  The values in a config are additive only. If you want to remove a flag
  you need to remove the corresponding config that sets it. The final
  set of flags, defines, etc. for a target is generated in this order:

   1. The values specified directly on the target (rather than using a
      config.
   2. The configs specified in the target's "configs" list, in order.
   3. Public_configs from a breadth-first traversal of the dependency
      tree in the order that the targets appear in "deps".
   4. All dependent configs from a breadth-first traversal of the
      dependency tree in the order that the targets appear in "deps".

```

### **Variables valid in a config definition**:
```
  Flags: cflags, cflags_c, cflags_cc, cflags_objc, cflags_objcc,
         defines, include_dirs, ldflags, lib_dirs, libs

```

### **Variables on a target used to apply configs**:
```
  all_dependent_configs, configs, public_configs,
  forward_dependent_configs_from

```

### **Example**:
```
  config("myconfig") {
    includes = [ "include/common" ]
    defines = [ "ENABLE_DOOM_MELON" ]
  }

  executable("mything") {
    configs = [ ":myconfig" ]
  }


```
## **copy**: Declare a target that copies files.

### **File name handling**

```
  All output files must be inside the output directory of the build.
  You would generally use |$target_out_dir| or |$target_gen_dir| to
  reference the output or generated intermediate file directories,
  respectively.

  Both "sources" and "outputs" must be specified. Sources can
  as many files as you want, but there can only be one item in the
  outputs list (plural is used for the name for consistency with
  other target types).

  If there is more than one source file, your output name should specify
  a mapping from each source files to output file names using source
  expansion (see "gn help source_expansion"). The placeholders will
  will look like "{{source_name_part}}", for example.

```

### **Examples**

```
  # Write a rule that copies a checked-in DLL to the output directory.
  copy("mydll") {
    sources = [ "mydll.dll" ]
    outputs = [ "$target_out_dir/mydll.dll" ]
  }

  # Write a rule to copy several files to the target generated files
  # directory.
  copy("myfiles") {
    sources = [ "data1.dat", "data2.dat", "data3.dat" ]

    # Use source expansion to generate output files with the
    # corresponding file names in the gen dir. This will just copy each
    # file.
    outputs = [ "$target_gen_dir/{{source_file_part}}" ]
  }


```
## **declare_args**: Declare build arguments.

```
  Introduces the given arguments into the current scope. If they are
  not specified on the command line or in a toolchain's arguments,
  the default values given in the declare_args block will be used.
  However, these defaults will not override command-line values.

  See also "gn help buildargs" for an overview.

```

### **Example**:
```
  declare_args() {
    enable_teleporter = true
    enable_doom_melon = false
  }

  If you want to override the (default disabled) Doom Melon:
    gn --args="enable_doom_melon=true enable_teleporter=false"
  This also sets the teleporter, but it's already defaulted to on so
  it will have no effect.


```
## **defined**: Returns whether an identifier is defined.

```
  Returns true if the given argument is defined. This is most useful in
  templates to assert that the caller set things up properly.

  You can pass an identifier:
    defined(foo)
  which will return true or false depending on whether foo is defined in
  the current scope.

  You can also check a named scope:
    defined(foo.bar)
  which will return true or false depending on whether bar is defined in
  the named scope foo. It will throw an error if foo is not defined or
  is not a scope.

```

### **Example**:

```
  template("mytemplate") {
    # To help users call this template properly...
    assert(defined(invoker.sources), "Sources must be defined")

    # If we want to accept an optional "values" argument, we don't
    # want to dereference something that may not be defined.
    if (defined(invoker.values)) {
      values = invoker.values
    } else {
      values = "some default value"
    }
  }


```
## **exec_script**: Synchronously run a script and return the output.

```
  exec_script(filename,
              arguments = [],
              input_conversion = "",
              file_dependencies = [])

  Runs the given script, returning the stdout of the script. The build
  generation will fail if the script does not exist or returns a nonzero
  exit code.

  The current directory when executing the script will be the root
  build directory. If you are passing file names, you will want to use
  the rebase_path() function to make file names relative to this
  path (see "gn help rebase_path").

```

### **Arguments**:

```
  filename:
      File name of python script to execute. Non-absolute names will
      be treated as relative to the current build file.

  arguments:
      A list of strings to be passed to the script as arguments.
      May be unspecified or the empty list which means no arguments.

  input_conversion:
      Controls how the file is read and parsed.
      See "gn help input_conversion".

      If unspecified, defaults to the empty string which causes the
      script result to be discarded. exec script will return None.

  dependencies:
      (Optional) A list of files that this script reads or otherwise
      depends on. These dependencies will be added to the build result
      such that if any of them change, the build will be regenerated and
      the script will be re-run.

      The script itself will be an implicit dependency so you do not
      need to list it.

```

### **Example**:

```
  all_lines = exec_script(
      "myscript.py", [some_input], "list lines",
      [ rebase_path("data_file.txt", root_build_dir) ])

  # This example just calls the script with no arguments and discards
  # the result.
  exec_script("//foo/bar/myscript.py")


```
## **executable**: Declare an executable target.

### **Variables**

```
  Flags: cflags, cflags_c, cflags_cc, cflags_objc, cflags_objcc,
         defines, include_dirs, ldflags, lib_dirs, libs
  Deps: data_deps, deps, forward_dependent_configs_from, public_deps
  Dependent configs: all_dependent_configs, public_configs
  General: check_includes, configs, data, inputs, output_name,
           output_extension, public, sources, testonly, visibility


```
## **foreach**: Iterate over a list.

```
  foreach(<loop_var>, <list>) {
    <loop contents>
  }

  Executes the loop contents block over each item in the list,
  assigning the loop_var to each item in sequence.

  The block does not introduce a new scope, so that variable assignments
  inside the loop will be visible once the loop terminates.

  The loop variable will temporarily shadow any existing variables with
  the same name for the duration of the loop. After the loop terminates
  the loop variable will no longer be in scope, and the previous value
  (if any) will be restored.

```

### **Example**

```
  mylist = [ "a", "b", "c" ]
  foreach(i, mylist) {
    print(i)
  }

  Prints:
  a
  b
  c


```
## **get_label_info**: Get an attribute from a target's label.

```
  get_label_info(target_label, what)

  Given the label of a target, returns some attribute of that target.
  The target need not have been previously defined in the same file,
  since none of the attributes depend on the actual target definition,
  only the label itself.

  See also "gn help get_target_outputs".

```

### **Possible values for the "what" parameter**

```
  "name"
      The short name of the target. This will match the value of the
      "target_name" variable inside that target's declaration. For the
      label "//foo/bar:baz" this will return "baz".

  "dir"
      The directory containing the target's definition, with no slash at
      the end. For the label "//foo/bar:baz" this will return
      "//foo/bar".

  "target_gen_dir"
      The generated file directory for the target. This will match the
      value of the "target_gen_dir" variable when inside that target's
      declaration.

  "root_gen_dir"
      The root of the generated file tree for the target. This will
      match the value of the "root_gen_dir" variable when inside that
      target's declaration.

  "target_out_dir
      The output directory for the target. This will match the
      value of the "target_out_dir" variable when inside that target's
      declaration.

  "root_out_dir"
      The root of the output file tree for the target. This will
      match the value of the "root_gen_dir" variable when inside that
      target's declaration.

  "label_no_toolchain"
      The fully qualified version of this label, not including the
      toolchain. For the input ":bar" it might return
      "//foo:bar".

  "label_with_toolchain"
      The fully qualified version of this label, including the
      toolchain. For the input ":bar" it might return
      "//foo:bar(//toolchain:x64)".

  "toolchain"
      The label of the toolchain. This will match the value of the
      "current_toolchain" variable when inside that target's
      declaration.

```

### **Examples**

```
  get_label_info(":foo", "name")
  # Returns string "foo".

  get_label_info("//foo/bar:baz", "gen_dir")
  # Returns string "//out/Debug/gen/foo/bar".


```
## **get_path_info**: Extract parts of a file or directory name.

```
  get_path_info(input, what)

  The first argument is either a string representing a file or
  directory name, or a list of such strings. If the input is a list
  the return value will be a list containing the result of applying the
  rule to each item in the input.

```

### **Possible values for the "what" parameter**

```
  "file"
      The substring after the last slash in the path, including the name
      and extension. If the input ends in a slash, the empty string will
      be returned.
        "foo/bar.txt" => "bar.txt"
        "bar.txt" => "bar.txt"
        "foo/" => ""
        "" => ""

  "name"
     The substring of the file name not including the extension.
        "foo/bar.txt" => "bar"
        "foo/bar" => "bar"
        "foo/" => ""

  "extension"
      The substring following the last period following the last slash,
      or the empty string if not found. The period is not included.
        "foo/bar.txt" => "txt"
        "foo/bar" => ""

  "dir"
      The directory portion of the name, not including the slash.
        "foo/bar.txt" => "foo"
        "//foo/bar" => "//foo"
        "foo" => "."

      The result will never end in a slash, so if the resulting
      is empty, the system ("/") or source ("//") roots, a "."
      will be appended such that it is always legal to append a slash
      and a filename and get a valid path.

  "out_dir"
      The output file directory corresponding to the path of the
      given file, not including a trailing slash.
        "//foo/bar/baz.txt" => "//out/Default/obj/foo/bar"
  "gen_dir"
      The generated file directory corresponding to the path of the
      given file, not including a trailing slash.
        "//foo/bar/baz.txt" => "//out/Default/gen/foo/bar"

  "abspath"
      The full absolute path name to the file or directory. It will be
      resolved relative to the currebt directory, and then the source-
      absolute version will be returned. If the input is system-
      absolute, the same input will be returned.
        "foo/bar.txt" => "//mydir/foo/bar.txt"
        "foo/" => "//mydir/foo/"
        "//foo/bar" => "//foo/bar"  (already absolute)
        "/usr/include" => "/usr/include"  (already absolute)

      If you want to make the path relative to another directory, or to
      be system-absolute, see rebase_path().

```

### **Examples**
```
  sources = [ "foo.cc", "foo.h" ]
  result = get_path_info(source, "abspath")
  # result will be [ "//mydir/foo.cc", "//mydir/foo.h" ]

  result = get_path_info("//foo/bar/baz.cc", "dir")
  # result will be "//foo/bar"

  # Extract the source-absolute directory name,
  result = get_path_info(get_path_info(path, "dir"), "abspath")


```
## **get_target_outputs**: [file list] Get the list of outputs from a target.

```
  get_target_outputs(target_label)

  Returns a list of output files for the named target. The named target
  must have been previously defined in the current file before this
  function is called (it can't reference targets in other files because
  there isn't a defined execution order, and it obviously can't
  reference targets that are defined after the function call).

  Only copy and action targets are supported. The outputs from binary
  targets will depend on the toolchain definition which won't
  necessarily have been loaded by the time a given line of code has run,
  and source sets and groups have no useful output file.

```

### **Return value**

```
  The names in the resulting list will be absolute file paths (normally
  like "//out/Debug/bar.exe", depending on the build directory).

  action targets: this will just return the files specified in the
  "outputs" variable of the target.

  action_foreach targets: this will return the result of applying
  the output template to the sources (see "gn help source_expansion").
  This will be the same result (though with guaranteed absolute file
  paths), as process_file_template will return for those inputs
  (see "gn help process_file_template").

  binary targets (executables, libraries): this will return a list
  of the resulting binary file(s). The "main output" (the actual
  binary or library) will always be the 0th element in the result.
  Depending on the platform and output type, there may be other output
  files as well (like import libraries) which will follow.

  source sets and groups: this will return a list containing the path of
  the "stamp" file that Ninja will produce once all outputs are
  generated. This probably isn't very useful.

```

### **Example**

```
  # Say this action generates a bunch of C source files.
  action_foreach("my_action") {
    sources = [ ... ]
    outputs = [ ... ]
  }

  # Compile the resulting source files into a source set.
  source_set("my_lib") {
    sources = get_target_outputs(":my_action")
  }


```
## **getenv**: Get an environment variable.

```
  value = getenv(env_var_name)

  Returns the value of the given enironment variable. If the value is
  not found, it will try to look up the variable with the "opposite"
  case (based on the case of the first letter of the variable), but
  is otherwise case-sensitive.

  If the environment variable is not found, the empty string will be
  returned. Note: it might be nice to extend this if we had the concept
  of "none" in the language to indicate lookup failure.

```

### **Example**:

```
  home_dir = getenv("HOME")


```
## **group**: Declare a named group of targets.

```
  This target type allows you to create meta-targets that just collect a
  set of dependencies into one named target. Groups can additionally
  specify configs that apply to their dependents.

  Depending on a group is exactly like depending directly on that
  group's deps. Direct dependent configs will get automatically
  forwarded through the group so you shouldn't need to use
  "forward_dependent_configs_from.

```

### **Variables**

```
  Deps: data_deps, deps, forward_dependent_configs_from, public_deps
  Dependent configs: all_dependent_configs, public_configs

```

### **Example**

```
  group("all") {
    deps = [
      "//project:runner",
      "//project:unit_tests",
    ]
  }


```
## **import**: Import a file into the current scope.

```
  The import command loads the rules and variables resulting from
  executing the given file into the current scope.

  By convention, imported files are named with a .gni extension.

  An import is different than a C++ "include". The imported file is
  executed in a standalone environment from the caller of the import
  command. The results of this execution are cached for other files that
  import the same .gni file.

  Note that you can not import a BUILD.gn file that's otherwise used
  in the build. Files must either be imported or implicitly loaded as
  a result of deps rules, but not both.

  The imported file's scope will be merged with the scope at the point
  import was called. If there is a conflict (both the current scope and
  the imported file define some variable or rule with the same name but
  different value), a runtime error will be thrown. Therefore, it's good
  practice to minimize the stuff that an imported file defines.

  Variables and templates beginning with an underscore '_' are
  considered private and will not be imported. Imported files can use
  such variables for internal computation without affecting other files.

```

### **Examples**:

```
  import("//build/rules/idl_compilation_rule.gni")

  # Looks in the current directory.
  import("my_vars.gni")


```
## **print**: Prints to the console.

```
  Prints all arguments to the console separated by spaces. A newline is
  automatically appended to the end.

  This function is intended for debugging. Note that build files are run
  in parallel so you may get interleaved prints. A buildfile may also
  be executed more than once in parallel in the context of different
  toolchains so the prints from one file may be duplicated or
  interleaved with itself.

```

### **Examples**:
```
  print("Hello world")

  print(sources, deps)


```
## **process_file_template**: Do template expansion over a list of files.

```
  process_file_template(source_list, template)

  process_file_template applies a template list to a source file list,
  returning the result of applying each template to each source. This is
  typically used for computing output file names from input files.

  In most cases, get_target_outputs() will give the same result with
  shorter, more maintainable code. This function should only be used
  when that function can't be used (like there's no target or the target
  is defined in another build file).

```

### **Arguments**:

```
  The source_list is a list of file names.

  The template can be a string or a list. If it is a list, multiple
  output strings are generated for each input.

  The template should contain source expansions to which each name in
  the source list is applied. See "gn help source_expansion".

```

### **Example**:

```
  sources = [
    "foo.idl",
    "bar.idl",
  ]
  myoutputs = process_file_template(
      sources,
      [ "$target_gen_dir/{{source_name_part}}.cc",
        "$target_gen_dir/{{source_name_part}}.h" ])

 The result in this case will be:
    [ "//out/Debug/foo.cc"
      "//out/Debug/foo.h"
      "//out/Debug/bar.cc"
      "//out/Debug/bar.h" ]


```
## **read_file**: Read a file into a variable.

```
  read_file(filename, input_conversion)

  Whitespace will be trimmed from the end of the file. Throws an error
  if the file can not be opened.

```

### **Arguments**:

```
  filename
      Filename to read, relative to the build file.

  input_conversion
      Controls how the file is read and parsed.
      See "gn help input_conversion".

```

### **Example**
```
  lines = read_file("foo.txt", "list lines")


```
## **rebase_path**: Rebase a file or directory to another location.

```
  converted = rebase_path(input,
                          new_base = "",
                          current_base = ".")

  Takes a string argument representing a file name, or a list of such
  strings and converts it/them to be relative to a different base
  directory.

  When invoking the compiler or scripts, GN will automatically convert
  sources and include directories to be relative to the build directory.
  However, if you're passing files directly in the "args" array or
  doing other manual manipulations where GN doesn't know something is
  a file name, you will need to convert paths to be relative to what
  your tool is expecting.

  The common case is to use this to convert paths relative to the
  current directory to be relative to the build directory (which will
  be the current directory when executing scripts).

  If you want to convert a file path to be source-absolute (that is,
  beginning with a double slash like "//foo/bar"), you should use
  the get_path_info() function. This function won't work because it will
  always make relative paths, and it needs to support making paths
  relative to the source root, so can't also generate source-absolute
  paths without more special-cases.

```

### **Arguments**:

```
  input
      A string or list of strings representing file or directory names
      These can be relative paths ("foo/bar.txt"), system absolute
      paths ("/foo/bar.txt"), or source absolute paths
      ("//foo/bar.txt").

  new_base
      The directory to convert the paths to be relative to. This can be
      an absolute path or a relative path (which will be treated
      as being relative to the current BUILD-file's directory).

      As a special case, if new_base is the empty string (the default),
      all paths will be converted to system-absolute native style paths
      with system path separators. This is useful for invoking external
      programs.

  current_base
      Directory representing the base for relative paths in the input.
      If this is not an absolute path, it will be treated as being
      relative to the current build file. Use "." (the default) to
      convert paths from the current BUILD-file's directory.

```

### **Return value**

```
  The return value will be the same type as the input value (either a
  string or a list of strings). All relative and source-absolute file
  names will be converted to be relative to the requested output
  System-absolute paths will be unchanged.

```

### **Example**

```
  # Convert a file in the current directory to be relative to the build
  # directory (the current dir when executing compilers and scripts).
  foo = rebase_path("myfile.txt", root_build_dir)
  # might produce "../../project/myfile.txt".

  # Convert a file to be system absolute:
  foo = rebase_path("myfile.txt")
  # Might produce "D:\source\project\myfile.txt" on Windows or
  # "/home/you/source/project/myfile.txt" on Linux.

  # Typical usage for converting to the build directory for a script.
  action("myscript") {
    # Don't convert sources, GN will automatically convert these to be
    # relative to the build directory when it constructs the command
    # line for your script.
    sources = [ "foo.txt", "bar.txt" ]

    # Extra file args passed manually need to be explicitly converted
    # to be relative to the build directory:
    args = [
      "--data",
      rebase_path("//mything/data/input.dat", root_build_dir),
      "--rel",
      rebase_path("relative_path.txt", root_build_dir)
    ] + rebase_path(sources, root_build_dir)
  }


```
## **set_default_toolchain**: Sets the default toolchain name.

```
  set_default_toolchain(toolchain_label)

  The given label should identify a toolchain definition (see
  "help toolchain"). This toolchain will be used for all targets
  unless otherwise specified.

  This function is only valid to call during the processing of the build
  configuration file. Since the build configuration file is processed
  separately for each toolchain, this function will be a no-op when
  called under any non-default toolchains.

  For example, the default toolchain should be appropriate for the
  current environment. If the current environment is 32-bit and 
  somebody references a target with a 64-bit toolchain, we wouldn't
  want processing of the build config file for the 64-bit toolchain to
  reset the default toolchain to 64-bit, we want to keep it 32-bits.

```

### **Argument**:

```
  toolchain_label
      Toolchain name.

```

### **Example**:

```
  set_default_toolchain("//build/config/win:vs32")

```
## **set_defaults**: Set default values for a target type.

```
  set_defaults(<target_type_name>) { <values...> }

  Sets the default values for a given target type. Whenever
  target_type_name is seen in the future, the values specified in
  set_default's block will be copied into the current scope.

  When the target type is used, the variable copying is very strict.
  If a variable with that name is already in scope, the build will fail
  with an error.

  set_defaults can be used for built-in target types ("executable",
  "shared_library", etc.) and custom ones defined via the "template"
  command.

```

### **Example**:
```
  set_defaults("static_library") {
    configs = [ "//tools/mything:settings" ]
  }

  static_library("mylib")
    # The configs will be auto-populated as above. You can remove it if
    # you don't want the default for a particular default:
    configs -= "//tools/mything:settings"
  }


```
## **set_sources_assignment_filter**: Set a pattern to filter source files.

```
  The sources assignment filter is a list of patterns that remove files
  from the list implicitly whenever the "sources" variable is
  assigned to. This is intended to be used to globally filter out files
  with platform-specific naming schemes when they don't apply, for
  example, you may want to filter out all "*_win.cc" files on non-
  Windows platforms.

  Typically this will be called once in the master build config script
  to set up the filter for the current platform. Subsequent calls will
  overwrite the previous values.

  If you want to bypass the filter and add a file even if it might
  be filtered out, call set_sources_assignment_filter([]) to clear the
  list of filters. This will apply until the current scope exits

```

### **How to use patterns**

```
  File patterns are VERY limited regular expressions. They must match
  the entire input string to be counted as a match. In regular
  expression parlance, there is an implicit "^...$" surrounding your
  input. If you want to match a substring, you need to use wildcards at
  the beginning and end.

  There are only two special tokens understood by the pattern matcher.
  Everything else is a literal.

   * Matches zero or more of any character. It does not depend on the
     preceding character (in regular expression parlance it is
     equivalent to ".*").

  \b Matches a path boundary. This will match the beginning or end of
     a string, or a slash.

```

### **Pattern examples**

```
  "*asdf*"
      Matches a string containing "asdf" anywhere.

  "asdf"
      Matches only the exact string "asdf".

  "*.cc"
      Matches strings ending in the literal ".cc".

  "\bwin/*"
      Matches "win/foo" and "foo/win/bar.cc" but not "iwin/foo".

```

### **Sources assignment example**

```
  # Filter out all _win files.
  set_sources_assignment_filter([ "*_win.cc", "*_win.h" ])
  sources = [ "a.cc", "b_win.cc" ]
  print(sources)
  # Will print [ "a.cc" ]. b_win one was filtered out.


```
## **shared_library**: Declare a shared library target.

```
  A shared library will be specified on the linker line for targets
  listing the shared library in its "deps". If you don't want this
  (say you dynamically load the library at runtime), then you should
  depend on the shared library via "data_deps" instead.

```

### **Variables**

```
  Flags: cflags, cflags_c, cflags_cc, cflags_objc, cflags_objcc,
         defines, include_dirs, ldflags, lib_dirs, libs
  Deps: data_deps, deps, forward_dependent_configs_from, public_deps
  Dependent configs: all_dependent_configs, public_configs
  General: check_includes, configs, data, inputs, output_name,
           output_extension, public, sources, testonly, visibility


```
## **source_set**: Declare a source set target.

```
  A source set is a collection of sources that get compiled, but are not
  linked to produce any kind of library. Instead, the resulting object
  files are implicitly added to the linker line of all targets that
  depend on the source set.

  In most cases, a source set will behave like a static library, except
  no actual library file will be produced. This will make the build go
  a little faster by skipping creation of a large static library, while
  maintaining the organizational benefits of focused build targets.

  The main difference between a source set and a static library is
  around handling of exported symbols. Most linkers assume declaring
  a function exported means exported from the static library. The linker
  can then do dead code elimination to delete code not reachable from
  exported functions.

  A source set will not do this code elimination since there is no link
  step. This allows you to link many sources sets into a shared library
  and have the "exported symbol" notation indicate "export from the
  final shared library and not from the intermediate targets." There is
  no way to express this concept when linking multiple static libraries
  into a shared library.

```

### **Variables**

```
  Flags: cflags, cflags_c, cflags_cc, cflags_objc, cflags_objcc,
         defines, include_dirs, ldflags, lib_dirs, libs
  Deps: data_deps, deps, forward_dependent_configs_from, public_deps
  Dependent configs: all_dependent_configs, public_configs
  General: check_includes, configs, data, inputs, output_name,
           output_extension, public, sources, testonly, visibility


```
## **static_library**: Declare a static library target.

```
  Make a ".a" / ".lib" file.

  If you only need the static library for intermediate results in the
  build, you should consider a source_set instead since it will skip
  the (potentially slow) step of creating the intermediate library file.

```

### **Variables**

```
  Flags: cflags, cflags_c, cflags_cc, cflags_objc, cflags_objcc,
         defines, include_dirs, ldflags, lib_dirs, libs
  Deps: data_deps, deps, forward_dependent_configs_from, public_deps
  Dependent configs: all_dependent_configs, public_configs
  General: check_includes, configs, data, inputs, output_name,
           output_extension, public, sources, testonly, visibility


```
## **template**: Define a template rule.

```
  A template defines a custom name that acts like a function. It
  provides a way to add to the built-in target types.

  The template() function is used to declare a template. To invoke the
  template, just use the name of the template like any other target
  type.

  Often you will want to declare your template in a special file that
  other files will import (see "gn help import") so your template
  rule can be shared across build files.

```

### **More details**:

```
  When you call template() it creates a closure around all variables
  currently in scope with the code in the template block. When the
  template is invoked, the closure will be executed.

  When the template is invoked, the code in the caller is executed and
  passed to the template code as an implicit "invoker" variable. The
  template uses this to read state out of the invoking code.

  One thing explicitly excluded from the closure is the "current
  directory" against which relative file names are resolved. The
  current directory will be that of the invoking code, since typically
  that code specifies the file names. This means all files internal
  to the template should use absolute names.

```

### **Target naming**:

```
  Your template should almost always define a built-in target with the
  name the template invoker specified. For example, if you have an IDL
  template and somebody does:
    idl("foo") {...
  you will normally want this to expand to something defining a
  source_set or static_library named "foo" (among other things you may
  need). This way, when another target specifies a dependency on
  "foo", the static_library or source_set will be linked.

  It is also important that any other targets your template expands to
  have globally unique names, or you will get collisions.

  Access the invoking name in your template via the implicit
  "target_name" variable. This should also be the basis of how other
  targets that a template expands to to ensure uniquness.

  A typical example would be a template that defines an action to
  generate some source files, and a source_set to compile that source.
  Your template would name the source_set "target_name" because
  that's what you want external targets to depend on to link your code.
  And you would name the action something like "${target_name}_action"
  to make it unique. The source set would have a dependency on the
  action to make it run.

```

### **Example of defining a template**:

```
  template("my_idl") {
    # Be nice and help callers debug problems by checking that the
    # variables the template requires are defined. This gives a nice
    # message rather than giving the user an error about an
    # undefined variable in the file defining the template
    #
    # You can also use defined() to give default values to variables
    # unspecified by the invoker.
    assert(defined(invoker.sources),
           "Need sources in $target_name listing the idl files.")

    # Name of the intermediate target that does the code gen. This must
    # incorporate the target name so it's unique across template
    # instantiations.
    code_gen_target_name = target_name + "_code_gen"

    # Intermediate target to convert IDL to C source. Note that the name
    # is based on the name the invoker of the template specified. This
    # way, each time the template is invoked we get a unique
    # intermediate action name (since all target names are in the global
    # scope).
    action_foreach(code_gen_target_name) {
      # Access the scope defined by the invoker via the implicit
      # "invoker" variable.
      sources = invoker.sources

      # Note that we need an absolute path for our script file name.
      # The current directory when executing this code will be that of
      # the invoker (this is why we can use the "sources" directly
      # above without having to rebase all of the paths). But if we need
      # to reference a script relative to the template file, we'll need
      # to use an absolute path instead.
      script = "//tools/idl/idl_code_generator.py"

      # Tell GN how to expand output names given the sources.
      # See "gn help source_expansion" for more.
      outputs = [ "$target_gen_dir/{{source_name_part}}.cc",
                  "$target_gen_dir/{{source_name_part}}.h" ]
    }

    # Name the source set the same as the template invocation so
    # instancing this template produces something that other targets
    # can link to in their deps.
    source_set(target_name) {
      # Generates the list of sources, we get these from the
      # action_foreach above.
      sources = get_target_outputs(":$code_gen_target_name")

      # This target depends on the files produced by the above code gen
      # target.
      deps = [ ":$code_gen_target_name" ]
    }
  }

```

### **Example of invoking the resulting template**:

```
  # This calls the template code above, defining target_name to be
  # "foo_idl_files" and "invoker" to be the set of stuff defined in
  # the curly brackets.
  my_idl("foo_idl_files") {
    # Goes into the template as "invoker.sources".
    sources = [ "foo.idl", "bar.idl" ]
  }

  # Here is a target that depends on our template.
  executable("my_exe") {
    # Depend on the name we gave the template call above. Internally,
    # this will produce a dependency from executable to the source_set
    # inside the template (since it has this name), which will in turn
    # depend on the code gen action.
    deps = [ ":foo_idl_files" ]
  }


```
## **tool**: Specify arguments to a toolchain tool.

### **Usage**:

```
  tool(<tool type>) {
    <tool variables...>
  }

```

### **Tool types**

```
    Compiler tools:
      "cc": C compiler
      "cxx": C++ compiler
      "objc": Objective C compiler
      "objcxx": Objective C++ compiler
      "rc": Resource compiler (Windows .rc files)
      "asm": Assembler

    Linker tools:
      "alink": Linker for static libraries (archives)
      "solink": Linker for shared libraries
      "link": Linker for executables

    Other tools:
      "stamp": Tool for creating stamp files
      "copy": Tool to copy files.

```

### **Tool variables**

```
    command  [string with substitutions]
        Valid for: all tools (required)

        The command to run.

    default_output_extension  [string]
        Valid for: linker tools

        Extension for the main output of a linkable tool. It includes
        the leading dot. This will be the default value for the
        {{output_extension}} expansion (discussed below) but will be
        overridden by by the "output extension" variable in a target,
        if one is specified. Empty string means no extension.

        GN doesn't actually do anything with this extension other than
        pass it along, potentially with target-specific overrides. One
        would typically use the {{output_extension}} value in the
        "outputs" to read this value.

        Example: default_output_extension = ".exe"

    depfile  [string]
        Valid for: compiler tools (optional)

        If the tool can write ".d" files, this specifies the name of
        the resulting file. These files are used to list header file
        dependencies (or other implicit input dependencies) that are
        discovered at build time. See also "depsformat".

        Example: depfile = "{{output}}.d"

    depsformat  [string]
        Valid for: compiler tools (when depfile is specified)

        Format for the deps outputs. This is either "gcc" or "msvc".
        See the ninja documentation for "deps" for more information.

        Example: depsformat = "gcc"

    description  [string with substitutions, optional]
        Valid for: all tools

        What to print when the command is run.

        Example: description = "Compiling {{source}}"

    lib_switch  [string, optional, link tools only]
    lib_dir_switch  [string, optional, link tools only]
        Valid for: Linker tools except "alink"

        These strings will be prepended to the libraries and library
        search directories, respectively, because linkers differ on how
        specify them. If you specified:
          lib_switch = "-l"
          lib_dir_switch = "-L"
        then the "{{libs}}" expansion for [ "freetype", "expat"]
        would be "-lfreetype -lexpat".

    outputs  [list of strings with substitutions]
        Valid for: Linker and compiler tools (required)

        An array of names for the output files the tool produces. These
        are relative to the build output directory. There must always be
        at least one output file. There can be more than one output (a
        linker might produce a library and an import library, for
        example).

        This array just declares to GN what files the tool will
        produce. It is your responsibility to specify the tool command
        that actually produces these files.

        If you specify more than one output for shared library links,
        you should consider setting link_output and depend_output.
        Otherwise, the first entry in the outputs list should always be
        the main output which will be linked to.

        Example for a compiler tool that produces .obj files:
          outputs = [
            "{{source_out_dir}}/{{source_name_part}}.obj"
          ]

        Example for a linker tool that produces a .dll and a .lib. The
        use of {{output_extension}} rather than hardcoding ".dll"
        allows the extension of the library to be overridden on a
        target-by-target basis, but in this example, it always
        produces a ".lib" import library:
          outputs = [
            "{{root_out_dir}}/{{target_output_name}}{{output_extension}}",
            "{{root_out_dir}}/{{target_output_name}}.lib",
          ]

    link_output  [string with substitutions]
    depend_output  [string with substitutions]
        Valid for: "solink" only (optional)

        These two files specify whch of the outputs from the solink
        tool should be used for linking and dependency tracking. These
        should match entries in the "outputs". If unspecified, the
        first item in the "outputs" array will be used for both. See
        "Separate linking and dependencies for shared libraries"
        below for more.

        On Windows, where the tools produce a .dll shared library and
        a .lib import library, you will want both of these to be the
        import library. On Linux, if you're not doing the separate
        linking/dependency optimization, both of these should be the
        .so output.

    output_prefix  [string]
        Valid for: Linker tools (optional)

        Prefix to use for the output name. Defaults to empty. This
        prefix will be prepended to the name of the target (or the
        output_name if one is manually specified for it) if the prefix
        is not already there. The result will show up in the
        {{output_name}} substitution pattern.

        This is typically used to prepend "lib" to libraries on
        Posix systems:
          output_prefix = "lib"

    restat  [boolean]
        Valid for: all tools (optional, defaults to false)

        Requests that Ninja check the file timestamp after this tool has
        run to determine if anything changed. Set this if your tool has
        the ability to skip writing output if the output file has not
        changed.

        Normally, Ninja will assume that when a tool runs the output
        be new and downstream dependents must be rebuild. When this is
        set to trye, Ninja can skip rebuilding downstream dependents for
        input changes that don't actually affect the output.

        Example:
          restat = true

    rspfile  [string with substitutions]
        Valid for: all tools (optional)

        Name of the response file. If empty, no response file will be
        used. See "rspfile_content".

    rspfile_content  [string with substitutions]
        Valid for: all tools (required when "rspfile" is specified)

        The contents to be written to the response file. This may
        include all or part of the command to send to the tool which
        allows you to get around OS command-line length limits.

        This example adds the inputs and libraries to a response file,
        but passes the linker flags directly on the command line:
          tool("link") {
            command = "link -o {{output}} {{ldflags}} @{{output}}.rsp"
            rspfile = "{{output}}.rsp"
            rspfile_content = "{{inputs}} {{solibs}} {{libs}}"
          }

```

### **Expansions for tool variables**
```
  All paths are relative to the root build directory, which is the
  current directory for running all tools. These expansions are
  available to all tools:

    {{label}}
        The label of the current target. This is typically used in the
        "description" field for link tools. The toolchain will be
        omitted from the label for targets in the default toolchain, and
        will be included for targets in other toolchains.

    {{output}}
        The relative path and name of the output(s) of the current
        build step. If there is more than one output, this will expand
        to a list of all of them.
        Example: "out/base/my_file.o"

    {{target_gen_dir}}
    {{target_out_dir}}
        The directory of the generated file and output directories,
        respectively, for the current target. There is no trailing
        slash.
        Example: "out/base/test"

    {{target_output_name}}
        The short name of the current target with no path information,
        or the value of the "output_name" variable if one is specified
        in the target. This will include the "output_prefix" if any.
        Example: "libfoo" for the target named "foo" and an
        output prefix for the linker tool of "lib".

  Compiler tools have the notion of a single input and a single output,
  along with a set of compiler-specific flags. The following expansions
  are available:

    {{cflags}}
    {{cflags_c}}
    {{cflags_cc}}
    {{cflags_objc}}
    {{cflags_objcc}}
    {{defines}}
    {{include_dirs}}
        Strings correspond that to the processed flags/defines/include
        directories specified for the target.
        Example: "--enable-foo --enable-bar"

        Defines will be prefixed by "-D" and include directories will
        be prefixed by "-I" (these work with Posix tools as well as
        Microsoft ones).

    {{source}}
        The relative path and name of the current input file.
        Example: "../../base/my_file.cc"

    {{source_file_part}}
        The file part of the source including the extension (with no
        directory information).
        Example: "foo.cc"

    {{source_name_part}}
        The filename part of the source file with no directory or
        extension.
        Example: "foo"

    {{source_gen_dir}}
    {{source_out_dir}}
        The directory in the generated file and output directories,
        respectively, for the current input file. If the source file
        is in the same directory as the target is declared in, they will
        will be the same as the "target" versions above.
        Example: "gen/base/test"

  Linker tools have multiple inputs and (potentially) multiple outputs
  The following expansions are available:

    {{inputs}}
    {{inputs_newline}}
        Expands to the inputs to the link step. This will be a list of
        object files and static libraries.
        Example: "obj/foo.o obj/bar.o obj/somelibrary.a"

        The "_newline" version will separate the input files with
        newlines instead of spaces. This is useful in response files:
        some linkers can take a "-filelist" flag which expects newline
        separated files, and some Microsoft tools have a fixed-sized
        buffer for parsing each line of a response file.

    {{ldflags}}
        Expands to the processed set of ldflags and library search paths
        specified for the target.
        Example: "-m64 -fPIC -pthread -L/usr/local/mylib"

    {{libs}}
        Expands to the list of system libraries to link to. Each will
        be prefixed by the "lib_prefix".

        As a special case to support Mac, libraries with names ending in
        ".framework" will be added to the {{libs}} with "-framework"
        preceeding it, and the lib prefix will be ignored.

        Example: "-lfoo -lbar"

    {{output_extension}}
        The value of the "output_extension" variable in the target,
        or the value of the "default_output_extension" value in the
        tool if the target does not specify an output extension.
        Example: ".so"

    {{solibs}}
        Extra libraries from shared library dependencide not specified
        in the {{inputs}}. This is the list of link_output files from
        shared libraries (if the solink tool specifies a "link_output"
        variable separate from the "depend_output").

        These should generally be treated the same as libs by your tool.
        Example: "libfoo.so libbar.so"

  The copy tool allows the common compiler/linker substitutions, plus
  {{source}} which is the source of the copy. The stamp tool allows
  only the common tool substitutions.

```

### **Separate linking and dependencies for shared libraries**

```
  Shared libraries are special in that not all changes to them require
  that dependent targets be re-linked. If the shared library is changed
  but no imports or exports are different, dependent code needn't be
  relinked, which can speed up the build.

  If your link step can output a list of exports from a shared library
  and writes the file only if the new one is different, the timestamp of
  this file can be used for triggering re-links, while the actual shared
  library would be used for linking.

  You will need to specify
    restat = true
  in the linker tool to make this work, so Ninja will detect if the
  timestamp of the dependency file has changed after linking (otherwise
  it will always assume that running a command updates the output):

    tool("solink") {
      command = "..."
      outputs = [
        "{{root_out_dir}}/{{target_output_name}}{{output_extension}}",
        "{{root_out_dir}}/{{target_output_name}}{{output_extension}}.TOC",
      ]
      link_output =
        "{{root_out_dir}}/{{target_output_name}}{{output_extension}}"
      depend_output =
        "{{root_out_dir}}/{{target_output_name}}{{output_extension}}.TOC"
      restat = true
    }

```

### **Example**

```
  toolchain("my_toolchain") {
    # Put these at the top to apply to all tools below.
    lib_prefix = "-l"
    lib_dir_prefix = "-L"

    tool("cc") {
      command = "gcc {{source}} -o {{output}}"
      outputs = [ "{{source_out_dir}}/{{source_name_part}}.o" ]
      description = "GCC {{source}}"
    }
    tool("cxx") {
      command = "g++ {{source}} -o {{output}}"
      outputs = [ "{{source_out_dir}}/{{source_name_part}}.o" ]
      description = "G++ {{source}}"
    }
  }


```
## **toolchain**: Defines a toolchain.

```
  A toolchain is a set of commands and build flags used to compile the
  source code. You can have more than one toolchain in use at once in
  a build.

```

### **Functions and variables**

```
  tool()
    The tool() function call specifies the commands commands to run for
    a given step. See "gn help tool".

  toolchain_args()
    List of arguments to pass to the toolchain when invoking this
    toolchain. This applies only to non-default toolchains. See
    "gn help toolchain_args" for more.

  deps
    Dependencies of this toolchain. These dependencies will be resolved
    before any target in the toolchain is compiled. To avoid circular
    dependencies these must be targets defined in another toolchain.

    This is expressed as a list of targets, and generally these targets
    will always specify a toolchain:
      deps = [ "//foo/bar:baz(//build/toolchain:bootstrap)" ]

    This concept is somewhat inefficient to express in Ninja (it
    requires a lot of duplicate of rules) so should only be used when
    absolutely necessary.

  concurrent_links
    In integer expressing the number of links that Ninja will perform in
    parallel. GN will create a pool for shared library and executable
    link steps with this many processes. Since linking is memory- and
    I/O-intensive, projects with many large targets may want to limit
    the number of parallel steps to avoid overloading the computer.
    Since creating static libraries is generally not as intensive
    there is no limit to "alink" steps.

    Defaults to 0 which Ninja interprets as "no limit".

    The value used will be the one from the default toolchain of the
    current build.

```

### **Invoking targets in toolchains**:

```
  By default, when a target depends on another, there is an implicit
  toolchain label that is inherited, so the dependee has the same one
  as the dependent.

  You can override this and refer to any other toolchain by explicitly
  labeling the toolchain to use. For example:
    data_deps = [ "//plugins:mine(//toolchains:plugin_toolchain)" ]
  The string "//build/toolchains:plugin_toolchain" is a label that
  identifies the toolchain declaration for compiling the sources.

  To load a file in an alternate toolchain, GN does the following:

   1. Loads the file with the toolchain definition in it (as determined
      by the toolchain label).
   2. Re-runs the master build configuration file, applying the
      arguments specified by the toolchain_args section of the toolchain
      definition (see "gn help toolchain_args").
   3. Loads the destination build file in the context of the
      configuration file in the previous step.

```

### **Example**:
```
  toolchain("plugin_toolchain") {
    concurrent_links = 8

    tool("cc") {
      command = "gcc {{source}}"
      ...
    }

    toolchain_args() {
      is_plugin = true
      is_32bit = true
      is_64bit = false
    }
  }


```
## **toolchain_args**: Set build arguments for toolchain build setup.

```
  Used inside a toolchain definition to pass arguments to an alternate
  toolchain's invocation of the build.

  When you specify a target using an alternate toolchain, the master
  build configuration file is re-interpreted in the context of that
  toolchain (see "gn help toolchain"). The toolchain_args function
  allows you to control the arguments passed into this alternate
  invocation of the build.

  Any default system arguments or arguments passed in on the command-
  line will also be passed to the alternate invocation unless explicitly
  overridden by toolchain_args.

  The toolchain_args will be ignored when the toolchain being defined
  is the default. In this case, it's expected you want the default
  argument values.

  See also "gn help buildargs" for an overview of these arguments.

```

### **Example**:
```
  toolchain("my_weird_toolchain") {
    ...
    toolchain_args() {
      # Override the system values for a generic Posix system.
      is_win = false
      is_posix = true

      # Pass this new value for specific setup for my toolchain.
      is_my_weird_system = true
    }
  }


```
## **write_file**: Write a file to disk.

```
  write_file(filename, data)

  If data is a list, the list will be written one-item-per-line with no
  quoting or brackets.

  If the file exists and the contents are identical to that being
  written, the file will not be updated. This will prevent unnecessary
  rebuilds of targets that depend on this file.

  TODO(brettw) we probably need an optional third argument to control
  list formatting.

```

### **Arguments**:

```
  filename
      Filename to write. This must be within the output directory.

  data:
      The list or string to write.


```
## **current_cpu**: The processor architecture of the current toolchain.

```
  The build configuration usually sets this value based on the value
  of "host_cpu" (see "gn help host_cpu") and then threads
  this through the toolchain definitions to ensure that it always
  reflects the appropriate value.

  This value is not used internally by GN for any purpose. It is
  set it to the empty string ("") by default but is declared so
  that it can be overridden on the command line if so desired.

  See "gn help target_cpu" for a list of common values returned.


```
## **current_os**: The operating system of the current toolchain.

```
  The build configuration usually sets this value based on the value
  of "target_os" (see "gn help target_os"), and then threads this
  through the toolchain definitions to ensure that it always reflects
  the appropriate value.

  This value is not used internally by GN for any purpose. It is
  set it to the empty string ("") by default but is declared so
  that it can be overridden on the command line if so desired.

  See "gn help target_os" for a list of common values returned.


```
## **current_toolchain**: Label of the current toolchain.

```
  A fully-qualified label representing the current toolchain. You can
  use this to make toolchain-related decisions in the build. See also
  "default_toolchain".

```

### **Example**:

```
  if (current_toolchain == "//build:64_bit_toolchain") {
    executable("output_thats_64_bit_only") {
      ...


```
## **default_toolchain**: [string] Label of the default toolchain.

```
  A fully-qualified label representing the default toolchain, which may
  not necessarily be the current one (see "current_toolchain").


```
## **host_cpu**: The processor architecture that GN is running on.

```
  This is value is exposed so that cross-compile toolchains can
  access the host architecture when needed.

  The value should generally be considered read-only, but it can be
  overriden in order to handle unusual cases where there might
  be multiple plausible values for the host architecture (e.g., if
  you can do either 32-bit or 64-bit builds). The value is not used
  internally by GN for any purpose.

```

### **Some possible values**:
```
  - "x64"
  - "x86"


```
## **host_os**: [string] The operating system that GN is running on.

```
  This value is exposed so that cross-compiles can access the host
  build system's settings.

  This value should generally be treated as read-only. It, however,
  is not used internally by GN for any purpose.

```

### **Some possible values**:
```
  - "linux"
  - "mac"
  - "win"


```
## **python_path**: Absolute path of Python.

```
  Normally used in toolchain definitions if running some command
  requires Python. You will normally not need this when invoking scripts
  since GN automatically finds it for you.


```
## **root_build_dir**: [string] Directory where build commands are run.

```
  This is the root build output directory which will be the current
  directory when executing all compilers and scripts.

  Most often this is used with rebase_path (see "gn help rebase_path")
  to convert arguments to be relative to a script's current directory.


```
## **root_gen_dir**: Directory for the toolchain's generated files.

```
  Absolute path to the root of the generated output directory tree for
  the current toolchain. An example would be "//out/Debug/gen" for the
  default toolchain, or "//out/Debug/arm/gen" for the "arm"
  toolchain.

  This is primarily useful for setting up include paths for generated
  files. If you are passing this to a script, you will want to pass it
  through rebase_path() (see "gn help rebase_path") to convert it
  to be relative to the build directory.

  See also "target_gen_dir" which is usually a better location for
  generated files. It will be inside the root generated dir.


```
## **root_out_dir**: [string] Root directory for toolchain output files.

```
  Absolute path to the root of the output directory tree for the current
  toolchain. It will not have a trailing slash.

  For the default toolchain this will be the same as the root_build_dir.
  An example would be "//out/Debug" for the default toolchain, or
  "//out/Debug/arm" for the "arm" toolchain.

  This is primarily useful for setting up script calls. If you are
  passing this to a script, you will want to pass it through
  rebase_path() (see "gn help rebase_path") to convert it
  to be relative to the build directory.

  See also "target_out_dir" which is usually a better location for
  output files. It will be inside the root output dir.

```

### **Example**:

```
  action("myscript") {
    # Pass the output dir to the script.
    args = [ "-o", rebase_path(root_out_dir, root_build_dir) ]
  }


```
## **target_cpu**: The desired cpu architecture for the build.

```
  This value should be used to indicate the desired architecture for
  the primary objects of the build. It will match the cpu architecture
  of the default toolchain.

  In many cases, this is the same as "host_cpu", but in the case
  of cross-compiles, this can be set to something different. This 
  value is different from "current_cpu" in that it can be referenced
  from inside any toolchain. This value can also be ignored if it is
  not needed or meaningful for a project.

  This value is not used internally by GN for any purpose, so it
  may be set to whatever value is needed for the build.
  GN defaults this value to the empty string ("") and the
  configuration files should set it to an appropriate value
  (e.g., setting it to the value of "host_cpu") if it is not
  overridden on the command line or in the args.gn file.

  Where practical, use one of the following list of common values:

```

### **Possible values**:
```
  - "x86"
  - "x64"
  - "arm"
  - "arm64"
  - "mipsel"


```
## **target_gen_dir**: Directory for a target's generated files.

```
  Absolute path to the target's generated file directory. This will be
  the "root_gen_dir" followed by the relative path to the current
  build file. If your file is in "//tools/doom_melon" then
  target_gen_dir would be "//out/Debug/gen/tools/doom_melon". It will
  not have a trailing slash.

  This is primarily useful for setting up include paths for generated
  files. If you are passing this to a script, you will want to pass it
  through rebase_path() (see "gn help rebase_path") to convert it
  to be relative to the build directory.

  See also "gn help root_gen_dir".

```

### **Example**:

```
  action("myscript") {
    # Pass the generated output dir to the script.
    args = [ "-o", rebase_path(target_gen_dir, root_build_dir) ]
  }


```
## **target_os**: The desired operating system for the build.

```
  This value should be used to indicate the desired operating system
  for the primary object(s) of the build. It will match the OS of
  the default toolchain.

  In many cases, this is the same as "host_os", but in the case of
  cross-compiles, it may be different. This variable differs from
  "current_os" in that it can be referenced from inside any
  toolchain and will always return the initial value.

  This should be set to the most specific value possible. So,
  "android" or "chromeos" should be used instead of "linux"
  where applicable, even though Android and ChromeOS are both Linux
  variants. This can mean that one needs to write

      if (target_os == "android" || target_os == "linux") {
          # ...
      }

  and so forth.

  This value is not used internally by GN for any purpose, so it
  may be set to whatever value is needed for the build.
  GN defaults this value to the empty string ("") and the
  configuration files should set it to an appropriate value
  (e.g., setting it to the value of "host_os") if it is not
  set via the command line or in the args.gn file.

  Where practical, use one of the following list of common values:

```

### **Possible values**:
```
  - "android"
  - "chromeos"
  - "ios"
  - "linux"
  - "nacl"
  - "mac"
  - "win"


```
## **target_out_dir**: [string] Directory for target output files.

```
  Absolute path to the target's generated file directory. If your
  current target is in "//tools/doom_melon" then this value might be
  "//out/Debug/obj/tools/doom_melon". It will not have a trailing
  slash.

  This is primarily useful for setting up arguments for calling
  scripts. If you are passing this to a script, you will want to pass it
  through rebase_path() (see "gn help rebase_path") to convert it
  to be relative to the build directory.

  See also "gn help root_out_dir".

```

### **Example**:

```
  action("myscript") {
    # Pass the output dir to the script.
    args = [ "-o", rebase_path(target_out_dir, root_build_dir) ]
  }


```
## **all_dependent_configs**: Configs to be forced on dependents.

```
  A list of config labels.

  All targets depending on this one, and recursively, all targets
  depending on those, will have the configs listed in this variable
  added to them. These configs will also apply to the current target.

  This addition happens in a second phase once a target and all of its
  dependencies have been resolved. Therefore, a target will not see
  these force-added configs in their "configs" variable while the
  script is running, and then can not be removed. As a result, this
  capability should generally only be used to add defines and include
  directories necessary to compile a target's headers.

  See also "public_configs".

```

### **Ordering of flags and values**:

```
  1. Those set on the current target (not in a config).
  2. Those set on the "configs" on the target in order that the
     configs appear in the list.
  3. Those set on the "all_dependent_configs" on the target in order
     that the configs appear in the list.
  4. Those set on the "public_configs" on the target in order that
     those configs appear in the list.
  5. all_dependent_configs pulled from dependencies, in the order of
     the "deps" list. This is done recursively. If a config appears
     more than once, only the first occurance will be used.
  6. public_configs pulled from dependencies, in the order of the
     "deps" list. If a dependency is public, they will be applied
     recursively.


```
## **allow_circular_includes_from**: Permit includes from deps.

```
  A list of target labels. Must be a subset of the target's "deps".
  These targets will be permitted to include headers from the current
  target despite the dependency going in the opposite direction.

```

### **Tedious exposition**

```
  Normally, for a file in target A to include a file from target B,
  A must list B as a dependency. This invariant is enforced by the
  "gn check" command (and the --check flag to "gn gen").

  Sometimes, two targets might be the same unit for linking purposes
  (two source sets or static libraries that would always be linked
  together in a final executable or shared library). In this case,
  you want A to be able to include B's headers, and B to include A's
  headers.

  This list, if specified, lists which of the dependencies of the
  current target can include header files from the current target.
  That is, if A depends on B, B can only include headers from A if it is
  in A's allow_circular_includes_from list.

```

### **Example**

```
  source_set("a") {
    deps = [ ":b", ":c" ]
    allow_circular_includes_from = [ ":b" ]
    ...
  }


```
## **args**: Arguments passed to an action.

```
  For action and action_foreach targets, args is the list of arguments
  to pass to the script. Typically you would use source expansion (see
  "gn help source_expansion") to insert the source file names.

  See also "gn help action" and "gn help action_foreach".


```
## **cflags***: Flags passed to the C compiler.

```
  A list of strings.

  "cflags" are passed to all invocations of the C, C++, Objective C,
  and Objective C++ compilers.

  To target one of these variants individually, use "cflags_c",
  "cflags_cc", "cflags_objc", and "cflags_objcc", respectively.
  These variant-specific versions will be appended to the "cflags".

```

### **Ordering of flags and values**:

```
  1. Those set on the current target (not in a config).
  2. Those set on the "configs" on the target in order that the
     configs appear in the list.
  3. Those set on the "all_dependent_configs" on the target in order
     that the configs appear in the list.
  4. Those set on the "public_configs" on the target in order that
     those configs appear in the list.
  5. all_dependent_configs pulled from dependencies, in the order of
     the "deps" list. This is done recursively. If a config appears
     more than once, only the first occurance will be used.
  6. public_configs pulled from dependencies, in the order of the
     "deps" list. If a dependency is public, they will be applied
     recursively.


```
## **cflags***: Flags passed to the C compiler.

```
  A list of strings.

  "cflags" are passed to all invocations of the C, C++, Objective C,
  and Objective C++ compilers.

  To target one of these variants individually, use "cflags_c",
  "cflags_cc", "cflags_objc", and "cflags_objcc", respectively.
  These variant-specific versions will be appended to the "cflags".

```

### **Ordering of flags and values**:

```
  1. Those set on the current target (not in a config).
  2. Those set on the "configs" on the target in order that the
     configs appear in the list.
  3. Those set on the "all_dependent_configs" on the target in order
     that the configs appear in the list.
  4. Those set on the "public_configs" on the target in order that
     those configs appear in the list.
  5. all_dependent_configs pulled from dependencies, in the order of
     the "deps" list. This is done recursively. If a config appears
     more than once, only the first occurance will be used.
  6. public_configs pulled from dependencies, in the order of the
     "deps" list. If a dependency is public, they will be applied
     recursively.


```
## **cflags***: Flags passed to the C compiler.

```
  A list of strings.

  "cflags" are passed to all invocations of the C, C++, Objective C,
  and Objective C++ compilers.

  To target one of these variants individually, use "cflags_c",
  "cflags_cc", "cflags_objc", and "cflags_objcc", respectively.
  These variant-specific versions will be appended to the "cflags".

```

### **Ordering of flags and values**:

```
  1. Those set on the current target (not in a config).
  2. Those set on the "configs" on the target in order that the
     configs appear in the list.
  3. Those set on the "all_dependent_configs" on the target in order
     that the configs appear in the list.
  4. Those set on the "public_configs" on the target in order that
     those configs appear in the list.
  5. all_dependent_configs pulled from dependencies, in the order of
     the "deps" list. This is done recursively. If a config appears
     more than once, only the first occurance will be used.
  6. public_configs pulled from dependencies, in the order of the
     "deps" list. If a dependency is public, they will be applied
     recursively.


```
## **cflags***: Flags passed to the C compiler.

```
  A list of strings.

  "cflags" are passed to all invocations of the C, C++, Objective C,
  and Objective C++ compilers.

  To target one of these variants individually, use "cflags_c",
  "cflags_cc", "cflags_objc", and "cflags_objcc", respectively.
  These variant-specific versions will be appended to the "cflags".

```

### **Ordering of flags and values**:

```
  1. Those set on the current target (not in a config).
  2. Those set on the "configs" on the target in order that the
     configs appear in the list.
  3. Those set on the "all_dependent_configs" on the target in order
     that the configs appear in the list.
  4. Those set on the "public_configs" on the target in order that
     those configs appear in the list.
  5. all_dependent_configs pulled from dependencies, in the order of
     the "deps" list. This is done recursively. If a config appears
     more than once, only the first occurance will be used.
  6. public_configs pulled from dependencies, in the order of the
     "deps" list. If a dependency is public, they will be applied
     recursively.


```
## **cflags***: Flags passed to the C compiler.

```
  A list of strings.

  "cflags" are passed to all invocations of the C, C++, Objective C,
  and Objective C++ compilers.

  To target one of these variants individually, use "cflags_c",
  "cflags_cc", "cflags_objc", and "cflags_objcc", respectively.
  These variant-specific versions will be appended to the "cflags".

```

### **Ordering of flags and values**:

```
  1. Those set on the current target (not in a config).
  2. Those set on the "configs" on the target in order that the
     configs appear in the list.
  3. Those set on the "all_dependent_configs" on the target in order
     that the configs appear in the list.
  4. Those set on the "public_configs" on the target in order that
     those configs appear in the list.
  5. all_dependent_configs pulled from dependencies, in the order of
     the "deps" list. This is done recursively. If a config appears
     more than once, only the first occurance will be used.
  6. public_configs pulled from dependencies, in the order of the
     "deps" list. If a dependency is public, they will be applied
     recursively.


```
## **check_includes**: [boolean] Controls whether a target's files are checked.

```
  When true (the default), the "gn check" command (as well as
  "gn gen" with the --check flag) will check this target's sources
  and headers for proper dependencies.

  When false, the files in this target will be skipped by default.
  This does not affect other targets that depend on the current target,
  it just skips checking the includes of the current target's files.

```

### **Example**

```
  source_set("busted_includes") {
    # This target's includes are messed up, exclude it from checking.
    check_includes = false
    ...
  }


```
## **complete_static_lib**: [boolean] Links all deps into a static library.

```
  A static library normally doesn't include code from dependencies, but
  instead forwards the static libraries and source sets in its deps up
  the dependency chain until a linkable target (an executable or shared
  library) is reached. The final linkable target only links each static
  library once, even if it appears more than once in its dependency
  graph.

  In some cases the static library might be the final desired output.
  For example, you may be producing a static library for distribution to
  third parties. In this case, the static library should include code
  for all dependencies in one complete package. Since GN does not unpack
  static libraries to forward their contents up the dependency chain,
  it is an error for complete static libraries to depend on other static
  libraries.

```

### **Example**

```
  static_library("foo") {
    complete_static_lib = true
    deps = [ "bar" ]
  }


```
## **configs**: Configs applying to this target.

```
  A list of config labels.

  The include_dirs, defines, etc. in each config are appended in the
  order they appear to the compile command for each file in the target.
  They will appear after the include_dirs, defines, etc. that the target
  sets directly.

  The build configuration script will generally set up the default
  configs applying to a given target type (see "set_defaults").
  When a target is being defined, it can add to or remove from this
  list.

```

### **Ordering of flags and values**:

```
  1. Those set on the current target (not in a config).
  2. Those set on the "configs" on the target in order that the
     configs appear in the list.
  3. Those set on the "all_dependent_configs" on the target in order
     that the configs appear in the list.
  4. Those set on the "public_configs" on the target in order that
     those configs appear in the list.
  5. all_dependent_configs pulled from dependencies, in the order of
     the "deps" list. This is done recursively. If a config appears
     more than once, only the first occurance will be used.
  6. public_configs pulled from dependencies, in the order of the
     "deps" list. If a dependency is public, they will be applied
     recursively.

```

### **Example**:
```
  static_library("foo") {
    configs -= "//build:no_rtti"  # Don't use the default RTTI config.
    configs += ":mysettings"      # Add some of our own settings.
  }


```
## **data**: Runtime data file dependencies.

```
  Lists files required to run the given target. These are typically
  data files.

  Appearing in the "data" section does not imply any special handling
  such as copying them to the output directory. This is just used for
  declaring runtime dependencies. There currently isn't a good use for
  these but it is envisioned that test data can be listed here for use
  running automated tests.

  See also "gn help inputs" and "gn help data_deps", both of
  which actually affect the build in concrete ways.


```
## **data_deps**: Non-linked dependencies.

```
  A list of target labels.

  Specifies dependencies of a target that are not actually linked into
  the current target. Such dependencies will built and will be available
  at runtime.

  This is normally used for things like plugins or helper programs that
  a target needs at runtime.

  See also "gn help deps" and "gn help data".

```

### **Example**:
```
  executable("foo") {
    deps = [ "//base" ]
    data_deps = [ "//plugins:my_runtime_plugin" ]
  }


```
## **defines**: C preprocessor defines.

```
  A list of strings

  These strings will be passed to the C/C++ compiler as #defines. The
  strings may or may not include an "=" to assign a value.

```

### **Ordering of flags and values**:

```
  1. Those set on the current target (not in a config).
  2. Those set on the "configs" on the target in order that the
     configs appear in the list.
  3. Those set on the "all_dependent_configs" on the target in order
     that the configs appear in the list.
  4. Those set on the "public_configs" on the target in order that
     those configs appear in the list.
  5. all_dependent_configs pulled from dependencies, in the order of
     the "deps" list. This is done recursively. If a config appears
     more than once, only the first occurance will be used.
  6. public_configs pulled from dependencies, in the order of the
     "deps" list. If a dependency is public, they will be applied
     recursively.

```

### **Example**:
```
  defines = [ "AWESOME_FEATURE", "LOG_LEVEL=3" ]


```
## **depfile**: [string] File name for input dependencies for actions.

```
  If nonempty, this string specifies that the current action or
  action_foreach target will generate the given ".d" file containing
  the dependencies of the input. Empty or unset means that the script
  doesn't generate the files.

  The .d file should go in the target output directory. If you have more
  than one source file that the script is being run over, you can use
  the output file expansions described in "gn help action_foreach" to
  name the .d file according to the input.
  The format is that of a Makefile, and all of the paths should be
  relative to the root build directory.

```

### **Example**:
```
  action_foreach("myscript_target") {
    script = "myscript.py"
    sources = [ ... ]

    # Locate the depfile in the output directory named like the
    # inputs but with a ".d" appended.
    depfile = "$relative_target_output_dir/{{source_name}}.d"

    # Say our script uses "-o <d file>" to indicate the depfile.
    args = [ "{{source}}", "-o", depfile ]
  }


```
## **deps**: Private linked dependencies.

```
  A list of target labels.

  Specifies private dependencies of a target. Shared and dynamic
  libraries will be linked into the current target. Other target types
  that can't be linked (like actions and groups) listed in "deps" will
  be treated as "data_deps". Likewise, if the current target isn't
  linkable, then all deps will be treated as "data_deps".

  These dependencies are private in that it does not grant dependent
  targets the ability to include headers from the dependency, and direct
  dependent configs are not forwarded.

  See also "public_deps" and "data_deps".


```
## **forward_dependent_configs_from**

```
  A list of target labels.

  DEPRECATED. Use public_deps instead which will have the same effect.

  Exposes the public_configs from a private dependent target as
  public_configs of the current one. Each label in this list
  must also be in the deps.

  Generally you should use public_deps instead of this variable to
  express the concept of exposing a dependency as part of a target's
  public API. We're considering removing this variable.

```

### **Discussion**

```
  Sometimes you depend on a child library that exports some necessary
  configuration via public_configs. If your target in turn exposes the
  child library's headers in its public headers, it might mean that
  targets that depend on you won't work: they'll be seeing the child
  library's code but not the necessary configuration. This list
  specifies which of your deps' direct dependent configs to expose as
  your own.

```

### **Examples**

```
  If we use a given library "a" from our public headers:

    deps = [ ":a", ":b", ... ]
    forward_dependent_configs_from = [ ":a" ]

  This example makes a "transparent" target that forwards a dependency
  to another:

    group("frob") {
      if (use_system_frob) {
        deps = ":system_frob"
      } else {
        deps = "//third_party/fallback_frob"
      }
      forward_dependent_configs_from = deps
    }


```
## **include_dirs**: Additional include directories.

```
  A list of source directories.

  The directories in this list will be added to the include path for
  the files in the affected target.

```

### **Ordering of flags and values**:

```
  1. Those set on the current target (not in a config).
  2. Those set on the "configs" on the target in order that the
     configs appear in the list.
  3. Those set on the "all_dependent_configs" on the target in order
     that the configs appear in the list.
  4. Those set on the "public_configs" on the target in order that
     those configs appear in the list.
  5. all_dependent_configs pulled from dependencies, in the order of
     the "deps" list. This is done recursively. If a config appears
     more than once, only the first occurance will be used.
  6. public_configs pulled from dependencies, in the order of the
     "deps" list. If a dependency is public, they will be applied
     recursively.

```

### **Example**:
```
  include_dirs = [ "src/include", "//third_party/foo" ]


```
## **inputs**: Additional compile-time dependencies.

```
  Inputs are compile-time dependencies of the current target. This means
  that all inputs must be available before compiling any of the sources
  or executing any actions.

  Inputs are typically only used for action and action_foreach targets.

```

### **Inputs for actions**

```
  For action and action_foreach targets, inputs should be the inputs to
  script that don't vary. These should be all .py files that the script
  uses via imports (the main script itself will be an implcit dependency
  of the action so need not be listed).

  For action targets, inputs should be the entire set of inputs the
  script needs. For action_foreach targets, inputs should be the set of
  dependencies that don't change. These will be applied to each script
  invocation over the sources.

  Note that another way to declare input dependencies from an action
  is to have the action write a depfile (see "gn help depfile"). This
  allows the script to dynamically write input dependencies, that might
  not be known until actually executing the script. This is more
  efficient than doing processing while running GN to determine the
  inputs, and is easier to keep in-sync than hardcoding the list.

```

### **Inputs for binary targets**

```
  Any input dependencies will be resolved before compiling any sources.
  Normally, all actions that a target depends on will be run before any
  files in a target are compiled. So if you depend on generated headers,
  you do not typically need to list them in the inputs section.

```

### **Example**

```
  action("myscript") {
    script = "domything.py"
    inputs = [ "input.data" ]
  }


```
## **ldflags**: Flags passed to the linker.

```
  A list of strings.

  These flags are passed on the command-line to the linker and generally
  specify various linking options. Most targets will not need these and
  will use "libs" and "lib_dirs" instead.

  ldflags are NOT pushed to dependents, so applying ldflags to source
  sets or static libraries will be a no-op. If you want to apply ldflags
  to dependent targets, put them in a config and set it in the
  all_dependent_configs or public_configs.


```
## **lib_dirs**: Additional library directories.

```
  A list of directories.

  Specifies additional directories passed to the linker for searching
  for the required libraries. If an item is not an absolute path, it
  will be treated as being relative to the current build file.

  libs and lib_dirs work differently than other flags in two respects.
  First, then are inherited across static library boundaries until a
  shared library or executable target is reached. Second, they are
  uniquified so each one is only passed once (the first instance of it
  will be the one used).

```

### **Ordering of flags and values**:

```
  1. Those set on the current target (not in a config).
  2. Those set on the "configs" on the target in order that the
     configs appear in the list.
  3. Those set on the "all_dependent_configs" on the target in order
     that the configs appear in the list.
  4. Those set on the "public_configs" on the target in order that
     those configs appear in the list.
  5. all_dependent_configs pulled from dependencies, in the order of
     the "deps" list. This is done recursively. If a config appears
     more than once, only the first occurance will be used.
  6. public_configs pulled from dependencies, in the order of the
     "deps" list. If a dependency is public, they will be applied
     recursively.

```

### **Example**:
```
  lib_dirs = [ "/usr/lib/foo", "lib/doom_melon" ]


```
## **libs**: Additional libraries to link.

```
  A list of strings.

  These files will be passed to the linker, which will generally search
  the library include path. Unlike a normal list of files, they will be
  passed to the linker unmodified rather than being treated as file
  names relative to the current build file. Generally you would set
  the "lib_dirs" so your library is found. If you need to specify
  a path, you can use "rebase_path" to convert a path to be relative
  to the build directory.

  When constructing the linker command, the "lib_prefix" attribute of
  the linker tool in the current toolchain will be prepended to each
  library. So your BUILD file should not specify the switch prefix
  (like "-l"). On Mac, libraries ending in ".framework" will be
  special-cased: the switch "-framework" will be prepended instead of
  the lib_prefix, and the ".framework" suffix will be trimmed.

  libs and lib_dirs work differently than other flags in two respects.
  First, then are inherited across static library boundaries until a
  shared library or executable target is reached. Second, they are
  uniquified so each one is only passed once (the first instance of it
  will be the one used).

```

### **Ordering of flags and values**:

```
  1. Those set on the current target (not in a config).
  2. Those set on the "configs" on the target in order that the
     configs appear in the list.
  3. Those set on the "all_dependent_configs" on the target in order
     that the configs appear in the list.
  4. Those set on the "public_configs" on the target in order that
     those configs appear in the list.
  5. all_dependent_configs pulled from dependencies, in the order of
     the "deps" list. This is done recursively. If a config appears
     more than once, only the first occurance will be used.
  6. public_configs pulled from dependencies, in the order of the
     "deps" list. If a dependency is public, they will be applied
     recursively.

```

### **Examples**:
```
  On Windows:
    libs = [ "ctl3d.lib" ]
  On Linux:
    libs = [ "ld" ]


```
## **output_extension**: Value to use for the output's file extension.

```
  Normally the file extension for a target is based on the target
  type and the operating system, but in rare cases you will need to
  override the name (for example to use "libfreetype.so.6" instead
  of libfreetype.so on Linux).

```
## **output_name**: Define a name for the output file other than the default.

```
  Normally the output name of a target will be based on the target name,
  so the target "//foo/bar:bar_unittests" will generate an output
  file such as "bar_unittests.exe" (using Windows as an example).

  Sometimes you will want an alternate name to avoid collisions or
  if the internal name isn't appropriate for public distribution.

  The output name should have no extension or prefixes, these will be
  added using the default system rules. For example, on Linux an output
  name of "foo" will produce a shared library "libfoo.so".

  This variable is valid for all binary output target types.

```

### **Example**:
```
  static_library("doom_melon") {
    output_name = "fluffy_bunny"
  }


```
## **outputs**: Output files for actions and copy targets.

```
  Outputs is valid for "copy", "action", and "action_foreach"
  target types and indicates the resulting files. The values may contain
  source expansions to generate the output names from the sources (see
  "gn help source_expansion").

  For copy targets, the outputs is the destination for the copied
  file(s). For actions, the outputs should be the list of files
  generated by the script.


```
## **public**: Declare public header files for a target.

```
  A list of files that other targets can include. These permissions are
  checked via the "check" command (see "gn help check").

  If no public files are declared, other targets (assuming they have
  visibility to depend on this target can include any file in the
  sources list. If this variable is defined on a target, dependent
  targets may only include files on this whitelist.

  Header file permissions are also subject to visibility. A target
  must be visible to another target to include any files from it at all
  and the public headers indicate which subset of those files are
  permitted. See "gn help visibility" for more.

  Public files are inherited through the dependency tree. So if there is
  a dependency A -> B -> C, then A can include C's public headers.
  However, the same is NOT true of visibility, so unless A is in C's
  visibility list, the include will be rejected.

  GN only knows about files declared in the "sources" and "public"
  sections of targets. If a file is included that is not known to the
  build, it will be allowed.

```

### **Examples**:
```
  These exact files are public:
    public = [ "foo.h", "bar.h" ]

  No files are public (no targets may include headers from this one):
    public = []


```
## **public_configs**: Configs to be applied on dependents.

```
  A list of config labels.

  Targets directly depending on this one will have the configs listed in
  this variable added to them. These configs will also apply to the
  current target.

  This addition happens in a second phase once a target and all of its
  dependencies have been resolved. Therefore, a target will not see
  these force-added configs in their "configs" variable while the
  script is running, and then can not be removed. As a result, this
  capability should generally only be used to add defines and include
  directories necessary to compile a target's headers.

  See also "all_dependent_configs".

```

### **Ordering of flags and values**:

```
  1. Those set on the current target (not in a config).
  2. Those set on the "configs" on the target in order that the
     configs appear in the list.
  3. Those set on the "all_dependent_configs" on the target in order
     that the configs appear in the list.
  4. Those set on the "public_configs" on the target in order that
     those configs appear in the list.
  5. all_dependent_configs pulled from dependencies, in the order of
     the "deps" list. This is done recursively. If a config appears
     more than once, only the first occurance will be used.
  6. public_configs pulled from dependencies, in the order of the
     "deps" list. If a dependency is public, they will be applied
     recursively.


```
## **public_deps**: Declare public dependencies.

```
  Public dependencies are like private dependencies ("deps") but
  additionally express that the current target exposes the listed deps
  as part of its public API.

  This has several ramifications:

    - public_configs that are part of the dependency are forwarded
      to direct dependents.

    - Public headers in the dependency are usable by dependents
      (includes do not require a direct dependency or visibility).

    - If the current target is a shared library, other shared libraries
      that it publicly depends on (directly or indirectly) are
      propagated up the dependency tree to dependents for linking.

```

### **Discussion**

```
  Say you have three targets: A -> B -> C. C's visibility may allow
  B to depend on it but not A. Normally, this would prevent A from
  including any headers from C, and C's public_configs would apply
  only to B.

  If B lists C in its public_deps instead of regular deps, A will now
  inherit C's public_configs and the ability to include C's public
  headers.

  Generally if you are writing a target B and you include C's headers
  as part of B's public headers, or targets depending on B should
  consider B and C to be part of a unit, you should use public_deps
  instead of deps.

```

### **Example**

```
  # This target can include files from "c" but not from
  # "super_secret_implementation_details".
  executable("a") {
    deps = [ ":b" ]
  }

  shared_library("b") {
    deps = [ ":super_secret_implementation_details" ]
    public_deps = [ ":c" ]
  }


```
## **script**: Script file for actions.

```
  An absolute or buildfile-relative file name of a Python script to run
  for a action and action_foreach targets (see "gn help action" and
  "gn help action_foreach").


```
## **sources**: Source files for a target

```
  A list of files relative to the current buildfile.


```
## **testonly**: Declares a target must only be used for testing.

```
  Boolean. Defaults to false.

  When a target is marked "testonly = true", it must only be depended
  on by other test-only targets. Otherwise, GN will issue an error
  that the depenedency is not allowed.

  This feature is intended to prevent accidentally shipping test code
  in a final product.

```

### **Example**

```
  source_set("test_support") {
    testonly = true
    ...
  }


```
## **visibility**: A list of labels that can depend on a target.

```
  A list of labels and label patterns that define which targets can
  depend on the current one. These permissions are checked via the
  "check" command (see "gn help check").

  If visibility is not defined, it defaults to public ("*").

  If visibility is defined, only the targets with labels that match it
  can depend on the current target. The empty list means no targets
  can depend on the current target.

  Tip: Often you will want the same visibility for all targets in a
  BUILD file. In this case you can just put the definition at the top,
  outside of any target, and the targets will inherit that scope and see
  the definition.

```

### **Patterns**

```
  See "gn help label_pattern" for more details on what types of
  patterns are supported. If a toolchain is specified, only targets
  in that toolchain will be matched. If a toolchain is not specified on
  a pattern, targets in all toolchains will be matched.

```

### **Examples**

```
  Only targets in the current buildfile ("private"):
    visibility = [ ":*" ]

  No targets (used for targets that should be leaf nodes):
    visibility = []

  Any target ("public", the default):
    visibility = [ "*" ]

  All targets in the current directory and any subdirectory:
    visibility = [ "./*" ]

  Any target in "//bar/BUILD.gn":
    visibility = [ "//bar:*" ]

  Any target in "//bar/" or any subdirectory thereof:
    visibility = [ "//bar/*" ]

  Just these specific targets:
    visibility = [ ":mything", "//foo:something_else" ]

  Any target in the current directory and any subdirectory thereof, plus
  any targets in "//bar/" and any subdirectory thereof.
    visibility = [ "./*", "//bar/*" ]


```
## **Build Arguments Overview**

```
  Build arguments are variables passed in from outside of the build
  that build files can query to determine how the build works.

```

### **How build arguments are set**

```
  First, system default arguments are set based on the current system.
  The built-in arguments are:
   - host_cpu
   - host_os
   - current_cpu
   - current_os
   - target_cpu
   - target_os

  If specified, arguments from the --args command line flag are used. If
  that flag is not specified, args from previous builds in the build
  directory will be used (this is in the file args.gn in the build
  directory).

  Last, for targets being compiled with a non-default toolchain, the
  toolchain overrides are applied. These are specified in the
  toolchain_args section of a toolchain definition. The use-case for
  this is that a toolchain may be building code for a different
  platform, and that it may want to always specify Posix, for example.
  See "gn help toolchain_args" for more.

  If you specify an override for a build argument that never appears in
  a "declare_args" call, a nonfatal error will be displayed.

```

### **Examples**

```
  gn args out/FooBar
      Create the directory out/FooBar and open an editor. You would type
      something like this into that file:
          enable_doom_melon=false
          os="android"

  gn gen out/FooBar --args="enable_doom_melon=true os=\"android\""
      This will overwrite the build directory with the given arguments.
      (Note that the quotes inside the args command will usually need to
      be escaped for your shell to pass through strings values.)

```

### **How build arguments are used**

```
  If you want to use an argument, you use declare_args() and specify
  default values. These default values will apply if none of the steps
  listed in the "How build arguments are set" section above apply to
  the given argument, but the defaults will not override any of these.

  Often, the root build config file will declare global arguments that
  will be passed to all buildfiles. Individual build files can also
  specify arguments that apply only to those files. It is also useful
  to specify build args in an "import"-ed file if you want such
  arguments to apply to multiple buildfiles.


```
## **.gn file**

```
  When gn starts, it will search the current directory and parent
  directories for a file called ".gn". This indicates the source root.
  You can override this detection by using the --root command-line
  argument

  The .gn file in the source root will be executed. The syntax is the
  same as a buildfile, but with very limited build setup-specific
  meaning.

  If you specify --root, by default GN will look for the file .gn in
  that directory. If you want to specify a different file, you can
  additionally pass --dotfile:

    gn gen out/Debug --root=/home/build --dotfile=/home/my_gn_file.gn

```

### **Variables**

```
  buildconfig [required]
      Label of the build config file. This file will be used to set up
      the build file execution environment for each toolchain.

  check_targets [optional]
      A list of labels and label patterns that should be checked when
      running "gn check" or "gn gen --check". If unspecified, all
      targets will be checked. If it is the empty list, no targets will
      be checked.

      The format of this list is identical to that of "visibility"
      so see "gn help visibility" for examples.

  exec_script_whitelist [optional]
      A list of .gn/.gni files (not labels) that have permission to call
      the exec_script function. If this list is defined, calls to
      exec_script will be checked against this list and GN will fail if
      the current file isn't in the list.

      This is to allow the use of exec_script to be restricted since
      is easy to use inappropriately. Wildcards are not supported.
      Files in the secondary_source tree (if defined) should be
      referenced by ignoring the secondary tree and naming them as if
      they are in the main tree.

      If unspecified, the ability to call exec_script is unrestricted.

      Example:
        exec_script_whitelist = [
          "//base/BUILD.gn",
          "//build/my_config.gni",
        ]

  root [optional]
      Label of the root build target. The GN build will start by loading
      the build file containing this target name. This defaults to
      "//:" which will cause the file //BUILD.gn to be loaded.

  secondary_source [optional]
      Label of an alternate directory tree to find input files. When
      searching for a BUILD.gn file (or the build config file discussed
      above), the file will first be looked for in the source root.
      If it's not found, the secondary source root will be checked
      (which would contain a parallel directory hierarchy).

      This behavior is intended to be used when BUILD.gn files can't be
      checked in to certain source directories for whatever reason.

      The secondary source root must be inside the main source tree.

```

### **Example .gn file contents**

```
  buildconfig = "//build/config/BUILDCONFIG.gn"

  check_targets = [
    "//doom_melon/*",  # Check everything in this subtree.
    "//tools:mind_controlling_ant",  # Check this specific target.
  ]

  root = "//:root"

  secondary_source = "//build/config/temporary_buildfiles/"


```
## **GN build language grammar**

### **Tokens**

```
  GN build files are read as sequences of tokens.  While splitting the
  file into tokens, the next token is the longest sequence of characters
  that form a valid token.

```

### **White space and comments**

```
  White space is comprised of spaces (U+0020), horizontal tabs (U+0009),
  carriage returns (U+000D), and newlines (U+000A).

  Comments start at the character "#" and stop at the next newline.

  White space and comments are ignored except that they may separate
  tokens that would otherwise combine into a single token.

```

### **Identifiers**

```
  Identifiers name variables and functions.

      identifier = letter { letter | digit } .
      letter     = "A" ... "Z" | "a" ... "z" | "_" .
      digit      = "0" ... "9" .

```

### **Keywords**

```
  The following keywords are reserved and may not be used as
  identifiers:

          else    false   if      true

```

### **Integer literals**

```
  An integer literal represents a decimal integer value.

      integer = [ "-" ] digit { digit } .

  Leading zeros and negative zero are disallowed.

```

### **String literals**

```
  A string literal represents a string value consisting of the quoted
  characters with possible escape sequences and variable expansions.

      string    = `"` { char | escape | expansion } `"` .
      escape    = `\` ( "$" | `"` | char ) .
      expansion = "$" ( identifier | "{" identifier "}" ) .
      char      = /* any character except "$", `"`, or newline */ .

  After a backslash, certain sequences represent special characters:

          \"    U+0022    quotation mark
          \$    U+0024    dollar sign
          \\    U+005C    backslash

  All other backslashes represent themselves.

```

### **Punctuation**

```
  The following character sequences represent punctuation:

          +       +=      ==      !=      (       )
          -       -=      <       <=      [       ]
          !       =       >       >=      {       }
                          &&      ||      .       ,

```

### **Grammar**

```
  The input tokens form a syntax tree following a context-free grammar:

      File = StatementList .

      Statement     = Assignment | Call | Condition .
      Assignment    = identifier AssignOp Expr .
      Call          = identifier "(" [ ExprList ] ")" [ Block ] .
      Condition     = "if" "(" Expr ")" Block
                      [ "else" ( Condition | Block ) ] .
      Block         = "{" StatementList "}" .
      StatementList = { Statement } .

      Expr        = UnaryExpr | Expr BinaryOp Expr .
      UnaryExpr   = PrimaryExpr | UnaryOp UnaryExpr .
      PrimaryExpr = identifier | integer | string | Call
                  | identifier "[" Expr "]"
                  | identifier "." identifier
                  | "(" Expr ")"
                  | "[" [ ExprList [ "," ] ] "]" .
      ExprList    = Expr { "," Expr } .

      AssignOp = "=" | "+=" | "-=" .
      UnaryOp  = "!" .
      BinaryOp = "+" | "-"                  // highest priority
               | "<" | "<=" | ">" | ">="
               | "==" | "!="
               | "&&"
               | "||" .                     // lowest priority

  All binary operators are left-associative.


```
## **input_conversion**: Specifies how to transform input to a variable.

```
  input_conversion is an argument to read_file and exec_script that
  specifies how the result of the read operation should be converted
  into a variable.

  "" (the default)
      Discard the result and return None.

  "list lines"
      Return the file contents as a list, with a string for each line.
      The newlines will not be present in the result. The last line may
      or may not end in a newline.

      After splitting, each individual line will be trimmed of
      whitespace on both ends.

  "scope"
      Execute the block as GN code and return a scope with the
      resulting values in it. If the input was:
        a = [ "hello.cc", "world.cc" ]
        b = 26
      and you read the result into a variable named "val", then you
      could access contents the "." operator on "val":
        sources = val.a
        some_count = val.b

  "string"
      Return the file contents into a single string.

  "value"
      Parse the input as if it was a literal rvalue in a buildfile.
      Examples of typical program output using this mode:
        [ "foo", "bar" ]     (result will be a list)
      or
        "foo bar"            (result will be a string)
      or
        5                    (result will be an integer)

      Note that if the input is empty, the result will be a null value
      which will produce an error if assigned to a variable.

  "trim ..."
      Prefixing any of the other transformations with the word "trim"
      will result in whitespace being trimmed from the beginning and end
      of the result before processing.

      Examples: "trim string" or "trim list lines"

      Note that "trim value" is useless because the value parser skips
      whitespace anyway.


```
## **Label patterns**

```
  A label pattern is a way of expressing one or more labels in a portion
  of the source tree. They are not general regular expressions.

  They can take the following forms only:

   - Explicit (no wildcard):
       "//foo/bar:baz"
       ":baz"

   - Wildcard target names:
       "//foo/bar:*" (all targets in the //foo/bar/BUILD.gn file)
       ":*"  (all targets in the current build file)

   - Wildcard directory names ("*" is only supported at the end)
       "*"  (all targets)
       "//foo/bar/*"  (all targets in any subdir of //foo/bar)
       "./*"  (all targets in the current build file or sub dirs)

  Any of the above forms can additionally take an explicit toolchain.
  In this case, the toolchain must be fully qualified (no wildcards
  are supported in the toolchain name).

    "//foo:bar(//build/toochain:mac)"
        An explicit target in an explicit toolchain.

    ":*(//build/toolchain/linux:32bit)"
        All targets in the current build file using the 32-bit Linux
        toolchain.

    "//foo/*(//build/toolchain:win)"
        All targets in //foo and any subdirectory using the Windows
        toolchain.


```
## **How Source Expansion Works**

```
  Source expansion is used for the action_foreach and copy target types
  to map source file names to output file names or arguments.

  To perform source expansion in the outputs, GN maps every entry in the
  sources to every entry in the outputs list, producing the cross
  product of all combinations, expanding placeholders (see below).

  Source expansion in the args works similarly, but performing the
  placeholder substitution produces a different set of arguments for
  each invocation of the script.

  If no placeholders are found, the outputs or args list will be treated
  as a static list of literal file names that do not depend on the
  sources.

  See "gn help copy" and "gn help action_foreach" for more on how
  this is applied.

```

### **Placeholders**

```
  {{source}}
      The name of the source file including directory (*). This will
      generally be used for specifying inputs to a script in the
      "args" variable.
        "//foo/bar/baz.txt" => "../../foo/bar/baz.txt"

  {{source_file_part}}
      The file part of the source including the extension.
        "//foo/bar/baz.txt" => "baz.txt"

  {{source_name_part}}
      The filename part of the source file with no directory or
      extension. This will generally be used for specifying a
      transformation from a soruce file to a destination file with the
      same name but different extension.
        "//foo/bar/baz.txt" => "baz"

  {{source_dir}}
      The directory (*) containing the source file with no
      trailing slash.
        "//foo/bar/baz.txt" => "../../foo/bar"

  {{source_root_relative_dir}}
      The path to the source file's directory relative to the source
      root, with no leading "//" or trailing slashes. If the path is
      system-absolute, (beginning in a single slash) this will just
      return the path with no trailing slash. This value will always
      be the same, regardless of whether it appears in the "outputs"
      or "args" section.
        "//foo/bar/baz.txt" => "foo/bar"

  {{source_gen_dir}}
      The generated file directory (*) corresponding to the source
      file's path. This will be different than the target's generated
      file directory if the source file is in a different directory
      than the BUILD.gn file.
        "//foo/bar/baz.txt" => "gen/foo/bar"

  {{source_out_dir}}
      The object file directory (*) corresponding to the source file's
      path, relative to the build directory. this us be different than
      the target's out directory if the source file is in a different
      directory than the build.gn file.
        "//foo/bar/baz.txt" => "obj/foo/bar"

```

### **(*) Note on directories**

```
  Paths containing directories (except the source_root_relative_dir)
  will be different depending on what context the expansion is evaluated
  in. Generally it should "just work" but it means you can't
  concatenate strings containing these values with reasonable results.

  Details: source expansions can be used in the "outputs" variable,
  the "args" variable, and in calls to "process_file_template". The
  "args" are passed to a script which is run from the build directory,
  so these directories will relative to the build directory for the
  script to find. In the other cases, the directories will be source-
  absolute (begin with a "//") because the results of those expansions
  will be handled by GN internally.

```

### **Examples**

```
  Non-varying outputs:
    action("hardcoded_outputs") {
      sources = [ "input1.idl", "input2.idl" ]
      outputs = [ "$target_out_dir/output1.dat",
                  "$target_out_dir/output2.dat" ]
    }
  The outputs in this case will be the two literal files given.

  Varying outputs:
    action_foreach("varying_outputs") {
      sources = [ "input1.idl", "input2.idl" ]
      outputs = [ "{{source_gen_dir}}/{{source_name_part}}.h",
                  "{{source_gen_dir}}/{{source_name_part}}.cc" ]
    }
  Performing source expansion will result in the following output names:
    //out/Debug/obj/mydirectory/input1.h
    //out/Debug/obj/mydirectory/input1.cc
    //out/Debug/obj/mydirectory/input2.h
    //out/Debug/obj/mydirectory/input2.cc


```
**Available global switches
**  Do "gn help --the_switch_you_want_help_on" for more. Individual
  commands may take command-specific switches not listed here. See the
  help on your specific command for more.

```

**  --args**: Specifies build arguments overrides.
**  --color**: Force colored output.
**  --dotfile**: override the name of the ".gn" file.
**  --markdown**: write the output in the Markdown format.
**  --nocolor**: Force non-colored output.
**  -q**: Quiet mode. Don't print output on success.
**  --root**: Explicitly specify source root.
**  --time**: Outputs a summary of how long everything took.
**  --tracelog**: Writes a Chrome-compatible trace log to the given file.
**  -v**: Verbose logging.
**  --version**: Prints the GN version number and exits.

```