# GYP (Generate Your Projects) User Documentation

Status: Draft (as of 2009-05-19)

Mark Mentovai <mark@chromium.org>,
Steven Knight <sgk@chromium.org>
_et al._

Modified: 2009-05-19

[TOC]

## Introduction

This document is intended to provide a user-level guide to GYP.  The
emphasis here is on how to use GYP to accomplish specific tasks, not on
the complete technical language specification.  (For that, see the
[LanguageSpecification](LanguageSpecification).)

The document below starts with some overviews to provide context: an
overview of the structure of a `.gyp` file itself, an overview of a
typical executable-program target in a `.gyp` file, an an overview of a
typical library target in a `.gyp` file.

After the overviews, there are examples of `gyp` patterns for different
common use cases.

## Skeleton of a typical Chromium .gyp file

Here is the skeleton of a typical `.gyp` file in the Chromium tree:

```
  {
    'variables': {
      .
      .
      .
    },
    'includes': [
      '../build/common.gypi',
    ],
    'target_defaults': {
      .
      .
      .
    },
    'targets': [
      {
        'target_name': 'target_1',
          .
          .
          .
      },
      {
        'target_name': 'target_2',
          .
          .
          .
      },
    ],
    'conditions': [
      ['OS=="linux"', {
        'targets': [
          {
            'target_name': 'linux_target_3',
              .
              .
              .
          },
        ],
      }],
      ['OS=="win"', {
        'targets': [
          {
            'target_name': 'windows_target_4',
              .
              .
              .
          },
        ],
      }, { # OS != "win"
        'targets': [
          {
            'target_name': 'non_windows_target_5',
              .
              .
              .
          },
      }],
    ],
  }
```

The entire file just contains a Python dictionary.  (It's actually JSON,
with two small Pythonic deviations: comments are introduced with `#`,
and a `,` (comma)) is legal after the last element in a list or
dictionary.)

The top-level pieces in the `.gyp` file are as follows:

`'variables'`:  Definitions of variables that can be interpolated and
used in various other parts of the file.

`'includes'`:  A list of of other files that will be included in this
file.  By convention, included files have the suffix `.gypi` (gyp
include).

`'target_defaults'`:  Settings that will apply to _all_ of the targets
defined in this `.gyp` file.

`'targets'`:  The list of targets for which this `.gyp` file can
generate builds.  Each target is a dictionary that contains settings
describing all the information necessary to build the target.

`'conditions'`:  A list of condition specifications that can modify the
contents of the items in the global dictionary defined by this `.gyp`
file based on the values of different variablwes.  As implied by the
above example, the most common use of a `conditions` section in the
top-level dictionary is to add platform-specific targets to the
`targets` list.

## Skeleton of a typical executable target in a .gyp file

The most straightforward target is probably a simple executable program.
Here is an example `executable` target that demonstrates the features
that should cover most simple uses of gyp:

```
  {
    'targets': [
      {
        'target_name': 'foo',
        'type': 'executable',
        'msvs_guid': '5ECEC9E5-8F23-47B6-93E0-C3B328B3BE65',
        'dependencies': [
          'xyzzy',
          '../bar/bar.gyp:bar',
        ],
        'defines': [
          'DEFINE_FOO',
          'DEFINE_A_VALUE=value',
        ],
        'include_dirs': [
          '..',
        ],
        'sources': [
          'file1.cc',
          'file2.cc',
        ],
        'conditions': [
          ['OS=="linux"', {
            'defines': [
              'LINUX_DEFINE',
            ],
            'include_dirs': [
              'include/linux',
            ],
          }],
          ['OS=="win"', {
            'defines': [
              'WINDOWS_SPECIFIC_DEFINE',
            ],
          }, { # OS != "win",
            'defines': [
              'NON_WINDOWS_DEFINE',
            ],
          }]
        ],
      },
    ],
  }
```

The top-level settings in the target include:

`'target_name'`: The name by which the target should be known, which
should be unique across all `.gyp` files.  This name will be used as the
project name in the generated Visual Studio solution, as the target name
in the generated XCode configuration, and as the alias for building this
target from the command line of the generated SCons configuration.

`'type'`: Set to `executable`, logically enough.

`'msvs_guid'`: THIS IS ONLY TRANSITIONAL.  This is a hard-coded GUID
values that will be used in the generated Visual Studio solution
file(s).  This allows us to check in a `chrome.sln` file that
interoperates with gyp-generated project files.  Once everything in
Chromium is being generated by gyp, it will no longer be important that
the GUIDs stay constant across invocations, and we'll likely get rid of
these settings,

`'dependencies'`: This lists other targets that this target depends on.
The gyp-generated files will guarantee that the other targets are built
before this target.  Any library targets in the `dependencies` list will
be linked with this target.  The various settings (`defines`,
`include_dirs`, etc.) listed in the `direct_dependent_settings` sections
of the targets in this list will be applied to how _this_ target is
built and linked.  See the more complete discussion of
`direct_dependent_settings`, below.

`'defines'`: The C preprocessor definitions that will be passed in on
compilation command lines (using `-D` or `/D` options).

`'include_dirs'`: The directories in which included header files live.
These will be passed in on compilation command lines (using `-I` or `/I`
options).

`'sources'`: The source files for this target.

`'conditions'`: A block of conditions that will be evaluated to update
the different settings in the target dictionary.

## Skeleton of a typical library target in a .gyp file

The vast majority of targets are libraries.  Here is an example of a
library target including the additional features that should cover most
needs of libraries:

```
  {
    'targets': [
      {
        'target_name': 'foo',
        'type': '<(library)'
        'msvs_guid': '5ECEC9E5-8F23-47B6-93E0-C3B328B3BE65',
        'dependencies': [
          'xyzzy',
          '../bar/bar.gyp:bar',
        ],
        'defines': [
          'DEFINE_FOO',
          'DEFINE_A_VALUE=value',
        ],
        'include_dirs': [
          '..',
        ],
        'direct_dependent_settings': {
          'defines': [
            'DEFINE_FOO',
            'DEFINE_ADDITIONAL',
          ],
          'linkflags': [
          ],
        },
        'export_dependent_settings': [
          '../bar/bar.gyp:bar',
        ],
        'sources': [
          'file1.cc',
          'file2.cc',
        ],
        'conditions': [
          ['OS=="linux"', {
            'defines': [
              'LINUX_DEFINE',
            ],
            'include_dirs': [
              'include/linux',
            ],
          ],
          ['OS=="win"', {
            'defines': [
              'WINDOWS_SPECIFIC_DEFINE',
            ],
          }, { # OS != "win",
            'defines': [
              'NON_WINDOWS_DEFINE',
            ],
          }]
        ],
    ],
  }
```

The possible entries in a library target are largely the same as those
that can be specified for an executable target (`defines`,
`include_dirs`, etc.).  The differences include:

`'type'`: This should almost always be set to '<(library)', which allows
the user to define at gyp time whether libraries are to be built static
or shared.  (On Linux, at least, linking with shared libraries saves
significant link time.) If it's necessary to pin down the type of
library to be built, the `type` can be set explicitly to
`static_library` or `shared_library`.

`'direct_dependent_settings'`: This defines the settings that will be
applied to other targets that _directly depend_ on this target--that is,
that list _this_ target in their `'dependencies'` setting.  This is
where you list the `defines`, `include_dirs`, `cflags` and `linkflags`
that other targets that compile or link against this target need to
build consistently.

`'export_dependent_settings'`: This lists the targets whose
`direct_dependent_settings` should be "passed on" to other targets that
use (depend on) this target.  `TODO:  expand on this description.`

## Use Cases

These use cases are intended to cover the most common actions performed
by developers using GYP.

Note that these examples are _not_ fully-functioning, self-contained
examples (or else they'd be way too long).  Each example mostly contains
just the keywords and settings relevant to the example, with perhaps a
few extra keywords for context.  The intent is to try to show the
specific pieces you need to pay attention to when doing something.
[NOTE:  if practical use shows that these examples are confusing without
additional context, please add what's necessary to clarify things.]

### Add new source files

There are similar but slightly different patterns for adding a
platform-independent source file vs. adding a source file that only
builds on some of the supported platforms.

#### Add a source file that builds on all platforms

**Simplest possible case**: You are adding a file(s) that builds on all
platforms.

Just add the file(s) to the `sources` list of the appropriate dictionary
in the `targets` list:

```
  {
    'targets': [
      {
        'target_name': 'my_target',
        'type': 'executable',
        'sources': [
          '../other/file_1.cc',
          'new_file.cc',
          'subdir/file3.cc',
        ],
      },
    ],
  },
```

File path names are relative to the directory in which the `.gyp` file lives.

Keep the list sorted alphabetically (unless there's a really, really,
_really_ good reason not to).

#### Add a platform-specific source file

##### Your platform-specific file is named `*_linux.{ext}`, `*_mac.{ext}`, `*_posix.{ext}` or `*_win.{ext}` 

The simplest way to add a platform-specific source file, assuming you're
adding a completely new file and get to name it, is to use one of the
following standard suffixes:

  * `_linux`  (e.g. `foo_linux.cc`)
  * `_mac`    (e.g. `foo_mac.cc`)
  * `_posix`  (e.g. `foo_posix.cc`)
  * `_win`    (e.g. `foo_win.cc`)

Simply add the file to the `sources` list of the appropriate dict within
the `targets` list, like you would any other source file.

```
  {
    'targets': [
      {
        'target_name': 'foo',
        'type': 'executable',
        'sources': [
          'independent.cc',
          'specific_win.cc',
        ],
      },
    ],
  },
```

The Chromium `.gyp` files all have appropriate `conditions` entries to
filter out the files that aren't appropriate for the current platform.
In the above example, the `specific_win.cc` file will be removed
automatically from the source-list on non-Windows builds.

##### Your platform-specific file does not use an already-defined pattern

If your platform-specific file does not contain a
`*_{linux,mac,posix,win}` substring (or some other pattern that's
already in the `conditions` for the target), and you can't change the
file name, there are two patterns that can be used.

**Prefererred**:  Add the file to the `sources` list of the appropriate
dictionary within the `targets` list.  Add an appropriate `conditions`
section to exclude the specific files name:

```
  {
    'targets': [
      {
        'target_name': 'foo',
        'type': 'executable',
        'sources': [
          'linux_specific.cc',
        ],
        'conditions': [
          ['OS != "linux"', {
            'sources!': [
              # Linux-only; exclude on other platforms.
              'linux_specific.cc',
            ]
          }[,
        ],
      },
    ],
  },
```

Despite the duplicate listing, the above is generally preferred because
the `sources` list contains a useful global list of all sources on all
platforms with consistent sorting on all platforms.

**Non-preferred**: In some situations, however, it might make sense to
list a platform-specific file only in a `conditions` section that
specifically _includes_ it in the `sources` list:

```
  {
    'targets': [
      {
        'target_name': 'foo',
        'type': 'executable',
        'sources': [],
        ['OS == "linux"', {
          'sources': [
            # Only add to sources list on Linux.
            'linux_specific.cc',
          ]
        }],
      },
    ],
  },
```

The above two examples end up generating equivalent builds, with the
small exception that the `sources` lists will list the files in
different orders.  (The first example defines explicitly where
`linux_specific.cc` appears in the list--perhaps in in the
middle--whereas the second example will always tack it on to the end of
the list.)

**Including or excluding files using patterns**: There are more
complicated ways to construct a `sources` list based on patterns.  See
`TODO(sgk)` below.

### Add a new executable

An executable program is probably the most straightforward type of
target, since all it typically needs is a list of source files, some
compiler/linker settings (probably varied by platform), and some library
targets on which it depends and which must be used in the final link.

#### Add an executable that builds on all platforms

Add a dictionary defining the new executable target to the `targets`
list in the appropriate `.gyp` file.  Example:

```
  {
    'targets': [
      {
        'target_name': 'new_unit_tests',
        'type': 'executable',
        'defines': [
          'FOO',
        ],
        'include_dirs': [
          '..',
        ],
        'dependencies': [
          'other_target_in_this_file',
          'other_gyp2:target_in_other_gyp2',
        ],
        'sources': [
          'new_additional_source.cc',
          'new_unit_tests.cc',
        ],
      },
    ],
  }
```

#### Add a platform-specific executable

Add a dictionary defining the new executable target to the `targets`
list within an appropriate `conditions` block for the platform.  The
`conditions` block should be a sibling to the top-level `targets` list:

```
  {
    'targets': [
    ],
    'conditions': [
      ['OS=="win"', {
        'targets': [
          {
            'target_name': 'new_unit_tests',
            'type': 'executable',
            'defines': [
              'FOO',
            ],
            'include_dirs': [
              '..',
            ],
            'dependencies': [
              'other_target_in_this_file',
              'other_gyp2:target_in_other_gyp2',
            ],
            'sources': [
              'new_additional_source.cc',
              'new_unit_tests.cc',
            ],
          },
        ],
      }],
    ],
  }
```

### Add settings to a target

There are several different types of settings that can be defined for
any given target.

#### Add new preprocessor definitions (`-D` or `/D` flags)

New preprocessor definitions are added by the `defines` setting:

```
  {
    'targets': [
      {
        'target_name': 'existing_target',
        'defines': [
          'FOO',
          'BAR=some_value',
        ],
      },
    ],
  },
```

These may be specified directly in a target's settings, as in the above
example, or in a `conditions` section.

#### Add a new include directory (`-I` or `/I` flags)

New include directories are added by the `include_dirs` setting:

```
  {
    'targets': [
      {
        'target_name': 'existing_target',
        'include_dirs': [
          '..',
          'include',
        ],
      },
    ],
  },
```

These may be specified directly in a target's settings, as in the above
example, or in a `conditions` section.

#### Add new compiler flags

Specific compiler flags can be added with the `cflags` setting:

```
  {
    'targets': [
      {
        'target_name': 'existing_target',
        'conditions': [
          ['OS=="win"', {
            'cflags': [
              '/WX',
            ],
          }, { # OS != "win"
            'cflags': [
              '-Werror',
            ],
          }],
        ],
      },
    ],
  },
```

Because these flags will be specific to the actual compiler involved,
they will almost always be only set within a `conditions` section.

#### Add new linker flags

Setting linker flags is OS-specific. On linux and most non-mac posix
systems, they can be added with the `ldflags` setting:

```
  {
    'targets': [
      {
        'target_name': 'existing_target',
        'conditions': [
          ['OS=="linux"', {
            'ldflags': [
              '-pthread',
            ],
          }],
        ],
      },
    ],
  },
```

Because these flags will be specific to the actual linker involved,
they will almost always be only set within a `conditions` section.

On OS X, linker settings are set via `xcode_settings`, on Windows via
`msvs_settings`.

#### Exclude settings on a platform

Any given settings keyword (`defines`, `include_dirs`, etc.) has a
corresponding form with a trailing `!` (exclamation point) to remove
values from a setting.  One useful example of this is to remove the
Linux `-Werror` flag from the global settings defined in
`build/common.gypi`:

```
  {
    'targets': [
      {
        'target_name': 'third_party_target',
        'conditions': [
          ['OS=="linux"', {
            'cflags!': [
              '-Werror',
            ],
          }],
        ],
      },
    ],
  },
```

### Cross-compiling

GYP has some (relatively limited) support for cross-compiling.

If the variable `GYP_CROSSCOMPILE` or one of the toolchain-related
variables (like `CC_host` or `CC_target`) is set, GYP will think that
you wish to do a cross-compile.

When cross-compiling, each target can be part of a "host" build, a
"target" build, or both. By default, the target is assumed to be (only)
part of the "target" build. The 'toolsets' property can be set on a
target to change the default.

A target's dependencies are assumed to match the build type (so, if A
depends on B, by default that means that a target build of A depends on
a target build of B). You can explicitly depend on targets across
toolchains by specifying "#host" or "#target" in the dependencies list.
If GYP is not doing a cross-compile, the "#host" and "#target" will be
stripped as needed, so nothing breaks.

### Add a new library

TODO:  write intro

#### Add a library that builds on all platforms

Add the a dictionary defining the new library target to the `targets`
list in the appropriate `.gyp` file.  Example:

```
  {
    'targets': [
      {
        'target_name': 'new_library',
        'type': '<(library)',
        'defines': [
          'FOO',
          'BAR=some_value',
        ],
        'include_dirs': [
          '..',
        ],
        'dependencies': [
          'other_target_in_this_file',
          'other_gyp2:target_in_other_gyp2',
        ],
        'direct_dependent_settings': {
          'include_dirs': '.',
        },
        'export_dependent_settings': [
          'other_target_in_this_file',
        ],
        'sources': [
          'new_additional_source.cc',
          'new_library.cc',
        ],
      },
    ],
  }
```

The use of the `<(library)` variable above should be the default `type`
setting for most library targets, as it allows the developer to choose,
at `gyp` time, whether to build with static or shared libraries.
(Building with shared libraries saves a _lot_ of link time on Linux.)

It may be necessary to build a specific library as a fixed type.  Is so,
the `type` field can be hard-wired appropriately.  For a static library:

```
        'type': 'static_library',
```

For a shared library:

```
        'type': 'shared_library',
```

#### Add a platform-specific library

Add a dictionary defining the new library target to the `targets` list
within a `conditions` block that's a sibling to the top-level `targets`
list:

```
  {
    'targets': [
    ],
    'conditions': [
      ['OS=="win"', {
        'targets': [
          {
            'target_name': 'new_library',
            'type': '<(library)',
            'defines': [
              'FOO',
              'BAR=some_value',
            ],
            'include_dirs': [
              '..',
            ],
            'dependencies': [
              'other_target_in_this_file',
              'other_gyp2:target_in_other_gyp2',
            ],
            'direct_dependent_settings': {
              'include_dirs': '.',
            },
            'export_dependent_settings': [
              'other_target_in_this_file',
            ],
            'sources': [
              'new_additional_source.cc',
              'new_library.cc',
            ],
          },
        ],
      }],
    ],
  }
```

### Dependencies between targets

GYP provides useful primitives for establishing dependencies between
targets, which need to be configured in the following situations.

#### Linking with another library target

```
  {
    'targets': [
      {
        'target_name': 'foo',
        'dependencies': [
          'libbar',
        ],
      },
      {
        'target_name': 'libbar',
        'type': '<(library)',
        'sources': [
        ],
      },
    ],
  }
```

Note that if the library target is in a different `.gyp` file, you have
to specify the path to other `.gyp` file, relative to this `.gyp` file's
directory:

```
  {
    'targets': [
      {
        'target_name': 'foo',
        'dependencies': [
          '../bar/bar.gyp:libbar',
        ],
      },
    ],
  }
```

Adding a library often involves updating multiple `.gyp` files, adding
the target to the approprate `.gyp` file (possibly a newly-added `.gyp`
file), and updating targets in the other `.gyp` files that depend on
(link with) the new library.

#### Compiling with necessary flags for a library target dependency

We need to build a library (often a third-party library) with specific
preprocessor definitions or command-line flags, and need to ensure that
targets that depend on the library build with the same settings.  This
situation is handled by a `direct_dependent_settings` block:

```
  {
    'targets': [
      {
        'target_name': 'foo',
        'type': 'executable',
        'dependencies': [
          'libbar',
        ],
      },
      {
        'target_name': 'libbar',
        'type': '<(library)',
        'defines': [
          'LOCAL_DEFINE_FOR_LIBBAR',
          'DEFINE_TO_USE_LIBBAR',
        ],
        'include_dirs': [
          '..',
          'include/libbar',
        ],
        'direct_dependent_settings': {
          'defines': [
            'DEFINE_TO_USE_LIBBAR',
          ],
          'include_dirs': [
            'include/libbar',
          ],
        },
      },
    ],
  }
```

In the above example, the sources of the `foo` executable will be
compiled with the options `-DDEFINE_TO_USE_LIBBAR -Iinclude/libbar`,
because of those settings' being listed in the
`direct_dependent_settings` block.

Note that these settings will likely need to be replicated in the
settings for the library target itsef, so that the library will build
with the same options.  This does not prevent the target from defining
additional options for its "internal" use when compiling its own source
files.  (In the above example, these are the `LOCAL_DEFINE_FOR_LIBBAR`
define, and the `..` entry in the `include_dirs` list.)

#### When a library depends on an additional library at final link time

```
  {
    'targets': [
      {
        'target_name': 'foo',
        'type': 'executable',
        'dependencies': [
          'libbar',
        ],
      },
      {
        'target_name': 'libbar',
        'type': '<(library)',
        'dependencies': [
          'libother'
        ],
        'export_dependent_settings': [
          'libother'
        ],
      },
      {
        'target_name': 'libother',
        'type': '<(library)',
        'direct_dependent_settings': {
          'defines': [
            'DEFINE_FOR_LIBOTHER',
          ],
          'include_dirs': [
            'include/libother',
          ],
        },
      },
    ],
  }
```

### Support for Mac OS X bundles

gyp supports building bundles on OS X (.app, .framework, .bundle, etc).
Here is an example of this:

```
    {
      'target_name': 'test_app',
      'product_name': 'Test App Gyp',
      'type': 'executable',
      'mac_bundle': 1,
      'sources': [
        'main.m',
        'TestAppAppDelegate.h',
        'TestAppAppDelegate.m',
      ],
      'mac_bundle_resources': [
        'TestApp/English.lproj/InfoPlist.strings',
        'TestApp/English.lproj/MainMenu.xib',
      ],
      'link_settings': {
        'libraries': [
          '$(SDKROOT)/System/Library/Frameworks/Cocoa.framework',
        ],
      },
      'xcode_settings': {
        'INFOPLIST_FILE': 'TestApp/TestApp-Info.plist',
      },
    },
```

The `mac_bundle` key tells gyp that this target should be a bundle.
`executable` targets get extension `.app` by default, `shared_library`
targets get `.framework` – but you can change the bundle extensions by
setting `product_extension` if you want. Files listed in
`mac_bundle_resources` will be copied to the bundle's `Resource` folder
of the bundle. You can also set
`process_outputs_as_mac_bundle_resources` to 1 in actions and rules to
let the output of actions and rules be added to that folder (similar to
`process_outputs_as_sources`). If `product_name` is not set, the bundle
will be named after `target_name`as usual.

### Move files (refactoring)

TODO(sgk)

### Custom build steps

TODO(sgk)

#### Adding an explicit build step to generate specific files

TODO(sgk)

#### Adding a rule to handle files with a new suffix

TODO(sgk)

### Build flavors

TODO(sgk)
