docs/Modules.rst

=======
Modules
=======

.. contents::
   :local:

.. warning::
   The functionality described on this page is still experimental! Please
   try it out and send us bug reports!

Introduction
============
Most software is built using a number of software libraries, including libraries supplied by the platform, internal libraries built as part of the software itself to provide structure, and third-party libraries. For each library, one needs to access both its interface (API) and its implementation. In the C family of languages, the interface to a library is accessed by including the appropriate header files(s):

.. code-block:: c

  #include <SomeLib.h>

The implementation is handled separately by linking against the appropriate library. For example, by passing ``-lSomeLib`` to the linker.

Modules provide an alternative, simpler way to use software libraries that provides better compile-time scalability and eliminates many of the problems inherent to using the C preprocessor to access the API of a library.

Problems with the current model
-------------------------------
The ``#include`` mechanism provided by the C preprocessor is a very poor way to access the API of a library, for a number of reasons:

* **Compile-time scalability**: Each time a header is included, the
  compiler must preprocess and parse the text in that header and every
  header it includes, transitively. This process must be repeated for
  every translation unit in the application, which involves a huge
  amount of redundant work. In a project with *N* translation units
  and *M* headers included in each translation unit, the compiler is
  performing *M x N* work even though most of the *M* headers are
  shared among multiple translation units. C++ is particularly bad,
  because the compilation model for templates forces a huge amount of
  code into headers.

* **Fragility**: ``#include`` directives are treated as textual
  inclusion by the preprocessor, and are therefore subject to any  
  active macro definitions at the time of inclusion. If any of the 
  active macro definitions happens to collide with a name in the 
  library, it can break the library API or cause compilation failures 
  in the library header itself. For an extreme example, 
  ``#define std "The C++ Standard"`` and then include a standard  
  library header: the result is a horrific cascade of failures in the
  C++ Standard Library's implementation. More subtle real-world
  problems occur when the headers for two different libraries interact
  due to macro collisions, and users are forced to reorder
  ``#include`` directives or introduce ``#undef`` directives to break
  the (unintended) dependency.

* **Conventional workarounds**: C programmers have
  adopted a number of conventions to work around the fragility of the
  C preprocessor model. Include guards, for example, are required for
  the vast majority of headers to ensure that multiple inclusion
  doesn't break the compile. Macro names are written with
  ``LONG_PREFIXED_UPPERCASE_IDENTIFIERS`` to avoid collisions, and some
  library/framework developers even use ``__underscored`` names
  in headers to avoid collisions with "normal" names that (by
  convention) shouldn't even be macros. These conventions are a
  barrier to entry for developers coming from non-C languages, are
  boilerplate for more experienced developers, and make our headers
  far uglier than they should be.

* **Tool confusion**: In a C-based language, it is hard to build tools
  that work well with software libraries, because the boundaries of
  the libraries are not clear. Which headers belong to a particular
  library, and in what order should those headers be included to
  guarantee that they compile correctly? Are the headers C, C++,
  Objective-C++, or one of the variants of these languages? What
  declarations in those headers are actually meant to be part of the
  API, and what declarations are present only because they had to be
  written as part of the header file?

Semantic import
---------------
Modules improve access to the API of software libraries by replacing the textual preprocessor inclusion model with a more robust, more efficient semantic model. From the user's perspective, the code looks only slightly different, because one uses an ``import`` declaration rather than a ``#include`` preprocessor directive:

.. code-block:: c

  import std.io; // pseudo-code; see below for syntax discussion

However, this module import behaves quite differently from the corresponding ``#include <stdio.h>``: when the compiler sees the module import above, it loads a binary representation of the ``std.io`` module and makes its API available to the application directly. Preprocessor definitions that precede the import declaration have no impact on the API provided by ``std.io``, because the module itself was compiled as a separate, standalone module. Additionally, any linker flags required to use the ``std.io`` module will automatically be provided when the module is imported [#]_
This semantic import model addresses many of the problems of the preprocessor inclusion model:

* **Compile-time scalability**: The ``std.io`` module is only compiled once, and importing the module into a translation unit is a constant-time operation (independent of module system). Thus, the API of each software library is only parsed once, reducing the *M x N* compilation problem to an *M + N* problem.

* **Fragility**: Each module is parsed as a standalone entity, so it has a consistent preprocessor environment. This completely eliminates the need for ``__underscored`` names and similarly defensive tricks. Moreover, the current preprocessor definitions when an import declaration is encountered are ignored, so one software library can not affect how another software library is compiled, eliminating include-order dependencies.

* **Tool confusion**: Modules describe the API of software libraries, and tools can reason about and present a module as a representation of that API. Because modules can only be built standalone, tools can rely on the module definition to ensure that they get the complete API for the library. Moreover, modules can specify which languages they work with, so, e.g., one can not accidentally attempt to load a C++ module into a C program.

Problems modules do not solve
-----------------------------
Many programming languages have a module or package system, and because of the variety of features provided by these languages it is important to define what modules do *not* do. In particular, all of the following are considered out-of-scope for modules:

* **Rewrite the world's code**: It is not realistic to require applications or software libraries to make drastic or non-backward-compatible changes, nor is it feasible to completely eliminate headers. Modules must interoperate with existing software libraries and allow a gradual transition.

* **Versioning**: Modules have no notion of version information. Programmers must still rely on the existing versioning mechanisms of the underlying language (if any exist) to version software libraries.

* **Namespaces**: Unlike in some languages, modules do not imply any notion of namespaces. Thus, a struct declared in one module will still conflict with a struct of the same name declared in a different module, just as they would if declared in two different headers. This aspect is important for backward compatibility, because (for example) the mangled names of entities in software libraries must not change when introducing modules.

* **Binary distribution of modules**: Headers (particularly C++ headers) expose the full complexity of the language. Maintaining a stable binary module format across architectures, compiler versions, and compiler vendors is technically infeasible.

Using Modules
=============
To enable modules, pass the command-line flag ``-fmodules`` [#]_. This will make any modules-enabled software libraries available as modules as well as introducing any modules-specific syntax. Additional `command-line parameters`_ are described in a separate section later.

Import declaration
------------------
The most direct way to import a module is with an *import declaration*, which imports the named module:

.. parsed-literal::

  import std;

The import declaration above imports the entire contents of the ``std`` module (which would contain, e.g., the entire C or C++ standard library) and make its API available within the current translation unit. To import only part of a module, one may use dot syntax to specific a particular submodule, e.g.,

.. parsed-literal::

  import std.io;

Redundant import declarations are ignored, and one is free to import modules at any point within the translation unit, so long as the import declaration is at global scope.

.. warning::
  The import declaration syntax described here does not actually exist. Rather, it is a straw man proposal that may very well change when modules are discussed in the C and C++ committees. See the section `Includes as imports`_ to see how modules get imported today.

Includes as imports
-------------------
The primary user-level feature of modules is the import operation, which provides access to the API of software libraries. However, today's programs make extensive use of ``#include``, and it is unrealistic to assume that all of this code will change overnight. Instead, modules automatically translate ``#include`` directives into the corresponding module import. For example, the include directive

.. code-block:: c

  #include <stdio.h>

will be automatically mapped to an import of the module ``std.io``. Even with specific ``import`` syntax in the language, this particular feature is important for both adoption and backward compatibility: automatic translation of ``#include`` to ``import`` allows an application to get the benefits of modules (for all modules-enabled libraries) without any changes to the application itself. Thus, users can easily use modules with one compiler while falling back to the preprocessor-inclusion mechanism with other compilers.

.. note::

  The automatic mapping of ``#include`` to ``import`` also solves an implementation problem: importing a module with a definition of some entity (say, a ``struct Point``) and then parsing a header containing another definition of ``struct Point`` would cause a redefinition error, even if it is the same ``struct Point``. By mapping ``#include`` to ``import``, the compiler can guarantee that it always sees just the already-parsed definition from the module.

Module maps
-----------
The crucial link between modules and headers is described by a *module map*, which describes how a collection of existing headers maps on to the (logical) structure of a module. For example, one could imagine a module ``std`` covering the C standard library. Each of the C standard library headers (``<stdio.h>``, ``<stdlib.h>``, ``<math.h>``, etc.) would contribute to the ``std`` module, by placing their respective APIs into the corresponding submodule (``std.io``, ``std.lib``, ``std.math``, etc.). Having a list of the headers that are part of the ``std`` module allows the compiler to build the ``std`` module as a standalone entity, and having the mapping from header names to (sub)modules allows the automatic translation of ``#include`` directives to module imports.

Module maps are specified as separate files (each named ``module.map``) alongside the headers they describe, which allows them to be added to existing software libraries without having to change the library headers themselves (in most cases [#]_). The actual `Module map language`_ is described in a later section.

.. note::

  To actually see any benefits from modules, one first has to introduce module maps for the underlying C standard library and the libraries and headers on which it depends. The section `Modularizing a Platform`_ describes the steps one must take to write these module maps.

Compilation model
-----------------
The binary representation of modules is automatically generated by the compiler on an as-needed basis. When a module is imported (e.g., by an ``#include`` of one of the module's headers), the compiler will spawn a second instance of itself [#]_, with a fresh preprocessing context [#]_, to parse just the headers in that module. The resulting Abstract Syntax Tree (AST) is then persisted into the binary representation of the module that is then loaded into translation unit where the module import was encountered.

The binary representation of modules is persisted in the *module cache*. Imports of a module will first query the module cache and, if a binary representation of the required module is already available, will load that representation directly. Thus, a module's headers will only be parsed once per language configuration, rather than once per translation unit that uses the module.

Modules maintain references to each of the headers that were part of the module build. If any of those headers changes, or if any of the modules on which a module depends change, then the module will be (automatically) recompiled. The process should never require any user intervention.

Command-line parameters
-----------------------
``-fmodules``
  Enable the modules feature (EXPERIMENTAL).

``-fcxx-modules``
  Enable the modules feature for C++ (EXPERIMENTAL and VERY BROKEN).

``-fmodules-cache-path=<directory>``
  Specify the path to the modules cache. If not provided, Clang will select a system-appropriate default.

``-f[no-]modules-autolink``
  Enable of disable automatic linking against the libraries associated with imported modules.

``-fmodules-ignore-macro=macroname``
  Instruct modules to ignore the named macro when selecting an appropriate module variant. Use this for macros defined on the command line that don't affect how modules are built, to improve sharing of compiled module files.

Module Map Language
===================

The module map language describes the mapping from header files to the
logical structure of modules. To enable support for using a library as
a module, one must write a ``module.map`` file for that library. The
``module.map`` file is placed alongside the header files themselves,
and is written in the module map language described below.

As an example, the module map file for the C standard library might look a bit like this:

.. parsed-literal::

  module std [system] {
    module complex {
      header "complex.h"
      export *
    }

    module ctype {
      header "ctype.h"
      export *
    }

    module errno {
      header "errno.h"
      header "sys/errno.h"
      export *
    }

    module fenv {
      header "fenv.h"
      export *
    }

    // ...more headers follow...
  }

Here, the top-level module ``std`` encompasses the whole C standard library. It has a number of submodules containing different parts of the standard library: ``complex`` for complex numbers, ``ctype`` for character types, etc. Each submodule lists one of more headers that provide the contents for that submodule. Finally, the ``export *`` command specifies that anything included by that submodule will be automatically re-exported. 

Lexical structure
-----------------
Module map files use a simplified form of the C99 lexer, with the same rules for identifiers, tokens, string literals, ``/* */`` and ``//`` comments. The module map language has the following reserved words; all other C identifiers are valid identifiers.

.. parsed-literal::

  ``config_macros`` ``export``     ``module``
  ``conflict``      ``framework``  ``requires``
  ``exclude``       ``header``     ``umbrella``
  ``explicit``      ``link``

Module map file
---------------
A module map file consists of a series of module declarations:

.. parsed-literal::

  *module-map-file*:
    *module-declaration**

Within a module map file, modules are referred to by a *module-id*, which uses periods to separate each part of a module's name:

.. parsed-literal::

  *module-id*:
    *identifier* (',' *identifier*)*

Module declaration
------------------
A module declaration describes a module, including the headers that contribute to that module, its submodules, and other aspects of the module.

.. parsed-literal::

  *module-declaration*:
    ``explicit``:sub:`opt` ``framework``:sub:`opt` ``module`` *module-id* *attributes*:sub:`opt` '{' *module-member** '}'

The *module-id* should consist of only a single *identifier*, which provides the name of the module being defined. Each module shall have a single definition. 

The ``explicit`` qualifier can only be applied to a submodule, i.e., a module that is nested within another module. The contents of explicit submodules are only made available when the submodule itself was explicitly named in an import declaration or was re-exported from an imported module.

The ``framework`` qualifier specifies that this module corresponds to a Darwin-style framework. A Darwin-style framework (used primarily on Mac OS X and iOS) is contained entirely in directory ``Name.framework``, where ``Name`` is the name of the framework (and, therefore, the name of the module). That directory has the following layout:

.. parsed-literal::

  Name.framework/
    module.map                Module map for the framework
    Headers/                  Subdirectory containing framework headers
    Frameworks/               Subdirectory containing embedded frameworks
    Resources/                Subdirectory containing additional resources
    Name                      Symbolic link to the shared library for the framework

The ``system`` attribute specifies that the module is a system module. When a system module is rebuilt, all of the module's header will be considered system headers, which suppresses warnings. This is equivalent to placing ``#pragma GCC system_header`` in each of the module's headers. The form of attributes is described in the section Attributes_, below.

Modules can have a number of different kinds of members, each of which is described below:

.. parsed-literal:

  *module-member*:
    *requires-declaration*
    *header-declaration*
    *umbrella-dir-declaration*
    *submodule-declaration*
    *export-declaration*
    *link-declaration*
    *config-macros-declaration*
    *conflict-declaration*

Requires declaration
~~~~~~~~~~~~~~~~~~~~
A *requires-declaration* specifies the requirements that an importing translation unit must satisfy to use the module.

.. parsed-literal::

  *requires-declaration*:
    ``requires`` *feature-list*

  *feature-list*:
    *identifier* (',' *identifier*)*

The requirements clause allows specific modules or submodules to specify that they are only accessible with certain language dialects or on certain platforms. The feature list is a set of identifiers, defined below. If any of the features is not available in a given translation unit, that translation unit shall not import the module.

The following features are defined:

altivec
  The target supports AltiVec.

blocks
  The "blocks" language feature is available.

cplusplus
  C++ support is available.

cplusplus11
  C++11 support is available.

objc
  Objective-C support is available.

objc_arc
  Objective-C Automatic Reference Counting (ARC) is available

opencl
  OpenCL is available

tls
  Thread local storage is available.

*target feature*
  A specific target feature (e.g., ``sse4``, ``avx``, ``neon``) is available.


**Example**: The ``std`` module can be extended to also include C++ and C++11 headers using a *requires-declaration*:

.. parsed-literal::

 module std {
    // C standard library...

    module vector {
      requires cplusplus
      header "vector"
    }

    module type_traits {
      requires cplusplus11
      header "type_traits"
    }
  }

Header declaration
~~~~~~~~~~~~~~~~~~
A header declaration specifies that a particular header is associated with the enclosing module.

.. parsed-literal::

  *header-declaration*:
    ``umbrella``:sub:`opt` ``header`` *string-literal*
    ``exclude`` ``header`` *string-literal*

A header declaration that does not contain ``exclude`` specifies a header that contributes to the enclosing module. Specifically, when the module is built, the named header will be parsed and its declarations will be (logically) placed into the enclosing submodule.

A header with the ``umbrella`` specifier is called an umbrella header. An umbrella header includes all of the headers within its directory (and any subdirectories), and is typically used (in the ``#include`` world) to easily access the full API provided by a particular library. With modules, an umbrella header is a convenient shortcut that eliminates the need to write out ``header`` declarations for every library header. A given directory can only contain a single umbrella header.

.. note::
    Any headers not included by the umbrella header should have
    explicit ``header`` declarations. Use the   
    ``-Wincomplete-umbrella`` warning option to ask Clang to complain
    about headers not covered by the umbrella header or the module map.

A header with the ``exclude`` specifier is excluded from the module. It will not be included when the module is built, nor will it be considered to be part of the module.

**Example**: The C header ``assert.h`` is an excellent candidate for an excluded header, because it is meant to be included multiple times (possibly with different ``NDEBUG`` settings).

.. parsed-literal::

  module std [system] {
    exclude header "assert.h"
  }

A given header shall not be referenced by more than one *header-declaration*.

Umbrella directory declaration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
An umbrella directory declaration specifies that all of the headers in the specified directory should be included within the module.

.. parsed-literal::

  *umbrella-dir-declaration*:
    ``umbrella`` *string-literal*
  
The *string-literal* refers to a directory. When the module is built, all of the header files in that directory (and its subdirectories) are included in the module.

An *umbrella-dir-declaration* shall not refer to the same directory as the location of an umbrella *header-declaration*. In other words, only a single kind of umbrella can be specified for a given directory.

.. note::

    Umbrella directories are useful for libraries that have a large number of headers but do not have an umbrella header.


Submodule declaration
~~~~~~~~~~~~~~~~~~~~~
Submodule declarations describe modules that are nested within their enclosing module.

.. parsed-literal::

  *submodule-declaration*:
    *module-declaration*
    *inferred-submodule-declaration*

A *submodule-declaration* that is a *module-declaration* is a nested module. If the *module-declaration* has a ``framework`` specifier, the enclosing module shall have a ``framework`` specifier; the submodule's contents shall be contained within the subdirectory ``Frameworks/SubName.framework``, where ``SubName`` is the name of the submodule.

A *submodule-declaration* that is an *inferred-submodule-declaration* describes a set of submodules that correspond to any headers that are part of the module but are not explicitly described by a *header-declaration*.

.. parsed-literal::

  *inferred-submodule-declaration*:
    ``explicit``:sub:`opt` ``framework``:sub:`opt` ``module`` '*' *attributes*:sub:`opt` '{' *inferred-submodule-member** '}'
  
  *inferred-submodule-member*:
    ``export`` '*'

A module containing an *inferred-submodule-declaration* shall have either an umbrella header or an umbrella directory. The headers to which the *inferred-submodule-declaration* applies are exactly those headers included by the umbrella header (transitively) or included in the module because they reside within the umbrella directory (or its subdirectories).

For each header included by the umbrella header or in the umbrella directory that is not named by a *header-declaration*, a module declaration is implicitly generated from the *inferred-submodule-declaration*. The module will:

* Have the same name as the header (without the file extension)
* Have the ``explicit`` specifier, if the *inferred-submodule-declaration* has the ``explicit`` specifier
* Have the ``framework`` specifier, if the    
  *inferred-submodule-declaration* has the ``framework`` specifier
* Have the attributes specified by the \ *inferred-submodule-declaration* 
* Contain a single *header-declaration* naming that header
* Contain a single *export-declaration* ``export *``, if the \ *inferred-submodule-declaration* contains the \ *inferred-submodule-member* ``export *``

**Example**: If the subdirectory "MyLib" contains the headers ``A.h`` and ``B.h``, then the following module map:

.. parsed-literal::

  module MyLib {
    umbrella "MyLib"
    explicit module * {
      export *
    }
  }

is equivalent to the (more verbose) module map:

.. parsed-literal::

  module MyLib {
    explicit module A {
      header "A.h"
      export *
    }

    explicit module B {
      header "B.h"
      export *
    }
  }

Export declaration
~~~~~~~~~~~~~~~~~~
An *export-declaration* specifies which imported modules will automatically be re-exported as part of a given module's API.

.. parsed-literal::

  *export-declaration*:
    ``export`` *wildcard-module-id*

  *wildcard-module-id*:
    *identifier*
    '*'
    *identifier* '.' *wildcard-module-id*

The *export-declaration* names a module or a set of modules that will be re-exported to any translation unit that imports the enclosing module. Each imported module that matches the *wildcard-module-id* up to, but not including, the first ``*`` will be re-exported.

**Example**:: In the following example, importing ``MyLib.Derived`` also provides the API for ``MyLib.Base``:

.. parsed-literal::

  module MyLib {
    module Base {
      header "Base.h"
    }

    module Derived {
      header "Derived.h"
      export Base
    }
  }

Note that, if ``Derived.h`` includes ``Base.h``, one can simply use a wildcard export to re-export everything ``Derived.h`` includes:

.. parsed-literal::

  module MyLib {
    module Base {
      header "Base.h"
    }

    module Derived {
      header "Derived.h"
      export *
    }
  }

.. note::

  The wildcard export syntax ``export *`` re-exports all of the
  modules that were imported in the actual header file. Because
  ``#include`` directives are automatically mapped to module imports,
  ``export *`` provides the same transitive-inclusion behavior
  provided by the C preprocessor, e.g., importing a given module
  implicitly imports all of the modules on which it depends.
  Therefore, liberal use of ``export *`` provides excellent backward
  compatibility for programs that rely on transitive inclusion (i.e.,
  all of them).

Link declaration
~~~~~~~~~~~~~~~~
A *link-declaration* specifies a library or framework against which a program should be linked if the enclosing module is imported in any translation unit in that program.

.. parsed-literal::

  *link-declaration*:
    ``link`` ``framework``:sub:`opt` *string-literal*

The *string-literal* specifies the name of the library or framework against which the program should be linked. For example, specifying "clangBasic" would instruct the linker to link with ``-lclangBasic`` for a Unix-style linker.

A *link-declaration* with the ``framework`` specifies that the linker should link against the named framework, e.g., with ``-framework MyFramework``.

.. note::

  Automatic linking with the ``link`` directive is not yet widely
  implemented, because it requires support from both the object file
  format and the linker. The notion is similar to Microsoft Visual
  Studio's ``#pragma comment(lib...)``.

Configuration macros declaration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The *config-macros-declaration* specifies the set of configuration macros that have an effect on the the API of the enclosing module.

.. parsed-literal::

  *config-macros-declaration*:
    ``config_macros`` *attributes*:sub:`opt` *config-macro-list*:sub:`opt`

  *config-macro-list*:
    *identifier* (',' *identifier*)*

Each *identifier* in the *config-macro-list* specifies the name of a macro. The compiler is required to maintain different variants of the given module for differing definitions of any of the named macros.

A *config-macros-declaration* shall only be present on a top-level module, i.e., a module that is not nested within an enclosing module.

The ``exhaustive`` attribute specifies that the list of macros in the *config-macros-declaration* is exhaustive, meaning that no other macro definition is intended to have an effect on the API of that module. 

.. note::

  The ``exhaustive`` attribute implies that any macro definitions 
  for macros not listed as configuration macros should be ignored
  completely when building the module. As an optimization, the
  compiler could reduce the number of unique module variants by not
  considering these non-configuration macros. This optimization is not
  yet implemented in Clang.

A translation unit shall not import the same module under different definitions of the configuration macros.

.. note::

  Clang implements a weak form of this requirement: the definitions
  used for configuration macros are fixed based on the definitions
  provided by the command line. If an import occurs and the definition
  of any configuration macro has changed, the compiler will produce a
  warning (under the control of ``-Wconfig-macros``).

**Example:** A logging library might provide different API (e.g., in the form of different definitions for a logging macro) based on the ``NDEBUG`` macro setting:

.. parsed-literal::

  module MyLogger {
    umbrella header "MyLogger.h"
    config_macros [exhaustive] NDEBUG
  }

Conflict declarations
~~~~~~~~~~~~~~~~~~~~~
A *conflict-declaration* describes a case where the presence of two different modules in the same translation unit is likely to cause a problem. For example, two modules may provide similar-but-incompatible functionality.

.. parsed-literal::

  *conflict-declaration*:
    ``conflict`` *module-id* ',' *string-literal*

The *module-id* of the *conflict-declaration* specifies the module with which the enclosing module conflicts. The specified module shall not have been imported in the translation unit when the enclosing module is imported.

The *string-literal* provides a message to be provided as part of the compiler diagnostic when two modules conflict.

.. note::

  Clang emits a warning (under the control of ``-Wmodule-conflict``)
  when a module conflict is discovered.

**Example:**

.. parsed-literal::

  module Conflicts {
    explicit module A {
      header "conflict_a.h"
      conflict B, "we just don't like B"
    }

    module B {
      header "conflict_b.h"
    }
  }


Attributes
----------
Attributes are used in a number of places in the grammar to describe specific behavior of other declarations. The format of attributes is fairly simple.

.. parsed-literal::

  *attributes*:
    *attribute* *attributes*:sub:`opt`

  *attribute*:
    '[' *identifier* ']'

Any *identifier* can be used as an attribute, and each declaration specifies what attributes can be applied to it.

Modularizing a Platform
=======================
To get any benefit out of modules, one needs to introduce module maps for software libraries starting at the bottom of the stack. This typically means introducing a module map covering the operating system's headers and the C standard library headers (in ``/usr/include``, for a Unix system). 

The module maps will be written using the `module map language`_, which provides the tools necessary to describe the mapping between headers and modules. Because the set of headers differs from one system to the next, the module map will likely have to be somewhat customized for, e.g., a particular distribution and version of the operating system. Moreover, the system headers themselves may require some modification, if they exhibit any anti-patterns that break modules. Such common patterns are described below.

**Macro-guarded copy-and-pasted definitions**
  System headers vend core types such as ``size_t`` for users. These types are often needed in a number of system headers, and are almost trivial to write. Hence, it is fairly common to see a definition such as the following copy-and-pasted throughout the headers:

  .. parsed-literal::

    #ifndef _SIZE_T
    #define _SIZE_T
    typedef __SIZE_TYPE__ size_t;
    #endif

  Unfortunately, when modules compiles all of the C library headers together into a single module, only the first actual type definition of ``size_t`` will be visible, and then only in the submodule corresponding to the lucky first header. Any other headers that have copy-and-pasted versions of this pattern will *not* have a definition of ``size_t``. Importing the submodule corresponding to one of those headers will therefore not yield ``size_t`` as part of the API, because it wasn't there when the header was parsed. The fix for this problem is either to pull the copied declarations into a common header that gets included everywhere ``size_t`` is part of the API, or to eliminate the ``#ifndef`` and redefine the ``size_t`` type. The latter works for C++ headers and C11, but will cause an error for non-modules C90/C99, where redefinition of ``typedefs`` is not permitted.

**Conflicting definitions**
  Different system headers may provide conflicting definitions for various macros, functions, or types. These conflicting definitions don't tend to cause problems in a pre-modules world unless someone happens to include both headers in one translation unit. Since the fix is often simply "don't do that", such problems persist. Modules requires that the conflicting definitions be eliminated or that they be placed in separate modules (the former is generally the better answer).

**Missing includes**
  Headers are often missing ``#include`` directives for headers that they actually depend on. As with the problem of conflicting definitions, this only affects unlucky users who don't happen to include headers in the right order. With modules, the headers of a particular module will be parsed in isolation, so the module may fail to build if there are missing includes.

**Headers that vend multiple APIs at different times**
  Some systems have headers that contain a number of different kinds of API definitions, only some of which are made available with a given include. For example, the header may vend ``size_t`` only when the macro ``__need_size_t`` is defined before that header is included, and also vend ``wchar_t`` only when the macro ``__need_wchar_t`` is defined. Such headers are often included many times in a single translation unit, and will have no include guards. There is no sane way to map this header to a submodule. One can either eliminate the header (e.g., by splitting it into separate headers, one per actual API) or simply ``exclude`` it in the module map.

To detect and help address some of these problems, the ``clang-tools-extra`` repository contains a ``modularize`` tool that parses a set of given headers and attempts to detect these problems and produce a report. See the tool's in-source documentation for information on how to check your system or library headers.

Future Directions
=================
Modules is an experimental feature, and there is much work left to do to make it both real and useful. Here are a few ideas:

**Detect unused module imports**
  Unlike with ``#include`` directives, it should be fairly simple to track whether a directly-imported module has ever been used. By doing so, Clang can emit ``unused import`` or ``unused #include`` diagnostics, including Fix-Its to remove the useless imports/includes.

**Fix-Its for missing imports**
  It's fairly common for one to make use of some API while writing code, only to get a compiler error about "unknown type" or "no function named" because the corresponding header has not been included. Clang should detect such cases and auto-import the required module (with a Fix-It!).

**Improve modularize**
  The modularize tool is both extremely important (for deployment) and extremely crude. It needs better UI, better detection of problems (especially for C++), and perhaps an assistant mode to help write module maps for you.

**C++ Support**
  Modules clearly has to work for C++, or we'll never get to use it for the Clang code base.

Where To Learn More About Modules
=================================
The Clang source code provides additional information about modules:

``clang/lib/Headers/module.map``
  Module map for Clang's compiler-specific header files.

``clang/test/Modules/``
  Tests specifically related to modules functionality.

``clang/include/clang/Basic/Module.h``
  The ``Module`` class in this header describes a module, and is used throughout the compiler to implement modules.

``clang/include/clang/Lex/ModuleMap.h``
  The ``ModuleMap`` class in this header describes the full module map, consisting of all of the module map files that have been parsed, and providing facilities for looking up module maps and mapping between modules and headers (in both directions).

PCHInternals_
  Information about the serialized AST format used for precompiled headers and modules. The actual implementation is in the ``clangSerialization`` library.

.. [#] Automatic linking against the libraries of modules requires specific linker support, which is not widely available.

.. [#] Modules are only available in C and Objective-C; a separate flag ``-fcxx-modules`` enables modules support for C++, which is even more experimental and broken.

.. [#] There are certain anti-patterns that occur in headers, particularly system headers, that cause problems for modules. The section `Modularizing a Platform`_ describes some of them.

.. [#] The second instance is actually a new thread within the current process, not a separate process. However, the original compiler instance is blocked on the execution of this thread.

.. [#] The preprocessing context in which the modules are parsed is actually dependent on the command-line options provided to the compiler, including the language dialect and any ``-D`` options. However, the compiled modules for different command-line options are kept distinct, and any preprocessor directives that occur within the translation unit are ignored. See the section on the `Configuration macros declaration`_ for more information.

.. _PCHInternals: PCHInternals.html