Clang 3.6 documentation


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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):

#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:

import; // 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 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, because the module itself was compiled as a separate, standalone module. Additionally, any linker flags required to use the module will automatically be provided when the module is imported [1] This semantic import model addresses many of the problems of the preprocessor inclusion model:

  • Compile-time scalability: The 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.

Objective-C Import declaration

Objective-C provides syntax for importing a module via an @import declaration, which imports the named module:

@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.,


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.

At present, there is no C or C++ syntax for import declarations. Clang will track the modules proposal in the C++ committee. 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

#include <stdio.h>

will be automatically mapped to an import of the module 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.


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.

While building a module, #include_next is also supported, with one caveat. The usual behavior of #include_next is to search for the specified filename in the list of include paths, starting from the path after the one in which the current file was found. Because files listed in module maps are not found through include paths, a different strategy is used for #include_next directives in such files: the list of include paths is searched for the specified header name, to find the first include path that would refer to the current file. #include_next is interpreted as if the current file had been found in that path. If this search finds a file named by a module map, the #include_next directive is translated into an import, just like for a #include directive.``

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.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.modulemap) 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 [2]). The actual Module map language is described in a later section.


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.

One can use module maps without modules to check the integrity of the use of header files. To do this, use the -fmodule-maps option instead of the -fmodules option.

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 [3], with a fresh preprocessing context [4], 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

Enable the modules feature.
Enable interpretation of module maps. This option is implied by -fmodules.
Specify the path to the modules cache. If not provided, Clang will select a system-appropriate default.
Disable automatic linking against the libraries associated with imported modules.
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.
Specify the minimum delay (in seconds) between attempts to prune the module cache. Module cache pruning attempts to clear out old, unused module files so that the module cache itself does not grow without bound. The default delay is large (604,800 seconds, or 7 days) because this is an expensive operation. Set this value to 0 to turn off pruning.
Specify the minimum time (in seconds) for which a file in the module cache must be unused (according to access time) before module pruning will remove it. The default delay is large (2,678,400 seconds, or 31 days) to avoid excessive module rebuilding.
-module-file-info <module file name>
Debugging aid that prints information about a given module file (with a .pcm extension), including the language and preprocessor options that particular module variant was built with.
Enable checking of module use declarations.
Consider a source file as a part of the given module.
Load the given module map file if a header from its directory or one of its subdirectories is loaded.
If a symbol is not found, search modules referenced in the current module maps but not imported for symbols, so the error message can reference the module by name. Note that if the global module index has not been built before, this might take some time as it needs to build all the modules. Note that this option doesn’t apply in module builds, to avoid the recursion.
Suppresses the implicit search for files called module.modulemap and similar. Instead, module files need to be explicitly specified via -fmodule-map-file or transitively used.

Module Semantics

Modules are modeled as if each submodule were a separate translation unit, and a module import makes names from the other translation unit visible. Each submodule starts with a new preprocessor state and an empty translation unit.


This behavior is currently only approximated when building a module with submodules. Entities within a submodule that has already been built are visible when building later submodules in that module. This can lead to fragile modules that depend on the build order used for the submodules of the module, and should not be relied upon. This behavior is subject to change.

As an example, in C, this implies that if two structs are defined in different submodules with the same name, those two types are distinct types (but may be compatible types if their definitions match. In C++, two structs defined with the same name in different submodules are the same type, and must be equivalent under C++’s One Definition Rule.


Clang currently only performs minimal checking for violations of the One Definition Rule.

If any submodule of a module is imported into any part of a program, the entire top-level module is considered to be part of the program. As a consequence of this, Clang may diagnose conflicts between an entity declared in an unimported submodule and an entity declared in the current translation unit, and Clang may inline or devirtualize based on knowledge from unimported submodules.


The C and C++ preprocessor assumes that the input text is a single linear buffer, but with modules this is not the case. It is possible to import two modules that have conflicting definitions for a macro (or where one #defines a macro and the other #undefines it). The rules for handling macro definitions in the presence of modules are as follows:

  • Each definition and undefinition of a macro is considered to be a distinct entity.
  • Such entities are visible if they are from the current submodule or translation unit, or if they were exported from a submodule that has been imported.
  • A #define X or #undef X directive overrides all definitions of X that are visible at the point of the directive.
  • A #define or #undef directive is active if it is visible and no visible directive overrides it.
  • A set of macro directives is consistent if it consists of only #undef directives, or if all #define directives in the set define the macro name to the same sequence of tokens (following the usual rules for macro redefinitions).
  • If a macro name is used and the set of active directives is not consistent, the program is ill-formed. Otherwise, the (unique) meaning of the macro name is used.

For example, suppose:

  • <stdio.h> defines a macro getc (and exports its #define)
  • <cstdio> imports the <stdio.h> module and undefines the macro (and exports its #undef)

The #undef overrides the #define, and a source file that imports both modules in any order will not see getc defined as a macro.

Module Map Language


The module map language is not currently guaranteed to be stable between major revisions of Clang.

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.modulemap file for that library. The module.modulemap file is placed alongside the header files themselves, and is written in the module map language described below.


For compatibility with previous releases, if a module map file named module.modulemap is not found, Clang will also search for a file named This behavior is deprecated and we plan to eventually remove it.

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

module std [system] [extern_c] {
  module assert {
    textual header "assert.h"
    header "bits/assert-decls.h"
    export *

  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.

config_macros export     private
conflict      framework  requires
exclude       header     textual
explicit      link       umbrella
extern        module     use

Module map file

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


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:

  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.

  explicitopt frameworkopt module module-id attributesopt '{' module-member* '}'
  extern module module-id string-literal

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:

  Modules/module.modulemap  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 headers 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.

The extern_c attribute specifies that the module contains C code that can be used from within C++. When such a module is built for use in C++ code, all of the module’s headers will be treated as if they were contained within an implicit extern "C" block. An import for a module with this attribute can appear within an extern "C" block. No other restrictions are lifted, however: the module currently cannot be imported within an extern "C" block in a namespace.

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


An extern module references a module defined by the module-id in a file given by the string-literal. The file can be referenced either by an absolute path or by a path relative to the current map file.

Requires declaration

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

  requires feature-list

  feature (',' feature)*

  !opt 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 optional ! indicates that a feature is incompatible with the module.

The following features are defined:

The target supports AltiVec.
The “blocks” language feature is available.
C++ support is available.
C++11 support is available.
Objective-C support is available.
Objective-C Automatic Reference Counting (ARC) is available
OpenCL is available
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:

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.

  privateopt textualopt header string-literal
  umbrella header string-literal
  exclude header string-literal

A header declaration that does not contain exclude nor textual 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.


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 private specifier may not be included from outside the module itself.

A header with the textual specifier will not be included when the module is built, and will be textually included if it is named by a #include directive. However, it is considered to be part of the module for the purpose of checking use-declarations.

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, even if an umbrella header or directory would otherwise make it part of the module.

Example: The C header assert.h is an excellent candidate for a textual header, because it is meant to be included multiple times (possibly with different NDEBUG settings). However, declarations within it should typically be split into a separate modular header.

module std [system] {
  textual 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.

  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.


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.


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.

  explicitopt frameworkopt module '*' attributesopt '{' 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:

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

is equivalent to the (more verbose) module map:

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.

  export wildcard-module-id

  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:

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:

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

  module Derived {
    header "Derived.h"
    export *


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).

Use declaration

A use-declaration specifies one of the other modules that the module is allowed to use. An import or include not matching one of these is rejected when the option -fmodules-decluse.

  use module-id

Example:: In the following example, use of A from C is not declared, so will trigger a warning.

module A {
  header "a.h"

module B {
  header "b.h"

module C {
  header "c.h"
  use B

When compiling a source file that implements a module, use the option -fmodule-name=module-id to indicate that the source file is logically part of that module.

The compiler at present only applies restrictions to the module directly being built.

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.

  config_macros attributesopt config-macro-listopt

  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.


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.


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:

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.

  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.


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


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

  module B {
    header "conflict_b.h"


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.

  attribute attributesopt

  '[' identifier ']'

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

Private Module Map Files

Module map files are typically named module.modulemap and live either alongside the headers they describe or in a parent directory of the headers they describe. These module maps typically describe all of the API for the library.

However, in some cases, the presence or absence of particular headers is used to distinguish between the “public” and “private” APIs of a particular library. For example, a library may contain the headers Foo.h and Foo_Private.h, providing public and private APIs, respectively. Additionally, Foo_Private.h may only be available on some versions of library, and absent in others. One cannot easily express this with a single module map file in the library:

module Foo {
  header "Foo.h"

  explicit module Private {
    header "Foo_Private.h"

because the header Foo_Private.h won’t always be available. The module map file could be customized based on whether Foo_Private.h is available or not, but doing so requires custom build machinery.

Private module map files, which are named module.private.modulemap (or, for backward compatibility,, allow one to augment the primary module map file with an additional submodule. For example, we would split the module map file above into two module map files:

/* module.modulemap */
module Foo {
  header "Foo.h"

/* module.private.modulemap */
explicit module Foo.Private {
  header "Foo_Private.h"

When a module.private.modulemap file is found alongside a module.modulemap file, it is loaded after the module.modulemap file. In our example library, the module.private.modulemap file would be available when Foo_Private.h is available, making it easier to split a library’s public and private APIs along header boundaries.

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:

#ifndef _SIZE_T
#define _SIZE_T
typedef __SIZE_TYPE__ size_t;

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 support is under active development, and there are many opportunities remaining to improve it. 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 can detect such cases and auto-import the required module, but should provide a Fix-It to add the import.
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.

Where To Learn More About Modules

The Clang source code provides additional information about modules:

Module map for Clang’s compiler-specific header files.
Tests specifically related to modules functionality.
The Module class in this header describes a module, and is used throughout the compiler to implement modules.
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).
Information about the serialized AST format used for precompiled headers and modules. The actual implementation is in the clangSerialization library.
[1]Automatic linking against the libraries of modules requires specific linker support, which is not widely available.
[2]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.
[3]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.
[4]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.

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