C++ Modules#

Modules are a C++20 language feature. As the name suggests, they provides a modular compilation system, intending to provide both faster builds and better library isolation. The 1103, provides the easiest to read set of changes to the standard, although it does not capture later changes.

G++’s modules support is not complete. Other than bugs, the known missing pieces are:

Private Module Fragment

The Private Module Fragment is recognized, but an error is emitted.

Partition definition visibility rules

Entities may be defined in implementation partitions, and those definitions are not available outside of the module. This is not implemented, and the definitions are available to extra-module use.

Textual merging of reachable GM entities

Entities may be multiply defined across different header-units. These must be de-duplicated, and this is implemented across imports, or when an import redefines a textually-defined entity. However the reverse is not implemented—textually redefining an entity that has been defined in an imported header-unit. A redefinition error is emitted.

Translation-Unit local referencing rules

Papers P1815 and P2003 add limitations on which entities an exported region may reference (for instance, the entities an exported template definition may reference). These are not fully implemented.

Standard Library Header Units

The Standard Library is not provided as importable header units. If you want to import such units, you must explicitly build them first. If you do not do this with care, you may have multiple declarations, which the module machinery must merge—compiler resource usage can be affected by how you partition header files into header units.

Modular compilation is not enabled with just the -std=c++20 option. You must explicitly enable it with the -fmodules-ts option. It is independent of the language version selected, although in pre-C++20 versions, it is of course an extension.

No new source file suffixes are required or supported. If you wish to use a non-standard suffix (see Options Controlling the Kind of Output), you also need to provide a -x c++ option too.Some users like to distinguish module interface files with a new suffix, such as naming the source module.cppm, which involves teaching all tools about the new suffix. A different scheme, such as naming module-m.cpp would be less invasive.

Compiling a module interface unit produces an additional output (to the assembly or object file), called a Compiled Module Interface (CMI). This encodes the exported declarations of the module. Importing a module reads in the CMI. The import graph is a Directed Acyclic Graph (DAG). You must build imports before the importer.

Header files may themselves be compiled to header units, which are a transitional ability aiming at faster compilation. The -fmodule-header option is used to enable this, and implies the -fmodules-ts option. These CMIs are named by the fully resolved underlying header file, and thus may be a complete pathname containing subdirectories. If the header file is found at an absolute pathname, the CMI location is still relative to a CMI root directory.

As header files often have no suffix, you commonly have to specify a -x option to tell the compiler the source is a header file. You may use -x c++-header, -x c++-user-header or -x c++-system-header. When used in conjunction with -fmodules-ts, these all imply an appropriate -fmodule-header option. The latter two variants use the user or system include path to search for the file specified. This allows you to, for instance, compile standard library header files as header units, without needing to know exactly where they are installed. Specifying the language as one of these variants also inhibits output of the object file, as header files have no associated object file.

The -fmodule-only option disables generation of the associated object file for compiling a module interface. Only the CMI is generated. This option is implied when using the -fmodule-header option.

The -flang-info-include-translate and -flang-info-include-translate-not options notes whether include translation occurs or not. With no argument, the first will note all include translation. The second will note all non-translations of include files not known to intentionally be textual. With an argument, queries about include translation of a header files with that particular trailing pathname are noted. You may repeat this form to cover several different header files. This option may be helpful in determining whether include translation is happening—if it is working correctly, it behaves as if it isn’t there at all.

The -flang-info-module-cmi option can be used to determine where the compiler is reading a CMI from. Without the option, the compiler is silent when such a read is successful. This option has an optional argument, which will restrict the notification to just the set of named modules or header units specified.

The -Winvalid-imported-macros option causes all imported macros to be resolved at the end of compilation. Without this, imported macros are only resolved when expanded or (re)defined. This option detects conflicting import definitions for all macros.

For details of the -fmodule-mapper family of options, see Module Mapper.

Module Mapper#

A module mapper provides a server or file that the compiler queries to determine the mapping between module names and CMI files. It is also used to build CMIs on demand. Mapper functionality is in its infancy and is intended for experimentation with build system interactions.

You can specify a mapper with the -fmodule-mapper=val option or CXX_MODULE_MAPPER environment variable. The value may have one of the following forms:


An optional hostname and a numeric port number to connect to. If the hostname is omitted, the loopback address is used. If the hostname corresponds to multiple IPV6 addresses, these are tried in turn, until one is successful. If your host lacks IPv6, this form is non-functional. If you must use IPv4 use -fmodule-mapper='|ncat ipv4hostport'.


A local domain socket. If your host lacks local domain sockets, this form is non-functional.


A program to spawn, and communicate with on its stdin/stdout streams. Your PATH environment variable is searched for the program. Arguments are separated by space characters, (it is not possible for one of the arguments delivered to the program to contain a space). An exception is if program begins with @. In that case program (sans @) is looked for in the compiler’s internal binary directory. Thus the sample mapper-server can be specified with @g++-mapper-server.



Named pipes or file descriptors to communicate over. The first form, <>, communicates over stdin and stdout. The other forms allow you to specify a file descriptor or name a pipe. A numeric value is interpreted as a file descriptor, otherwise named pipe is opened. The second form specifies a bidirectional pipe and the last form allows specifying two independent pipes. Using file descriptors directly in this manner is fragile in general, as it can require the cooperation of intermediate processes. In particular using stdin & stdout is fraught with danger as other compiler options might also cause the compiler to read stdin or write stdout, and it can have unfortunate interactions with signal delivery from the terminal.


A mapping file consisting of space-separated module-name, filename pairs, one per line. Only the mappings for the direct imports and any module export name need be provided. If other mappings are provided, they override those stored in any imported CMI files. A repository root may be specified in the mapping file by using $root as the module name in the first active line. Use of this option will disable any default module->CMI name mapping.

As shown, an optional ident may suffix the first word of the option, indicated by a ? prefix. The value is used in the initial handshake with the module server, or to specify a prefix on mapping file lines. In the server case, the main source file name is used if no ident is specified. In the file case, all non-blank lines are significant, unless a value is specified, in which case only lines beginning with ident are significant. The ident must be separated by whitespace from the module name. Be aware that <, >, ?, and | characters are often significant to the shell, and therefore may need quoting.

The mapper is connected to or loaded lazily, when the first module mapping is required. The networking protocols are only supported on hosts that provide networking. If no mapper is specified a default is provided.

A project-specific mapper is expected to be provided by the build system that invokes the compiler. It is not expected that a general-purpose server is provided for all compilations. As such, the server will know the build configuration, the compiler it invoked, and the environment (such as working directory) in which that is operating. As it may parallelize builds, several compilations may connect to the same socket.

The default mapper generates CMI files in a gcm.cache directory. CMI files have a .gcm suffix. The module unit name is used directly to provide the basename. Header units construct a relative path using the underlying header file name. If the path is already relative, a , directory is prepended. Internal .. components are translated to ,,. No attempt is made to canonicalize these filenames beyond that done by the preprocessor’s include search algorithm, as in general it is ambiguous when symbolic links are present.

The mapper protocol was published as A Module Mapper. The implementation is provided by libcody, https://github.com/urnathan/libcody, which specifies the canonical protocol definition. A proof of concept server implementation embedded in make was described in Make Me A Module.

Module Preprocessing#

Modules affect preprocessing because of header units and include translation. Some uses of the preprocessor as a separate step either do not produce a correct output, or require CMIs to be available.

Header units import macros. These macros can affect later conditional inclusion, which therefore can cascade to differing import sets. When preprocessing, it is necessary to load the CMI. If a header unit is unavailable, the preprocessor issues a warning and continue (when not just preprocessing, an error is emitted). Detecting such imports requires preprocessor tokenization of the input stream to phase 4 (macro expansion).

Include translation converts #include, #include_next and #import directives to internal import declarations. Whether a particular directive is translated is controlled by the module mapper. Header unit names are canonicalized during preprocessing.

Dependency information can be emitted for macro import, extending the functionality of -MD and -MMD options. Detection of import declarations also requires phase 4 preprocessing, and thus requires full preprocessing (or compilation).

The -M, -MM and -E -fdirectives-only options halt preprocessing before phase 4.

The -save-temps option uses -fdirectives-only for preprocessing, and preserve the macro definitions in the preprocessed output. Usually you also want to use this option when explicitly preprocessing a header-unit, or consuming such preprocessed output:

g++ -fmodules-ts -E -fdirectives-only my-header.hh -o my-header.ii
g++ -x c++-header -fmodules-ts -fpreprocessed -fdirectives-only my-header.ii

Compiled Module Interface#

CMIs are an additional artifact when compiling named module interfaces, partitions or header units. These are read when importing. CMI contents are implementation-specific, and in GCC’s case tied to the compiler version. Consider them a rebuildable cache artifact, not a distributable object.

When creating an output CMI, any missing directory components are created in a manner that is safe for concurrent builds creating multiple, different, CMIs within a common subdirectory tree.

CMI contents are written to a temporary file, which is then atomically renamed. Observers either see old contents (if there is an existing file), or complete new contents. They do not observe the CMI during its creation. This is unlike object file writing, which may be observed by an external process.

CMIs are read in lazily, if the host OS provides mmap functionality. Generally blocks are read when name lookup or template instantiation occurs. To inhibit this, the -fno-module-lazy option may be used.

The --param lazy-modules=n parameter controls the limit on the number of concurrently open module files during lazy loading. Should more modules be imported, an LRU algorithm is used to determine which files to close—until that file is needed again. This limit may be exceeded with deep module dependency hierarchies. With large code bases there may be more imports than the process limit of file descriptors. By default, the limit is a few less than the per-process file descriptor hard limit, if that is determinable.Where applicable the soft limit is incremented as needed towards the hard limit.

GCC CMIs use ELF32 as an architecture-neutral encapsulation mechanism. You may use readelf to inspect them, although section contents are largely undecipherable. There is a section named .gnu.c++.README, which contains human-readable text. Other than the first line, each line consists of tag: value tuples.

$ readelf -p.gnu.c++.README gcm.cache/foo.gcm

String dump of section '.gnu.c++.README':
  [     0]  GNU C++ primary module interface
  [    21]  compiler: 11.0.0 20201116 (experimental) [c++-modules revision 20201116-0454]
  [    6f]  version: 2020/11/16-04:54
  [    89]  module: foo
  [    95]  source: c_b.ii
  [    a4]  dialect: C++20/coroutines
  [    be]  cwd: /data/users/nathans/modules/obj/x86_64/gcc
  [    ee]  repository: gcm.cache
  [   104]  buildtime: 2020/11/16 15:03:21 UTC
  [   127]  localtime: 2020/11/16 07:03:21 PST
  [   14a]  export: foo:part1 foo-part1.gcm

Amongst other things, this lists the source that was built, C++ dialect used and imports of the module.The precise contents of this output may change.

The timestamp is the same value as that provided by the __DATE__ & __TIME__ macros, and may be explicitly specified with the environment variable SOURCE_DATE_EPOCH. For further details see Environment Variables Affecting GCC.

A set of related CMIs may be copied, provided the relative pathnames are preserved.

The .gnu.c++.README contents do not affect CMI integrity, and it may be removed or altered. The section numbering of the sections whose names do not begin with .gnu.c++., or are not the string section is significant and must not be altered.