Guidelines for #include directives

The following include order is used in GROMACS and enforced by clang-format. An empty line should appear between each group, and headers within each group sorted alphabetically.

  1. Each source file should include gmxpre.h first.

  2. If a source file has a corresponding header, it should be included next. If the header is in the same directory as the source, then it is included without any path (i.e., relative to the source). Otherwise, the canonical include path of libraryname/modulename/header.h is used.

  3. If the file depends on defines from config.h, that comes next.

  4. This is followed by standard C/C++ headers, grouped as follows:

    1. Standard C headers (e.g., <stdio.h>)

    2. C++ versions of the above (e.g., <cstdio>)

    3. Standard C++ headers (e.g., <vector>)

    Preferably, only one of the first two groups is present, but this is not enforced.

  5. This is followed by other system headers: platform-specific headers such as <unistd.h>, as well as external libraries such as <gtest/gtest.h>.

  6. GROMACS-specific libraries from src/external/, such as "thread_mpi/threads.h".

  7. GROMACS headers that are not part of the including module.

  8. Public GROMACS headers that are part of the including module.

  9. Finally, GROMACS headers that are internal to the including module, executable, or test target (typically at the same path as the source file).

All GROMACS headers are included with quotes ("gromacs/utility/path.h"), other headers with angle brackets (<stdio.h>). Headers under src/external/ are generally included with quotes (whenever the include path is relative to src/, as well as for thread-MPI and TNG), but larger third-party entities are included as if they were provided by the system. The latter group currently includes gtest/gmock.

In some cases, the include paths available to build targets may leak visibility of headers inappropriately. This is usually encountered as a header that can be used by an #include with an unusual or long path. If a header cannot be included as described above, check that the appropriate CMake target is referenced by a target_link_libraries() command. Many modules provide their own CMake target. Additionally, note

  • The common CMake target provides access to gmxpre.h, config.h, gmxpre-config.h, buildinfo.h, and contributors.h

  • legacy_api provides access to those of the old gromacs/modulename headers that are in api/legacy/include

  • legacy_modules adds src/ to the include path, exposing all headers in gromacs/ and gromacs/*/ for #include lines that would appear to comply with the guidelines above, but which may not be intended for “public” use. (This target was intended as a temporary measure while working towards Issue 3288.)

If there are any conditionally included headers (typically, only when some #defines from config.h are set), these should be included at the end of their respective group. Note that the automatic checker/sorter script does not act on such headers, nor on comments that are between #include statements; it is up to the author of the code to put the headers in proper order in such cases. Trailing comments on the same line as #include statements are preserved and do not affect the checker/sorter.

As part of the effort to build a proper API, a new scheme of separating between public, library and module functionality in header files is planned. See also Source tree checker scripts and API restructuring issues for details.

Enforcing a consistent order and style has a few advantages:

  • It makes it easy at a quick glance to find the dependencies of a file, without scanning through a long list of unorganized #includes.

  • Including the header corresponding to the source file first makes most headers included first in some source file, revealing potential problems where headers would not compile unless some other header would be included first. With this order, the person working on the header is most likely to see these problems instead of someone else seeing them later when refactoring unrelated code.

  • Consistent usage of paths in #include directives makes it easy to use grep to find all uses of a header, as well as all include dependencies between two modules.

  • An automatic script can be used to re-establish clean code after semi-automatic refactoring like renaming an include file with sed, without causing other unnecessary changes.