Guidelines for PROJ code contributors

This is a guide for PROJ, casual or regular, code contributors.

Code contributions.

Code contributions can be either bug fixes or new features. The process is the same for both, so they will be discussed together in this section.

Making Changes

  • Create a topic branch from where you want to base your work.

  • You usually should base your topic branch off of the master branch.

  • To quickly create a topic branch: git checkout -b my-topic-branch

  • Make commits of logical units.

  • Check for unnecessary whitespace with git diff --check before committing.

  • Make sure your commit messages are in the proper format.

  • Make sure you have added the necessary tests for your changes.

  • Make sure that all tests pass

Submitting Changes

  • Push your changes to a topic branch in your fork of the repository.

  • Submit a pull request to the PROJ repository in the OSGeo organization.

  • If your pull request fixes/references an issue, include that issue number in the pull request. For example:

Wiz the bang

Fixes #123.
  • PROJ developers will look at your patch and take an appropriate action.

Coding conventions

Programming language

PROJ was originally developed in ANSI C. Today PROJ is mostly developed in C++11, with a few parts of the code base still being C. Most of the older parts of the code base is effectively C with a few modifications so that it compiles better as C++.

Coding style

The parts of the code base that has started its life as C++ is formatted with clang-format using the script scripts/ This is mostly contained to the code in src/iso19111/ but a few other .cpp-files are covered as well.

For the rest of the code base, which has its origin in C, we don't enforce any particular coding style, but please try to keep it as simple as possible. If improving existing code, please try to conform with the style of the locally surrounding code.


Throughout the PROJ code base you will see differing whitespace use. The general rule is to keep whitespace in whatever form it is in the file you are currently editing. If the file has a mix of tabs and space please convert the tabs to space in a separate commit before making any other changes. This makes it a lot easier to see the changes in diffs when evaluating the changed code. New files should use spaces as whitespace.


Reformatting C++ code

The script in scripts/ will reformat C++ code in accordance to the project preference.

If you are writing a new .cpp-file it should be added to the list in the reformatting script.

cppcheck static analyzer

You can run locally scripts/ that is a wrapper script around the cppcheck utility. This tool is used as part of the quality control of the code.

cppcheck can have false positives. In general, it is preferable to rework the code a bit to make it more 'obvious' and avoid those false positives. When not possible, you can add a comment in the code like

/* cppcheck-suppress duplicateBreak */

in the preceding line. Replace duplicateBreak with the actual name of the violated rule emitted by cppcheck.

Clang Static Analyzer (CSA)

CSA is run by a GitHub Actions workflow. You may also run it locally.

Preliminary step: install clang tools. For example, on a Debian-like OS:

sudo apt install clang-tools libfindbin-libs-perl

Configure PROJ with the scan-build utility of clang:

mkdir csa_build
cd csa_build
scan-build cmake ..

Build using scan-build:

scan-build make [-j8]

If CSA finds errors, they will be emitted during the build. And in which case, at the end of the build process, scan-build will emit a warning message indicating errors have been found and how to display the error report. This is with something like

scan-view /tmp/scan-build-2021-03-15-121416-17476-1

This will open a web browser with the interactive report.

CSA may also have false positives. In general, this happens when the code is non-trivial / makes assumptions that hard to check at first sight. You will need to add extra checks or rework it a bit to make it more "obvious" for CSA. This will also help humans reading your code !

Typo detection and fixes

Run scripts/

Include What You Use (IWYU)

Managing C includes is a pain. IWYU makes updating headers a bit easier. IWYU scans the code for functions that are called and makes sure that the headers for all those functions are present and in sorted order. However, you cannot blindly apply IWYU to PROJ. It does not understand ifdefs, other platforms, or the order requirements of PROJ internal headers. So the way to use it is to run it on a copy of the source and merge in only the changes that make sense. Additions of standard headers should always be safe to merge. The rest require careful evaluation. See the IWYU documentation for motivation and details.

IWYU docs