librsync  2.3.4

Instructions and conventions for people wanting to work on librsync. Please consider these guidelines even if you're doing your own fork.


There are a bunch of tools and libraries that are useful for development or that librsync depends on. In addition to the standard cmake/gcc/make/etc C development tools, the following packages are recommended;

  • libpopt-dev - the cmdline argument parsing library used by rdiff. If this is not available rdiff cannot be compiled, and only the librsync library will be compiled.
  • libb2-dev - blake2 hash library librsync can use if USE_LIBB2 is set to ON in cmake. If this is not avalable librsync will use its included copy of blake2 (which may be older... or newer).
  • doxygen - documentation generator used to generate html documentation. If installed make doc can be run to generate all the docs.
  • graphviz - graph generator used by doxygen for generating diagrams. If not installed doxygen will not generate any diagrams.
  • indent - code reformatter for tidying code. If installed all the code can be tidied with make tidy.
  • tidyc - extension wrapper for indent that includes better formatting of doxygen comments. If installed code and comments can be tidied with make tidyc.
  • clang-tidy - code lint checker for potential problems. If installed the code can be checked with make clang-tidy.
  • iwyu - #include checker and fixer. If installed includes can be checked with make iwyu, and automatically fixed with make iwyu-fix. Note on some platforms this package is missing a dependency and also needs libclang-common-9-dev to be installed.

These can all be installed on Debian/Ubuntu systems by running;

$ apt-get update
$ apt-get install libpopt-dev libb2-dev doxygen graphviz indent clang-tidy iwyu
$ git clone
$ cp tidyc/tidyc ~/bin


Not all the recommended packages are easily available on windows. Cygwin and MSYS2 provide a development environment similar to Linux. Some packages can also be found on Chocolatey. For native development using standard MSVC tools, libpopt can be found on vcpkg and installed by running;

$ vcpkg update
$ vcpkg --triplet x64-windows install libpopt

For cmake to find the installed libpopt you need to add -D CMAKE_TOOLCHAIN_FILE=C:/vcpkg/scripts/buildsystems/vcpkg.cmake to the cmake cmdline. This configures cmake to correctly search the vcpkg install locations to find libraries.


MacOS is generally more similar to Linux than Windows, and several packages are available on homebrew. The libpopt library can be installed by running;

$ brew update
$ brew install popt


The minimal instructions to fetch, configure, compile, and test everything using a in-place default Debug build with trace enabled using the internal blake2 implementation is;

$ git clone
$ cd librsync
$ cmake .
$ make check

For cmake, -B can be used to select a separate build directory, and -G can select a different make system. Also the following settings can be changed with -D <setting>=<value> arguments when generating the project with cmake;

  • CMAKE_BUILD_TYPE=(Debug|Release|MinSizeRel|RelWithDebInfo) - the build type to use, which selects compiler options. The default is Debug.
  • CMAKE_C_COMPILER=(cc|gcc|clang|...) - The compiler to use. The default is to auto-detect the available compiler on the platform.
  • BUILD_RDIFF=(ON|OFF) - Whether to build and test the rdiff executable. Defaults to ON if libpopt is available.
  • BUILD_SHARED_LIBS=(ON|OFF) - Whether to build dynamic libraries or use static linking. Defaults to ON.
  • ENABLE_TRACE=(ON|OFF) - Whether to enable trace output in the library and for rdiff using -v. Trace output can help with debugging but its a little faster with ENABLE_TRACE=OFF. Defaults to ON for Debug builds, and OFF for others.
  • USE_LIBB2=(ON|OFF) - Whether to use libb2 instead of the included blake2 implementation. Defaults to OFF.

So for a Release build in a separate directory using Ninja, clang, static linking, and libb2 with trace enabled, do this instead;

$ cmake -B build -G Ninja
$ cmake --build build --config Release --target check

You can also use ccmake or cmake-gui to interactively configure and generate into a separate build directory with;

$ ccmake -B build


The prefered style for code is equivalent to using GNU indent with the following arguments;

$ indent -linux -nut -i4 -ppi2 -l80 -lc80 -fc1 -sob -T FILE -T Rollsum -T rs_result

The preferred style for non-docbook comments are as follows;

| A short poem that
| shall never ever be
| reformated or reindented
/* Single line comment indented to match code indenting. */
/* Blank line delimited paragraph multi-line comments.
Without leading stars, or blank line comment delimiters. */
int a; /* code line comments */

The preferred style for docbook comments is javadoc with autobrief as follows;

/** /file file.c
* Brief summary paragraph.
* With blank line paragraph delimiters and leading stars.
* /param foo parameter descriptions...
* /param bar separate blank-line delimited paragraphs.
* Example:/code
* code blocks that will never be reformated.
* /endcode
* Without blank-line comment delimiters. */
int a; /**< brief attribute description */
int b; /**< multiline attribute description
* With blank line delimited paragraphs.*/

There is a make tidy target that will use GNU indent to reformat all code and non-docbook comments, doing some pre/post processing with sed to handle some corner cases indent doesn't handle well.

There is a make tidyc target that will reformat all code and comments with tidyc. This will also correctly reformat all docbook comments, equivalent to running tidyc with the following arguments;

$ tidyc -R -C -l80 -T FILE -T Rollsum -T rs_result

There is make clang-tidy and make iwyu targets for checking for coding errors and incorrect #include statements. Note that the iwyu check gets confused by and will emit warnings about fileutil.c which has extra conditional includes necessary to find working functions on various platforms. Other than fileutil.c both checks should be clean.

If iwyu finds problems, make ifwu-fix can be run to automatically fix them, followed by make tidyc to reformat the result to our preferred style. Note that this doesn't always produce an ideal result and may require manual intervention.

Please try to update docs and tests in parallel with code changes.


Using make check will compile and run all tests. Additional code correctness checks can be run with make clang-tidy and make iwyu.

Note that assert() is used extensively within the code for verifying the correctness of internal operations using a roughly design-by-contract approach. These are only enabled for Debug builds, so testing with a Debug build will give a better chance of identifying problems during development. Once you are confident the code is correct, a Release build will turn these off giving faster execution.

There are also GitHub Actions configured for the librsync project to configure, build, test, and lint everything on a variety of different platforms with a variety of different settings. These are run against any pull request or commit, and are a good way to check things are not broken for other platforms.

Test results for builds of public github branches are at

Documenting contains a list of user-visible changes in the library between release versions. This includes changes to the way it's packaged, bug fixes, portability notes, changes to the API, and so on. Add and update items under a "Changes in X.Y.Z" heading at the top of the file. Do this as you go along, so that we don't need to work out what happened when it's time for a release. contains a list of ideas and proposals for the future. Ideally entries should be formated in a way that can be just moved into when they are done. Regularly check to see if there is anything that needs removing or updating.


Fixes or improvements in pull requests are welcome. Please:

  • [ ] Send small PRs that address one issues each.
  • [ ] Update to say what you changed.
  • [ ] Add a test as a self-contained C file in tests/ that passes or fails, and is hooked into CMakeLists.txt.
  • [ ] Keep the code style consistent with what's already there, especially in keeping symbols with an rs_ prefix.


If you are making a new tarball release of librsync, follow this checklist:

  • Make a "Release vx.x.x" pull request containing updates ready for the release including;
    • Review the changes included and decide if the release should be a major (non-backwards compatible change), minor (backwards compatible change), or micro (bugfix only change) version number change to get the new "X.Y.Z" version number.
    • - make sure the top "Changes in X.Y.Z" is correct, and the date is correct. Make sure the changes since the last release are documented.
    • - check if anything needs to be removed or updated.
    • CMakeLists.txt - version is correct.
    • librsync.spec - make sure version and URL are right.
  • Run make all doc check in a clean checkout of the release pull request branch. Also check the GitHub Actions check and lint status of the last commit on github. If it all looks good merge the release pull request on github.
  • Draft a new release on github, typing in the release details including an overview, included changes, and known issues. The overview should give an indication of the magnitude of the changes and their impact, and the relative urgency to upgrade. The included changes should come from the for the release. It's probably easiest to copy and edit the previous release.
  • After creating the release, download the Source code (tar.gz) release asset. Go to "Actions", find the workflow run for the "Check" corresponding to the merge of the release pull request, and download the install results windows-latest Release artifact renamed to Edit the release, and upload the source code and windows artifacts. This ensures that the release includes a stable tarball (See for details) and win64 install.
  • Run make doc on a clean checkout of the new release tag and cp -av html/* into a rm -Rf * emptied checkout of and check it in. This ensures it includes deletes of obsolete files as well as new and updated files. Push this to update the online docs.
  • Create and merge a "Prepare vX.Y.Z+1." pull request containing updates to prepare for the changes in the next release including;
    • Bump the minor version in CMakeLists.txt.
    • Add a NOT RELEASED YET version entry in
    • Bump the minor version and add a changelog entry in librsync.spec.