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Knowledge Base

Using CMake

CMake can be used to build more complex projects in build.sh or bld.bat scripts.

If you are using cmake, be sure to make it a build requirement in the build section. You may also need to include make or ninja depending on your platform and build tools. On Windows, you can also use nmake to build, but that does not need to be explicitly included.

requirements:
build:
- cmake
- make # [not win]
- ninja # [win]

For CMake projects using the FindPython module, you can tell CMake which Python to use by passing -DPython_EXECUTABLE="$PYTHON" (macOS or Linux) or -DPython_EXECUTABLE="%PYTHON%" (Windows) as a command line option. Older CMake projects may require similar, but slightly different options.

tip

Don't forget that depending on which CMake module you use you have to use a different command:

or if you are still on the deprecated FindPythonLibs: -DPYTHON_EXECUTABLE=....

Some optional, but useful CMake options:

  • -DCMAKE_BUILD_TYPE=Release Configure as release build. This is better done on the initial cmake call as some packages construct different build configurations depending on this flag.
  • -DCMAKE_INSTALL_PREFIX=$PREFIX Specify the install location.
  • -DCMAKE_INSTALL_LIBDIR=lib Libraries will land in $PREFIX/lib, sometimes projects install into lib64 or similar but on conda-forge we keep shared libraries in simply lib.
  • -DBUILD_SHARED_LIBS=ON Instruct CMake to build shared libraries instead of static ones.
  • -DCMAKE_FIND_FRAMEWORK=NEVER and -DCMAKE_FIND_APPBUNDLE=NEVER Prevent CMake from using system-wide macOS packages.
  • ${CMAKE_ARGS} Add variables defined by conda-forge internally. This is required to enable various conda-forge enhancements, like CUDA builds.

Here are some basic commands for you to get started. These are dependent on your source code layout and aren't intended to be used "as is".

CMake lines for build.sh (macOS/Linux):

cmake CMakeLists.txt -DPython3_EXECUTABLE="$PYTHON"
cmake --build . --config Release

CMake lines for bld.bat (Windows):

cmake -G "NMake Makefiles" -DCMAKE_BUILD_TYPE=Release -DPython3_EXECUTABLE="%PYTHON%"
if errorlevel 1 exit /b 1
cmake --build . --config Release
if errorlevel 1 exit /b 1

See also the bld.bat in the Windows section below for an additional example.

Other useful cmake options are -B<directory> and -S<directory> to specify build and source directories.

Moving from an autotools build to a CMake build

Some packages maintain an autotools build and a cmake build. Some maintainers would like to switch to a cmake build because that provides windows builds easily. These builds are mostly not ABI compatible with each other. Here are some things you should check,

  1. Check that both libraries have the same SONAME on linux

    Run readelf -d /path/to/lib.so

  2. Check that both libraries have the same install name and have the same compatibility and current versions.

    Run otool -L /path/to/lib.dylib. The second line should give you the three pieces of information

  3. Check that the file list is the same in both.

  4. Check that you use the same options as the same autoconf build.

  5. Check that the symbols exported are the same.

  6. Check that additional packaging information stays the same, e.g. is the same pkg-config information provided.

Particularities on Windows

This document presents conda-forge and conda-build information and examples while building on Windows.

Local testing

The first thing that you should know is that you can locally test Windows builds of your packages even if you don't own a Windows machine. Microsoft makes available free, official Windows virtual machines (VMs) at this website. If you are unfamiliar with VM systems or have trouble installing Microsoft's VMs, please use a general web search to explore — while these topics are beyond the scope of this documentation, there are ample discussions on them on the broader Internet.

To bootstrap a conda environment and install conda-build, consider miniforge.

Executing a build

The build-locally.py script does not support Windows (yet, PRs welcome!). You can use conda build recipe/ -m .ci_support/choose_your_config.yaml as a workaround for now.

Testing a local build

Because we're using conda-build directly instead of build-locally.py, we can use the local channel:

conda create -n my-new-env -c local my-package

Notes on native code

In order to compile native code (C, C++, etc.) on Windows, you will need to install Microsoft's Visual C++ build tools on your VM. You must install particular versions of these tools — this is to maintain compatibility between compiled libraries used in Python, as described on this Python wiki page. The current relevant versions are:

  • For Python 3.5–3.12+: Visual C++ 14.x

While you can obtain these tools by installing the right version of the full Visual Studio development environment, you can save a lot of time and bandwidth by installing standalone "build tools" packages. You can get them from Visual Studio Subscriptions. To download build tools, you'll need a Microsoft account. Once on the Visual Studio Subscriptions page, you may also need to join the Dev Essentials program. Once that's done, you can click the "Download" tab and search for "Build Tools for Visual Studio 2022".

If you need more information. Please refer the Python wiki page on Windows compilers.

Simple CMake-Based bld.bat

Some projects provide hooks for CMake to build the project. The following example bld.bat file demonstrates how to build a traditional, out-of-core build for such projects.

CMake-based bld.bat:

setlocal EnableDelayedExpansion

:: Make a build folder and change to it.
mkdir build
cd build

:: Configure using the CMakeFiles
cmake -G "NMake Makefiles" ^
-DCMAKE_INSTALL_PREFIX:PATH="%LIBRARY_PREFIX%" ^
-DCMAKE_PREFIX_PATH:PATH="%LIBRARY_PREFIX%" ^
-DCMAKE_BUILD_TYPE:STRING=Release ^
..
if errorlevel 1 exit 1

:: Build!
nmake
if errorlevel 1 exit 1

:: Install!
nmake install
if errorlevel 1 exit 1

The following feedstocks are examples of this build structure deployed:

Building for different VC versions

On Windows, different Visual C versions have different ABI and therefore a package needs to be built for different Visual C versions. Packages are tied to the VC version that they were built with and some packages have specific requirements of the VC version. For example, python 2.7 requires vc 9 and python 3.5 requires vc 14.

With conda-build 3.x, vc can be used as a selector when using the compiler jinja syntax.

requirements:
build:
- {{ compiler('cxx') }}

To skip building with a particular vc version, add a skip statement.

build:
skip: true # [win and vc<14]

requirements:
build:
- {{ compiler('cxx') }}

Using vs2022

In recipe/conda_build_config.yaml file:

c_compiler:    # [win]
- vs2022 # [win]
cxx_compiler: # [win]
- vs2022 # [win]

You can look at the changes in this PR.

After making these changes don't forget to rerender with conda-smithy (to rerender manually use conda smithy rerender from the command line).

Tips & tricks for CMD/Batch syntax

Windows recipes rely on CMD/Batch scripts (.bat) by default. Batch syntax is a bit different from Bash and friends on Unix, so we have collected some tips here to help you get started if you are not familiar with this scripting language.

  • Check if you need to write a Batch script first! Simple recipes might not need shell-specific code and can be written in an agnostic way. Use the build.script item in meta.yaml (see conda-build docs). This item can take a string or a list of strings (one per line).
  • SS64's CMD howto pages are the best resource for any kind of question regarding CMD/Batch syntax.
  • Search conda-forge for existing .bat scripts and learn with examples. See this example query for all Batchfiles.
  • You can free trial Windows VMs from Microsoft. Set one up with your favorite virtualization solution to debug your CMD syntax. There are also some minimal emulators online that might get you started with the basics, even if not all CMD features are present. For example, this Windows 95 emulator features a more or less okay MS-DOS prompt.

Special Dependencies and Packages

Compilers

Compilers are dependencies with a special syntax and are always added to requirements/build.

There are currently five supported compilers:

  • C
  • cxx
  • Fortran
  • Go
  • Rust

A package that needs all five compilers would define

requirements:
build:
- {{ compiler('c') }}
- {{ compiler('cxx') }}
- {{ compiler('fortran') }}
- {{ compiler('go') }}
- {{ compiler('rust') }}
note

Appropriate compiler runtime packages will be automatically added to the package's runtime requirements and therefore there's no need to specify libgcc or libgfortran. There are additional informations about how conda-build 3 treats compilers in the conda docs.

Cross-compilation

conda-forge defaults to native builds of packages for x86_64 on Linux, macOS and Windows, because that's the architecture powering the default CI runners. Other architectures are supported too, but they are not guaranteed to have native builds. In those platforms where we can't provide native CI runners, we can still resort to either cross-compilation or emulation.

Cross-compiling means building a package for a different architecture than the one the build process is running on. Given how abundant x86_64 runners are, most common cross-compilation setups will target non-x86_64 architectures from x86_64 runners.

Cross-compilation terminology usually distinguishes between two types of machine:

  • Build: The machine running the building process.
  • Host: The machine we are building packages for.
note

Some cross-compilation documentation might also distinguish between a third type of machine, the target machine. You can read more about it in this Stack Overflow question. For the purposes of conda-forge, we'll consider the target machine to be the same as the host.

How to enable cross-compilation

Cross-compilation settings depend on the build_platform and target_platform conda-build variables:

  • build_platform: The platform on which conda-build is running, which defines the build environment in $BUILD_PREFIX.
  • target_platform: The platform on which the package will be installed. Defines the platform of the host environment in $PREFIX. Defaults to the value of build_platform.

To change the value of target_platform and enable cross-compilation, you must use the build_platform mapping in conda-forge.yml and then rerender the feedstock. This will generate the appropriate CI workflows and conda-build input metadata. See also test for how to skip the test phase when cross-compiling. Provided the requirements metadata and build scripts are written correctly, the package should just work. However, in some cases, it'll need some adjustments; see examples below for some common cases.

note

The build_platform and target_platform variables are exposed as environment variables in the build scripts (e.g. $build_platform), and also as Jinja variables in the meta.yaml selectors (e.g. # [build_platform != target_platform]).

In addition to these two variables, there are some more environment variables that are set by conda-forge's automation (e.g. conda-forge-ci-setup, compiler activation packages, etc) that can aid in cross-compilation setups:

  • CONDA_BUILD_CROSS_COMPILATION: set to 1 when build_platform and target_platform differ.
  • CONDA_TOOLCHAIN_BUILD: the autoconf triplet expected for build platform.
  • CONDA_TOOLCHAIN_HOST: the autoconf triplet expected for host platform.
  • CMAKE_ARGS: arguments needed to cross-compile with CMake. Pass it to cmake in your build script.
  • MESON_ARGS: arguments needed to cross-compile with Meson. Pass it to meson in your build script. Note a cross build definition file is automatically created for you too.
  • CC_FOR_BUILD: C compilers targeting the build platform.
  • CXX_FOR_BUILD: C++ compilers targeting the build platform.
  • CROSSCOMPILING_EMULATOR: Path to the qemu binary for the host platform. Useful for running tests when cross-compiling.

This is all supported by two main conda-build features introduced in version 3:

Placing requirements in build or host

The rule of the thumb is:

  • If it needs to run during the build, it goes in build.
  • If it needs to be available on the target host, it goes in host.
  • If both conditions are true, it belongs in both.

However, there are some exceptions to this rule; most notably Python cross-compilation (see below).

Cross-compilation examples

A package needs to make a few changes in their recipe to be compatible with cross-compilation. Here are a few examples.

A simple C library using autotools for cross-compilation might look like this:

requirements:
build:
- {{ compiler("c") }}
- make
- pkg-config
- gnuconfig

In the build script, it would need to update the config files and guard any tests when cross-compiling:

# Get an updated config.sub and config.guess
cp $BUILD_PREFIX/share/gnuconfig/config.* .

# Skip ``make check`` when cross-compiling
if [[ "${CONDA_BUILD_CROSS_COMPILATION:-}" != "1" || "${CROSSCOMPILING_EMULATOR:-}" != "" ]]; then
make check
fi

A simple C++ library using CMake for cross-compilation might look like this:

requirements:
build:
- {{ compiler("cxx") }}
- cmake
- make

In the build script, it would need to update cmake call and guard any tests when cross-compiling:

# Pass ``CMAKE_ARGS`` to ``cmake``
cmake ${CMAKE_ARGS} ..

# Skip ``ctest`` when cross-compiling
if [[ "${CONDA_BUILD_CROSS_COMPILATION:-}" != "1" || "${CROSSCOMPILING_EMULATOR:-}" != "" ]]; then
ctest
fi

Similarly, with Meson, the meta.yaml needs:

requirements:
build:
- {{ compiler("c") }}
- {{ compiler("cxx") }}
- meson
- make

And this in build.sh:

# Pass ``MESON_ARGS`` to ``meson``
meson ${MESON_ARGS} builddir/

A simple Python extension using Cython and NumPy's C API would look like so:

requirements:
build:
- {{ compiler("c") }}
- cross-python_{{ target_platform }} # [build_platform != target_platform]
- python # [build_platform != target_platform]
- cython # [build_platform != target_platform]
- numpy # [build_platform != target_platform]
host:
- python
- pip
- cython
- numpy
run:
- python
- {{ pin_compatible("numpy") }}

With MPI, openmpi is required for the build platform as the compiler wrappers are binaries, but mpich is not required as the compiler wrappers are scripts (see example):

requirements:
build:
- {{ mpi }} # [build_platform != target_platform and mpi == "openmpi"]
host:
- {{ mpi }}
run:
- {{ mpi }}

In the build script, openmpi compiler wrappers can use host libraries by setting the environmental variable OPAL_PREFIX to $PREFIX.

if [[ "$CONDA_BUILD_CROSS_COMPILATION" == "1" && "${mpi}" == "openmpi" ]]; then
export OPAL_PREFIX="$PREFIX"
fi

There are more variations of this approach in the wild. So this is not meant to be exhaustive, but merely to provide a starting point with some guidelines. Please look at other recipes for more examples.

Details about cross-compiled Python packages

Cross-compiling Python packages is a bit more involved than other packages. The main pain point is that we need an executable Python interpreter (i.e. python in build) that knows how to provide accurate information about the target platform. Since this is not officially supported, a series of workarounds are required to make it work. Refer to PEP720 or the discussion in this issue for more information.

In practical terms, for conda-forge, this results into two extra metadata bits that are needed in meta.yaml:

  • Adding cross-python_{{ target_platform }} in build requirements, provided by the cross-python-feedstock. This is a wrapper for the crossenv Python interpreters with some activation logic that adjust some of the crossenv workarounds so they work better with the conda-build setup.
  • Copying some Python-related packages from host to build with a [build_platform != target_platform] selector:
    • python itself, to support crossenv.
    • Non-pure Python packages (i.e. they ship compiled libraries) that need to be present while the package is being built, like cython and numpy.

In the terms of the PEP720, the conda-forge setup implements the "faking the target environment" approach. More specifically, this will result in the following changes before the builds scripts run:

  • A modified crossenv installation in $BUILD_PREFIX/venv, mimicking the architecture of $PREFIX.
  • Forwarder binaries in $BUILD_PREFIX/bin that point to the crossenv installation.
  • Symlinks that expose the $BUILD_PREFIX site-packages in the crossenv installation, which is also included in $PYTHONPATH.
  • A copy of all $PREFIX site-packages to $BUILD_PREFIX (except the compiled libraries).

All in all, this results in a setup where conda-build can run a $BUILD_PREFIX-architecture python interpreter that can see the packages in $PREFIX (with the compiled bits provided by their corresponding counterparts in $BUILD_PREFIX) and sufficiently mimic that target architecture.

Emulated builds

When cross-compilation is not possible, one can resort to emulation. This is a technique that uses a virtual machine (QEMU) to emulate the target platform, which has a significant overhead. However, conda-build will see the target platform as native, so very little changes are usually needed in the recipe.

To enable emulated builds, you must use the provider mapping in conda-forge.yml. This key maps a build_platform to a provider that will be used to emulate the platform. conda-smithy will know how to detect whether the provider supports that platform natively or requires emulation, and will adjust the appropriate CI steps to ensure that QEMU runs the process. Ensure changes are applied by rerendering the feedstock.

Note that only Linux architectures are currently supported via emulation.

warning

Emulated builds are very slow and incur an additional strain on conda-forge CI resources. Whenever possible, please consider cross-compilation instead. Only use emulated builds as a last resort.

Emulation examples

Configure conda-forge.yml to emulate linux-ppc64le, but use native runners for linux-64 and linux-aarch64. This works because linux-ppc64le is not natively supported by Azure, so conda-smithy will add QEMU steps to emulate it. However, linux-64 and linux-aarch64 are natively supported by Azure and Travis CI, respectively, so no emulation is needed.

provider:
linux_aarch64: travis
linux_ppc64le: azure
linux_64: azure

Rust Nightly

Many rust packages rely on nightly versions of the rust compiler. Given this fast release cadence, conda-forge does not yet pull each release. Instead, rust nightly versions are pulled into the dev branch of the conda-forge/rust-feedstock on an as-needed basis. For a new version, please file an issue on that feedstock.

To enable the rust nightly compiler in your feedstock, follow the section above and then add the rust_dev channel in the conda_build_config.yaml file:

channel_sources:
- conda-forge/label/rust_dev,conda-forge

Core Dependency Tree Packages (CDTs)

Dependencies outside of the conda-forge channel should be avoided (see Avoid external dependencies). However, there are a few exceptions:

Some dependencies are so close to the system that they are not packaged with conda-forge. These dependencies have to be satisfied with Core Dependency Tree (CDT) packages.

A CDT package consists of repackaged CentOS binaries from the appropriate version, either 6 or 7 depending on user choice and platform. We manage the build of CDT packages using a centralized repo, conda-forge/cdt-builds, as opposed to generating feedstocks for them. (Note that historically we did use feedstocks but this practice has been deprecated.) To add a new CDT, make a PR on the conda-forge/cdt-builds repo.

Why are CDTs bad?

  1. CDTs repackage old versions of the library.
  2. As a result, newer functionality in the packages won't be used by downstream conda packages which check for the version of the library being built against. For example: OpenGL functionality from the CentOS 6/7 packaged library is available, but any newer functionality cannot be used.
  3. We have no guarantees that the version provided by the user's system is compatible. We only have the __glibc>=2.17 constraint and we assume that CentOS 6/7's lower bound of GLIBC and its corresponding lower bound of the CDT are equivalent.
  4. We have no guarantee that the library is provided by the user's system at all.

When should CDTs be used?

  1. When there are system specific configurations are used by the library. Some examples include:
    1. OpenGL: if we provided the OpenGL loader library libglvnd. and the user's system is not using libglvnd, then we cannot load the vendor-specific implementations losing out on accelerator/hardware optimized performance. (This is only on old distributions and we may finally be able to package libglvnd ourselves)
    2. linux-pam: This is a library that allows pluggable authentication modules and the configuration files for these modules usually live in /etc/pam.d. The issue is that the pluggable modules live in a distro specific location. For example: /usr/lib/x86_64-linux-gnu/security/. The default modules are built into the conda package in $CONDA_PREFIX/lib/security, but custom ones for system-wide configuration are installed into /usr/lib/x86_64-linux-gnu/security/. So, we would need to patch the module to look into both, but the directory /usr/lib/x86_64-linux-gnu/security/ is distro specific and will be hard to detect.
  2. When a conda packaged library will not work properly. For example: a new glibc package means we would have to edit the elf interpreter of all the conda package binaries.

What's are some good examples?

  1. The OpenCL loader (ocl-icd together with ocl-icd-system) provides an OpenCL loading library. The loader will look at OpenCL implementations given in $CONDA_PREFIX/etc/OpenCL/vendors. For example: Pocl is a conda packaged implementation that runs OpenCL on the CPU. Vendor specific implementations like the NVIDIA OpenCL or ROCm OpenCL are not conda packaged, so we have to rely on the system. By installing ocl-icd-system we enable the loader to look at the configuration in /etc/OpenCL/vendors, which is the configuration directory for all linux distributions. This gives us the best of both worlds. You don't need a system level package to run OpenCL because we have a conda packaged installation, but if there is a system wide implementation that is accelerated by specific hardware, we can use those.

In conda-forge the primary usages of CDTs is currently for packages that link against libGL.

libGL

In addition to the required compilers {{ compiler('c') }} and/or {{ compiler('cxx') }}, the following CDT packages are required for linking against libGL:

requirements:
build:
- {{ cdt('mesa-libgl-devel') }} # [linux]
- {{ cdt('mesa-dri-drivers') }} # [linux]
- {{ cdt('libselinux') }} # [linux]
- {{ cdt('libxdamage') }} # [linux]
- {{ cdt('libxxf86vm') }} # [linux]
- {{ cdt('libxext') }} # [linux]
host:
- xorg-libxfixes # [linux]

If you need a fully functional binary in the test phase, you have to also provide the shared libraries via yum_requirements.txt (see yum_requirements.txt).

mesa-libGL
mesa-dri-drivers
libselinux
libXdamage
libXxf86vm
libXext

You will need to re-render the feedstock after making these changes.

Building Against NumPy

Packages that link against NumPy need special treatment in the dependency section. Finding numpy.get_include() in setup.py or cimport statements in .pyx or .pyd files are a telltale sign that the package links against NumPy.

In the case of linking, you need to use the pin_compatible function to ensure having a compatible numpy version at run time:

host:
- numpy
run:
- {{ pin_compatible('numpy') }}

At the time of writing (January 22, 2022), above is equivalent to the following,

host:
- numpy 1.18 # [py==37]
- numpy 1.18 # [py==38]
- numpy 1.19 # [py==39]
run:
- numpy >=1.18.5,<2.0.a0 # [py==37]
- numpy >=1.18.5,<2.0.a0 # [py==38]
- numpy >=1.19.5,<2.0.a0 # [py==39]

See the pinning repository for what the pinning corresponds to at time of writing https://github.com/conda-forge/conda-forge-pinning-feedstock/blob/master/recipe/conda_build_config.yaml#L631

Notes
  1. You still need to respect minimum supported version of numpy for the package! That means you cannot use numpy 1.9 if the project requires at least numpy 1.12, adjust the minimum version accordingly!
host:
- numpy 1.12.*
run:
- {{ pin_compatible('numpy') }}
  1. if your package supports numpy 1.7, and you are brave enough :-), there are numpy packages for 1.7 available for Python 2.7 in the channel.

JupyterLab Extensions

A typical JupyterLab extension has both Python and JavaScript components. These should be packaged together, to prevent node from being needing to grab the JavaScript side of the package on the user's machine. To package an extension, the build should have the following meta.yaml snippet:

build:
noarch: python


requirements:
host:
- python
- nodejs
- pip
run:
- python
- nodejs
- jupyterlab >=2

Please use the following build.sh script in your recipe:

#!/usr/bin/env bash
set -ex

$PYTHON -m pip install . -vv
npm pack ${PKG_NAME}@${PKG_VERSION}
mkdir -p ${PREFIX}/share/jupyter/lab/extensions/js
cp ${PKG_NAME}-${PKG_VERSION}.tgz ${PREFIX}/share/jupyter/lab/extensions/js

Since this is a noarch recipe, the build script only needs to run on linux-64. Also note that we do not need to run jupyter labextension install or jupyter lab build as part of the package build or in any post-link scripts. This is because JupyterLab will run the build step itself when it is next run. The ${PREFIX}/share/jupyter/lab/extensions/js directory which JupyterLab knows to build from when performing this build step.

Message passing interface (MPI)

note

This section originates from Min's notes: https://hackmd.io/ry4uI0thTs2q_b4mAQd_qg

MPI Variants in conda-forge

How are MPI variants best handled in conda-forge?

There are a few broad cases:

  • package requires a specific MPI provider (easy!)
  • the package works with any MPI provider (e.g. mpich, openmpi)
  • the package works with/without MPI

Note that sometimes users want to use packages in conda-forge built against our MPI libraries but linked to external MPI libraries at runtime. If you are interested in this procedure, see Using External Message Passing Interface (MPI) Libraries for details.

Building MPI variants

In conda_build_config.yaml:

mpi:
- mpich
- openmpi

In meta.yaml:

requirements:
host:
- {{ mpi }}

And rerender with:

conda-smithy rerender -c auto

to produce the build matrices.

Including a no-mpi build

Some packages (e.g. hdf5) may want a no-mpi build, in addition to the mpi builds. To do this, add nompi to the mpi matrix:

mpi:
- nompi
- mpich
- openmpi

and apply the appropriate conditionals in your build:

requirements:
host:
- {{ mpi }} # [mpi != 'nompi']
run:
- {{ mpi }} # [mpi != 'nompi']

Preferring a provider (usually nompi)

Up to here, mpi providers have no explicit preference. When choosing an MPI provider, the mutual exclusivity of the mpi metapackage allows picking between mpi providers by installing an mpi provider, e.g.

conda install mpich ptscotch

or

conda install openmpi ptscotch

This doesn't extend to nompi, because there is no nompi variant of the mpi metapackage. And there probably shouldn't be, because some packages built with mpi don't preclude other packages in the env that may have an mpi variant from using the no-mpi variant of the library (e.g. for a long time, fenics used mpi with no-mpi hdf5 since there was no parallel hdf5 yet. This works fine, though some features may not be available).

Typically, if there is a preference it will be for the serial build, such that installers/requirers of the package only get the mpi build if explicitly requested. We use a higher build number for the nompi variant in this case.

Here is an example build section:

{% if mpi == 'nompi' %}
# prioritize nompi variant via build number
{% set build = build + 100 %}
{% endif %}
build:
number: {{ build }}

# add build string so packages can depend on
# mpi or nompi variants explicitly:
# `pkg * mpi_mpich_*` for mpich
# `pkg * mpi_*` for any mpi
# `pkg * nompi_*` for no mpi

{% if mpi != 'nompi' %}
{% set mpi_prefix = "mpi_" + mpi %}
{% else %}
{% set mpi_prefix = "nompi" %}
{% endif %}
string: "{{ mpi_prefix }}_h{{ PKG_HASH }}_{{ build }}"
note

{{ PKG_HASH }} avoids build string collisions on most variants, but not on packages that are excluded from the default build string, e.g. Python itself. If the package is built for multiple Python versions, use:

string: "{{ mpi_prefix }}_py{{ py }}h{{ PKG_HASH }}_{{ build }}"

as seen in mpi4py

This build section creates the following packages:

  • pkg-x.y.z-mpi_mpich_h12345_0
  • pkg-x.y.z-mpi_openmpi_h23456_0
  • pkg-x.y.z-nompi_h34567_100

Which has the following consequences:

  • The nompi variant is preferred, and will be installed by default unless an mpi variant is explicitly requested.
  • mpi variants can be explicitly requested with pkg=*=mpi_{{ mpi }}_*
  • any mpi variant, ignoring provider, can be requested with pkg=*=mpi_*
  • nompi variant can be explicitly requested with pkg=*=nompi_*

If building with this library creates a runtime dependency on the variant, the build string pinning can be added to run_exports.

For example, if building against the nompi variant will work with any installed version, but building with a given mpi provider requires running with that mpi:

build:
...
{% if mpi != 'nompi' %}
run_exports:
- {{ name }} * {{ mpi_prefix }}_*
{% endif %}

Remove the if mpi... condition if all variants should create a strict runtime dependency based on the variant chosen at build time (i.e. if the nompi build cannot be run against the mpich build).

Complete example

Combining all of the above, here is a complete recipe, with:

  • nompi, mpich, openmpi variants
  • run-exports to apply mpi choice made at build time to runtime where nompi builds can be run with mpi, but not vice versa.
  • nompi variant is preferred by default
  • only build nompi on Windows

This matches what is done in hdf5.

# conda_build_config.yaml
mpi:
- nompi
- mpich # [not win]
- openmpi # [not win]
# meta.yaml
{% set name = 'pkg' %}
{% set build = 0 %}

# ensure mpi is defined (needed for conda-smithy recipe-lint)
{% set mpi = mpi or 'nompi' %}

{% if mpi == 'nompi' %}
# prioritize nompi variant via build number
{% set build = build + 100 %}
{% endif %}

build:
number: {{ build }}

# add build string so packages can depend on
# mpi or nompi variants explicitly:
# `pkg * mpi_mpich_*` for mpich
# `pkg * mpi_*` for any mpi
# `pkg * nompi_*` for no mpi

{% if mpi != 'nompi' %}
{% set mpi_prefix = "mpi_" + mpi %}
{% else %}
{% set mpi_prefix = "nompi" %}
{% endif %}
string: "{{ mpi_prefix }}_h{{ PKG_HASH }}_{{ build }}"

{% if mpi != 'nompi' %}
run_exports:
- {{ name }} * {{ mpi_prefix }}_*
{% endif %}

requirements:
host:
- {{ mpi }} # [mpi != 'nompi']
run:
- {{ mpi }} # [mpi != 'nompi']

And then a package that depends on this one can explicitly pick the appropriate mpi builds:

# meta.yaml

requirements:
host:
- {{ mpi }} # [mpi != 'nompi']
- pkg
- pkg * mpi_{{ mpi }}_* # [mpi != 'nompi']
run:
- {{ mpi }} # [mpi != 'nompi']
- pkg * mpi_{{ mpi }}_* # [mpi != 'nompi']

mpi-metapackage exclusivity allows mpi_* to resolve the same as mpi_{{ mpi }}_* if {{ mpi }} is also a direct dependency, though it's probably nicer to be explicit.

Just mpi example

Without a preferred nompi variant, recipes that require mpi are much simpler. This is all that is needed:

# conda_build_config.yaml
mpi:
- mpich
- openmpi
# meta.yaml
requirements:
host:
- {{ mpi }}
run:
- {{ mpi }}

MPI Compiler Packages

Do not use the [openmpi,mpich]-[mpicc,mpicxx,mpifort] metapackages in the requirements/build section of a recipe; the MPI compiler wrappers are included in the main openmpi/mpich packages. As shown above, just add openmpi/mpich to the requirements/host section and use compiler directives for the corresponding compilers in requirements/build as normal.

OpenMP

You can enable OpenMP on macOS by adding the llvm-openmp package to the build section of the meta.yaml. For Linux OpenMP support is on by default, however it's better to explicitly depend on the libgomp package which is the OpenMP implementation from the GNU project.

# meta.yaml
requirements:
build:
- llvm-openmp # [osx]
- libgomp # [linux]

Switching OpenMP implementation

On macOS, only LLVM's OpenMP implementation llvm-openmp is supported. This implementation is used even in Fortran code compiled using GNU's gfortran.

On Linux (except aarch64), packages are linked against GNU's libgomp.so.1, but the OpenMP library at install time can be switched from GNU to LLVM by doing the following.

conda install _openmp_mutex=*=*_llvm

OpenMP library can be switched back to GNU's libgomp by doing the following.

conda install _openmp_mutex=*=*_gnu
note

OpenMP library switching is possible because LLVM's implementation has the symbol's from GNU in addition to the LLVM ones (originally from Intel). An object file generated by gcc, g++ or gfortran will have GNU's symbols and therefore the underlying library can be switched. However, an object file generated by clang or clang++ will have LLVM's symbols and therefore the underlying OpenMP library cannot be switched to GNU's library.

One reason you may wish to switch to LLVM is because the implementation is fork safe. One reason to keep using the GNU implementation is that the OpenMP target offloading symbols in libgomp like GOMP_target are empty stubs in LLVM and therefore does not work.

yum_requirements.txt

Dependencies can be installed into the build container with yum, by listing package names line by line in a file named yum_requirements.txt in the recipe directory of a feedstock.

There are only very few situations where dependencies installed by yum are acceptable. These cases include

  • satisfying the requirements of CDT packages during test phase
  • installing packages that are only required for testing

After changing yum_requirements.txt, rerender to update the configuration.

BLAS

If a package needs one of BLAS, CBLAS, LAPACK, LAPACKE, use the following in the host of the recipe,

requirements:
host:
- libblas
- libcblas
- liblapack
- liblapacke
note

You should specify only the libraries that the package needs. (i.e. if the package doesn't need LAPACK, remove liblapack and liblapacke)

At recipe build time, above requirements would download the NETLIB's reference implementations and build your recipe against those. At runtime, by default the following packages will be used.

- openblas   # [not win]
- mkl # [win]

If a package needs a specific implementation's internal API for more control you can have,

requirements:
host:
- {{ blas_impl }}
run:
- libblas * *{{ blas_impl }}
- {{ blas_impl }}

This would give you a matrix builds for different blas implementations. If you only want to support a specific blas implementation,

requirements:
host:
- openblas
run:
- libblas * *openblas
- openblas
note

blas_* features should not be used anymore.

Switching BLAS implementation

You can switch your BLAS implementation by doing,

conda install "libblas=*=*mkl"
conda install "libblas=*=*openblas"
conda install "libblas=*=*blis"
conda install "libblas=*=*accelerate"
conda install "libblas=*=*netlib"

This would change the BLAS implementation without changing the conda packages depending on BLAS.

The following legacy commands are also supported as well.

conda install "blas=*=mkl"
conda install "blas=*=openblas"
conda install "blas=*=blis"
conda install "blas=*=accelerate"
conda install "blas=*=netlib"
note

If you want to commit to a specific blas implementation, you can prevent conda from switching back by pinning the blas implementation in your environment. To commit to mkl, add blas=*=mkl to <conda-root>/envs/<env-name>/conda-meta/pinned, as described in the conda-docs.

How it works

At recipe build time, the netlib packages are used. This means that the downstream package will link to libblas.so.3 in the libblas=*=*netlib and will use only the reference implementation's symbols.

libblas and libcblas versioning is based on the Reference LAPACK versioning which at the time of writing is 3.8.0. Since the BLAS API is stable, a downstream package will only pin to 3.* of libblas and libcblas. On the other hand, liblapack and liblapacke pins to 3.8.*.

In addition to the above netlib package, there are other variants like libblas=*=*openblas, which has openblas as a dependency and has a symlink from libblas.so.3 to libopenblas.so. libblas=3.8.0=*openblas pins the openblas dependency to a version that is known to support the BLAS 3.8.0 API. This means that, at install time, the user can select what BLAS implementation they like without any knowledge of the version of the BLAS implementation needed.

Microarchitecture-optimized builds

conda virtual packages include __archspec, which expose the processor architecture to the solver. However, __archspec should not be used directly in recipes; instead, users should rely on the microarch-level helper packages (contributed in staged-recipes#24306).

Before learning how to use it, please read these considerations:

  • Adding microarchitecture variants can result in too many entries in the build matrix. Do not overuse it.
  • These optimized builds should only be used when the performance improvements are significant.
  • Preferrably, the project should rely on runtime dispatch for arch-specific optimizations.
  • If the package is already too large, consider using smaller outputs for the arch-optimized variants.

To implement microarchitecture-optimized builds in your feedstock, you'll end up with something like:

recipe/conda_build_config.yaml
microarch_level:  # [unix and x86_64]
- 1 # [unix and x86_64]
- 3 # [unix and x86_64]
- 4 # [unix and x86_64]
recipe/meta.yaml
# ...
{% set build = 0 %}

build:
number: {{ build }} # [not (unix and x86_64)]
number: {{ build + 100 }} # [unix and x86_64 and microarch_level == 1]
number: {{ build + 300 }} # [unix and x86_64 and microarch_level == 3]
number: {{ build + 400 }} # [unix and x86_64 and microarch_level == 4]

requirements:
build:
- x86_64-microarch-level {{ microarch_level }} # [unix and x86_64]
- {{ compiler('c') }}
# ...
# ...
Prioritize your preferred microarchitecture

The run_exports metadata is only set up with lower bounds to allow in-CI testing. This means that level=2 package can be installed in a level=3 machine. Make sure to assign a higher build number to the preferred microarchitecture (usually the highest level).

That's it! The activation scripts behind the microarch-level packages are already injecting the necessary compiler flags for you. Since they also have run_exports entries, your package will have the necessary runtime requirements to ensure the most adequate variant gets installed. Refer to this comment and the microarch-level-feedstock README for more information.

Matplotlib

matplotlib on conda-forge comes in two parts. The core library is in matplotlib-base. The actual matplotlib package is this core library plus pyqt. Most, if not all, packages that have dependence at runtime on matplotlib should list this dependence as matplotlib-base unless they explicitly need pyqt. The idea is that a user installing matplotlib explicitly would get a full featured installation with pyqt. However, pyqt is a rather large package, so not requiring it indirectly is better for performance. Note that you may need to include a yum_requirements.txt file in your recipe with

xorg-x11-server-Xorg

if you import parts of matplotlib that link to libX11.

pybind11 ABI Constraints

Sometimes when different python libraries using pybind11 interact via lower-level C++ interfaces, the underlying ABI between the two libraries has to match. To ease this use case, we have a pybind11-abi metapackage that can be used in the host section of a build. Its version is pinned globally and it has a run export on itself, meaning that builds with this package in host will have a runtime constraint on it. Further, the pybind11 has a run constraint on the ABI metapackage to help ensure consistent usage.

To use this package in a build, put it in the host environment like so

requirements:
host:
- pybind11-abi

Empty Python packages

For some features introduced in later Python versions, the Python community creates backports, which makes these features available for earlier versions of Python as well. One example here is dataclasses which was introduced with Python3.7 but is available as a backport for Python3.6 too. Therefore, most upstream packages make those backports only mandatory for specific versions of Python and exclude them otherwise.

Implementing this restriction in conda-forge is currently only possible through the use of skips which restricts the corresponding conda-forge recipes from becoming noarch.

Therefore, some conda-forge recipes only create an actual package on specific Python versions and are otherwise an empty placeholder. This allows them to be safely installed under all Python versions and makes using skips unnecessary.

Similarly, some packages are only platform-specific dependency of a package, such as pywin32, and have helper metapackages which can help recipes stay noarch. The version of the actual package required can be controlled with run_constrained, even for packages not available on all platforms.

Currently available packages:

NameAvailable on:Empty on:
backports.strenumpython >=3.8,<3.11python >=3.12
dataclassespython >=3.6,<3.7python >=3.7
enum34python =2.7python >=3.4
pywin32-on-windowswindowsunix
typingpython >=3

Non-version-specific Python packages

For some dependencies, upstream maintainers list Python versions where those packages are needed, even if the packages can actually be installed under all Python versions.

Implementing this restriction in conda-forge is currently only possible through the use of skips which restricts the corresponding conda-forge recipes from becoming noarch.

Therefore, the conda-forge community maintains a list of packages that are safe to be installed under all Python versions, even if the original package only requires it for some versions.

For example, the package pyquil only requires importlib-metadata for python <3.8 but it is actually save to be installed under python >=3.8 as well.

Currently available packages:

  • exceptiongroup
  • importlib-metadata

Noarch builds

Noarch packages are packages that are not architecture specific and therefore only have to be built once.

Declaring these packages as noarch in the build section of the meta.yaml, reduces shared CI resources. Therefore all packages that qualify to be noarch packages should be declared as such.

Noarch python

The noarch: python directive, in the build section, makes pure-Python packages that only need to be built once.

In order to qualify as a noarch python package, all of the following criteria must be fulfilled:

  • No compiled extensions
  • No post-link or pre-link or pre-unlink scripts
  • No OS-specific build scripts
  • No python version specific requirements
  • No skips except for python version. If the recipe is py3 only, remove skip statement and add version constraint on python in host and run section.
  • 2to3 is not used
  • scripts argument in setup.py is not used
  • If console_scripts entry_points are defined in setup.py or setup.cfg, they are also listed in the build section of meta.yaml
  • No activate scripts
note

While noarch: python does not work with selectors, it does work with version constraints. skip: True # [py2k] can be replaced with a constrained python version in the host and run subsections: say python >=3 instead of just python.

note

Only console_scripts entry points have to be listed in meta.yaml. Other entry points do not conflict with noarch and therefore do not require extra treatment.

note

noarch is a statement about the package's source code and not its install environment. A package is still considered noarch even if one of its dependencies is not available on a given platform. If this is the case, conda will display a helpful error message describing which dependency couldn't be found when it tries to install the package. If the dependency is later made available, your package will be installable on that platform without having to make any changes to the feedstock.

By default, noarch packages are built on Linux, and all dependencies must be available on Linux.

Hint

If a noarch package cannot be built on Linux, one or more noarch_platforms can be provided in conda-forge.yml. One example is pywin32-on-windows, which builds on Linux and Windows, with build_number offsets to create a pair packages, like dataclasses.

Hint

You can build platform-specific noarch packages to include runtime requirements depending on the target OS. See mini-tutorial below.

If an existing python package qualifies to be converted to a noarch package, you can request the required changes by opening a new issue and including @conda-forge-admin, please add noarch: python.

Noarch packages with OS-specific dependencies

It is possible to build noarch packages with runtime requirements that depend on the target OS (Linux, Windows, MacOS), regardless the architecture (amd64, ARM, PowerPC, etc). This approach relies on three concepts:

  1. Virtual packages. Prefixed with a double underscore, they are used by conda to represent system properties as constraints for the solver at install-time. We will use __linux, __win or __osx, which are only present when the running platform is Linux, Windows, or MacOS, respectively. __unix is present in both Linux and MacOS. Note that this feature is only fully available on conda 4.10 or above.
  2. conda-forge.yml's noarch_platforms option.
  3. conda-build 3.25.0 or above changing the build hash depending on virtual packages used.

The idea is to generate different noarch packages for each OS needing different dependencies. Let's say you have a pure Python package, perfectly eligible for noarch: python, but on Windows it requires windows-only-dependency. You might have something like:

recipe/meta.yaml (original)
name: package
source:
# ...
build:
number: 0
requirements:
# ...
run:
- python
- numpy
- windows-only-dependency # [win]

Being non-noarch, this means that the build matrix will include at least 12 outputs: three platforms, times four Python versions. This gets worse with arm64, aarch64 and ppc64le in the mix. We can get it down to two outputs if replace it with this other approach!

recipe/meta.yaml (modified)
name: package
source:
# ...
build:
number: 0
noarch: python
requirements:
host:
- python >=3.7
# ...
run:
- python >=3.7
- numpy
- __unix # [unix]
- __win # [win]
- windows-only-dependency # [win]

Do not forget to specify the platform virtual packages with their selectors! Otherwise, the solver will not be able to choose the variants correctly.

By default, conda-forge will only build noarch packages on a linux_64 CI runner, so only the # [unix] selectors would be true. However, we can change this behaviour using the noarch_platforms option in conda-forge.yml:

conda-forge.yml
noarch_platforms:
- linux_64
- win_64

This will provide two runners per package! Perfect! All these changes require a feedstock rerender to be applied. See Rerendering feedstocks.

If you need conditional dependencies on all three operating systems, this is how you do it:

recipe/meta.yaml
name: package
source:
# ...
build:
number: 0
noarch: python
requirements:
# ...
run:
- python
- numpy
- __linux # [linux]
- __osx # [osx]
- __win # [win]
- linux-only-dependency # [linux]
- osx-only-dependency # [osx]
- windows-only-dependency # [win]
conda-forge.yml
noarch_platforms:
- linux_64
- osx_64
- win_64

Again, remember to rerender after adding / modifying these files so the changes are applied.

Noarch generic

Todo

add some information on r packages which make heavy use of noarch: generic

Multi-output recipes

conda-build has the ability to create multiple package artifacts from a single recipe via the outputs section in meta.yaml. This is useful in several scenarios, including:

Common pitfalls with outputs

This is a non-exhaustive list of common pitfalls when using outputs.

  • It's usually simpler to use a top-level name that does not match any output names. If the top-level name is different than the feedstock name, make sure to set the extra.feedstock-name in meta.yaml. See rich-feedstock. Note how the top-level name is rich-split, the feedstock name is rich and the main output is rich too.
  • The build.sh and bld.bat scripts are only automatically used for the top-level package. Consider using other file names for the scripts in the outputs. See gdal-feedstock for an example.
  • The outputs[].script field can only be set to a script name. If you prefer passing shell commands, you have to use outputs[].build.script. Compare geopandas-feedstock to gym-feedstock, respectively.
  • Some PIP_* environment variables that are usually set for the top-level scripts are not automatically set for the outputs. If you are invoking pip in an output, you may need to pass additional flags. See napari-feedstock. This issue is tracked in conda/conda-build#3993.

Build matrices

Currently, python, vc, r-base will create a matrix of jobs for each supported version. If python is only a build dependency and not a runtime dependency (eg: build script of the package is written in Python, but the package is not dependent on Python), use build section

Following implies that python is only a build dependency and no Python matrix will be created.

build:
- python
host:
- some_other_package

Note that host should be non-empty or compiler jinja syntax used or build/merge_build_host set to True for the build section to be treated as different from host.

Following implies that python is a runtime dependency and a Python matrix for each supported Python version will be created.

host:
- python

conda-forge.yml's build matrices is removed in conda-smithy=3. To get a build matrix, create a conda_build_config.yaml file inside the recipe folder. For example, the following will give you 2 builds and you can use the selector vtk_with_osmesa in the meta.yaml

vtk_with_osmesa:
- False
- True

You need to rerender the feedstock after this change.

Requiring newer macOS SDKs

conda-forge uses macOS SDK 10.9 to build software so that they can be deployed to all macOS versions newer than 10.9. Sometimes, some packages require a newer SDK to build with. While the default version 10.9 can be overridden using the following changes to the recipe, it should be done as a last resort. Please consult with core team if this is something you think you need.

To use a new SDK, add the following in recipe/conda_build_config.yaml

# Please consult conda-forge/core before doing this
MACOSX_SDK_VERSION: # [osx and x86_64]
- "10.12" # [osx and x86_64]

Note that this should be done if the error you are getting says that a header is not found or a macro is not defined. This will make your package compile with a newer SDK but with 10.9 as the deployment target. WARNING: some packages might use features from 10.12 if you do the above due to buggy symbol availability checks. For example packages looking for clock_gettime will see it as it will be a weak symbol, but the package might not have a codepath to handle the weak symbol, in that case, you need to update the MACOSX_DEPLOYMENT_TARGET as described below.

After increasing the SDK version, if you are getting an error that says that a function is available only for macOS x.x, then do the following in recipe/conda_build_config.yaml,

# Please consult conda-forge/core before doing this
MACOSX_DEPLOYMENT_TARGET: # [osx and x86_64]
- "10.12" # [osx and x86_64]
MACOSX_SDK_VERSION: # [osx and x86_64]
- "10.12" # [osx and x86_64]

In recipe/meta.yaml, add the following to ensure that the user's system is compatible.

requirements:
run:
- __osx >={{ MACOSX_DEPLOYMENT_TARGET|default("10.9") }} # [osx and x86_64]

Note that this requires conda>=4.8. If you want to support older conda versions the requirement should be changed from run to run_constrained. Note that conda<4.8 will ignore the condition if it's a run_constrained on __osx.

Newer C++ features with old SDK

The libc++ library uses Clang availability annotations to mark certain symbols as unavailable when targeting versions of macOS that ship with a system libc++ that do not contain them. Clang always assumes that the system libc++ is used.

The conda-forge build infrastructure targets macOS 10.9 and some newer C++ features such as fs::path are marked as unavailable on that platform, so the build aborts:

...
error: 'path' is unavailable: introduced in macOS 10.15
...
note: 'path' has been explicitly marked unavailable here
class _LIBCPP_TYPE_VIS path {

However, since conda-forge ships its own (modern) libcxx we can ignore these checks because these symbols are in fact available. To do so, add _LIBCPP_DISABLE_AVAILABILITY to the defines. For example

CXXFLAGS="${CXXFLAGS} -D_LIBCPP_DISABLE_AVAILABILITY"

PyPy builds

See Using PyPy as an interpreter in the user docs for more info about PyPy and conda-forge.

To build your python package for pypy, wait for the bot to send a PR and contact conda-forge/bot team if a PR is not sent after the dependencies have been built.

To add a dependency just for pypy or cpython, do,

requirements:
run:
- spam # [python_impl == 'cpython']
- ham # [python_impl == 'pypy']
note

You'll need to rerender the feedstocks after making the above change in order for the python_impl variable to be available to conda-build

To skip the pypy builds, do the following,

build:
skip: True # [python_impl == 'pypy']

If something is failing the PyPy build when it passes the CPython one, reach out to @conda-forge/help-pypy.

Using setuptools_scm

The Python module setuptools_scm can be used to manage a package's version automatically from metadata, such as git tags. The package's version string is thus not specified anywhere in the package, but encoded in it at install-time.

For conda-build this means that setuptools_scm must be included as a host dependency. Additionally, some attention because the metadata is often not available in the sources. There are two options for how to proceed:

  • For Python package also available on PyPI: Use the PyPi tarball as a source, as it will have the metadata encoded (in such a way that setuptools_scm knows how to find it).

  • Specify the environment variable SETUPTOOLS_SCM_PRETEND_VERSION with the version string. If specified this environment variable is the principle source for setuptools_scm. There are two ways how to do this:

    • If you are using build scripts, in build.sh specify:

      export SETUPTOOLS_SCM_PRETEND_VERSION="$PKG_VERSION"

      and in bld.bat specify:

      set SETUPTOOLS_SCM_PRETEND_VERSION=%PKG_VERSION%

      Whereby you use that PKG_VERSION has been set with the version string, see Environment variables.

    • Otherwise, if you are directly building from meta.yaml, use for example:

      build:
      # [...]
      script_env:
      - SETUPTOOLS_SCM_PRETEND_VERSION={{version}}
      script: "{{ PYTHON }} -m pip install . -vv"

Using CentOS 7

To use the newer CentOS 7 sysroot with glibc 2.17 on linux-64, put the following in your build section.

requirements:
build:
- {{ compiler('c') }}
- sysroot_linux-64 2.17 # [linux64]

You also need to use a newer docker image by setting the following in the conda-forge.yml of your recipe and rerendering.

os_version:
linux_64: cos7

Finally, note that the aarch64 and ppc64le platforms already use CentOS 7.

CUDA builds

Although the provisioned CI machines do not feature a GPU, conda-forge does provide mechanisms to build CUDA-enabled packages. These mechanisms involve several packages:

  • cudatoolkit: The runtime libraries for the CUDA toolkit. This is what end-users will end up installing next to your package.
  • nvcc: Nvidia's EULA does not allow the redistribution of compilers and drivers. Instead, we provide a wrapper package that locates the CUDA installation in the system. The main role of this package is to set some environment variables (CUDA_HOME, CUDA_PATH, CFLAGS and others), as well as wrapping the real nvcc executable to set some extra command line arguments.

In practice, to enable CUDA on your package, add {{ compiler('cuda') }} to the build section of your requirements and rerender. The matching cudatoolkit will be added to the run requirements automatically.

On Linux, CMake users are required to use ${CMAKE_ARGS} so CMake can find CUDA correctly. For example:

mkdir build && cd build
cmake ${CMAKE_ARGS} ${SRC_DIR}
make
note

How is CUDA provided at the system level?

  • On Linux, Nvidia provides official Docker images, which we then adapt to conda-forge's needs.
  • On Windows, the compilers need to be installed for every CI run. This is done through the conda-forge-ci-setup scripts. Do note that the Nvidia executable won't install the drivers because no GPU is present in the machine.

How is cudatoolkit selected at install time?

Conda exposes the maximum CUDA version supported by the installed Nvidia drivers through a virtual package named __cuda. By default, conda will install the highest version available for the packages involved. To override this behaviour, you can define a CONDA_OVERRIDE_CUDA environment variable. More details in the Conda docs.

Note that prior to v4.8.4, __cuda versions would not be part of the constraints, so you would always get the latest one, regardless the supported CUDA version.

If for some reason you want to install a specific version, you can use:

conda install your-gpu-package cudatoolkit=10.1

Testing the packages

Since the CI machines do not feature a GPU, you won't be able to test the built packages as part of the conda recipe. That does not mean you can't test your package locally. To do so:

  1. Enable the Azure artifacts for your feedstock (see here).
  2. Include the test files and requirements in the recipe like this.
  3. Provide the test instructions. Take into account that the GPU tests will fail in the CI run, so you need to ignore them to get the package built and uploaded as an artifact. Example.
  4. Once you have downloaded the artifacts, you will be able to run:
    conda build --test <pkg file>.tar.bz2

Common problems and known issues

nvcuda.dll cannot be found on Windows

The scripts used to install the CUDA Toolkit on Windows cannot provide nvcuda.dll as part of the installation because no GPU is physically present in the CI machines. As a result, you might get linking errors in the postprocessing steps of conda build:

WARNING (arrow-cpp,Library/bin/arrow_cuda.dll): $RPATH/nvcuda.dll not found in packages,
sysroot(s) nor the missing_dso_whitelist.

.. is this binary repackaging?

For now, you will have to add nvcuda.dll to the missing_dso_whitelist

build:
...
missing_dso_whitelist:
- "*/nvcuda.dll" # [win]

My feedstock is not building old CUDA versions anymore

With the addition of CUDA 11.1 and 11.2, the default build matrix for CUDA versions was trimmed down to versions 10.2, 11.0, 11.1, 11.2.

If you really need it, you can re-add support for 9.2, 10.0 and 10.1. However, this is not recommended. Adding more CUDA versions to the build matrix will dramatically increase the number of jobs and will place a large burden on our CI resources. Only proceed if there's a known use case for the extra packages.

  1. Download this migration file.
  2. In your feedstock fork, create a new branch and place the migration file under .ci_support/migrations.
  3. Open a PR and re-render. CUDA 9.2, 10.0 and 10.1 will appear in the CI checks now. Merge when ready!

Adding support for a new CUDA version

Providing a new CUDA version involves five repositores:

The steps involved are, roughly:

  1. Add the cudatoolkit packages in cudatoolkit-feedstock.
  2. Submit the version migrator to conda-forge-pinning-feedstock. This will stay open during the following steps.
  3. For Linux, add the corresponding Docker images at docker-images. Copy the migration file manually to .ci_support/migrations. This copy should not specify a timestamp. Comment it out and rerender.
  4. For Windows, add the installer URLs and hashes to the conda-forge-ci-setup script. The migration file must also be manually copied here. Rerender.
  5. Create the new nvcc packages for the new version. Again, manual migration must be added. Rerender.
  6. When everything else has been merged and testing has taken place, consider merging the PR opened at step 2 now so it can apply to all the downstream feedstocks.

Packages that require a GPU or long-running builds

conda-forge has access to an OpenStack server that provides GPU builds and long-running builds (beyond the usual 6h limit). If your package needs a GPU to be built or tested, or its compilation times are so long that they are currently done manually off-CI, you can request access to these runners. To do so:

  1. Open a PR in conda-forge/admin-requests. Follow the instructions in the repository README. Note you need to request the type of resource you want access to (e.g. GPU runners, or long-running CPU builds) Once merged, this will enable the self-hosted Github Actions runners for your feedstock.
  2. In order to trigger jobs for these runners, the maintainer must have read and agreed to the open-gpu-server terms of use. You will need to open a PR in the open-gpu-server repository, as instructed in their README. You only need to do this once per maintainer (e.g. if you maintain multiple feedstocks).
  3. Finally, you can configure your feedstock to use the self-hosted runners. A PR will have been created by admin-requests after the PR in step (1) is merged. However, due to security measurements imposed by Github, automated re-rendering is not possible when they modify Github Actions workflows. You will need to rerender it manually by running conda-smithy rerender in your machine and then commit and push the result.
note

Due to some technical and legal limitations, some of the usual automation infrastructure is not available for these runners. As mentioned above, the conda-forge bots won't be able to rerender your feedstock automatically anymore. Automerge will not function properly either. Also note that the conda-forge bots won't be able to trigger the self-hosted runners. Closing and reopening the PR won't work, but a maintainer with sufficient permissions can trigger it manually by pushing an empty commit.

Apple Silicon builds

The new Apple M1 processor is the first Apple Silicon supported by conda-forge osx-arm64 builds. For new builds to be available, via cross-compilation, a migration is required for the package and its dependencies. These builds are experimental as many of them are untested.

To request a migration for a particular package and all its dependencies:

  1. It may be that your package is already in the process of being migrated. Please check the status of the arm osx addition migration. If your package is already in the process of being migrated, it will appear under the appropriate heading (done, in-pr, awaiting-parents, etc.).
  2. Check the feedstock in question to see if there is already an issue or pull request. Opening an issue here is fine, as it might take a couple iterations of the below, especially if many dependencies need to be built as well.
  3. If nothing is under way, look at the current conda-forge-pinning.
  4. If the package is not listed there, make a PR, adding the package name to a random location in osx_arm64.txt. The migration bot should start making automated pull requests to the repo and its dependencies.
  5. Within a few hours, the status page should reflect the progress of the package in question, and help you keep track of progress. Help out if you can!
  6. The feedstock maintainers (who might not have an M1) will work to make any changes required to pass continuous integration. If you have insight into the particular package, please chime in, but most of all be patient and polite.
  7. Once the new builds are available from anaconda.org, please help the maintainers by testing the packages, and reporting back with any problems… but also successes!

Pre-release builds

Recipe maintainers can make pre-release builds available on conda-forge by adding them to the dev or rc label.

The semantics of these labels should generally follow the guidelines that Python itself follows.

  • rc: Beta and Release Candidate (RC). No new features. Bugfix only.
  • dev: Pre-Alpha and Alpha. These are still packages that could see substantial changes between the dev version and the final release.
note

alpha and beta labels aren't used. Given the light usage of labels on the conda-forge channel thus far, it seems rather unnecessary to introduce many labels. dev and rc seem like a nice compromise.

note

Certain packages (for example black) follow a release cycle in which they have never had a non-beta/alpha release. In these cases the conda packages for those do not need to be published to a prerelease label.

Creating a pre-release build

To create a dev or rc package, a PR can be issued into the dev or rc branch of the feedstock. This branch must change the recipe/conda_build_config.yaml file to point to the <package_name>_dev or <package_name>_rc label.

For example, matplotlib rc releases would include:

channel_targets:
- conda-forge matplotlib_rc

If a pre-release build of B depends on a pre-release build of A, then A should have,

channel_targets:
- conda-forge A_rc

while B should have,

channel_sources:
- conda-forge/label/A_rc,conda-forge
channel_targets:
- conda-forge B_rc

in recipe/conda_build_config.yaml in their respective feedstocks.

note

A rerender needs to happen for these changes to reflect in CI files. The channel_targets entries map

Installing a pre-release build

Using the conda CLI

Use the following command, but replace PACKAGE_NAME with the package you want to install and replace LABEL with rc or dev:

conda install -c conda-forge/label/PACKAGE_NAME_LABEL -c conda-forge PACKAGE_NAME

For example, let's install matplotlib from the rc label:

conda install -c conda-forge/label/matplotlib_rc -c conda-forge matplotlib

Using environment.yml

Use MatchSpec to specify your package:

dependencies:
- conda-forge/label/matplotlib_rc::matplotlib=3.7.0rc1

Alternately, you can use the channels section to enable the matplotlib_rc channel:

channels:
- conda-forge/label/matplotlib_rc
dependencies:
- matplotlib=3.7.0.rc1

Pre-release version sorting

If you wish to add numbers to your dev or rc build, you should follow the guidelines put forth by Continuum regarding version sorting in conda. Also see the source code for conda 4.2.13. The tl;dr here is that conda sorts as follows:

< 1.0
< 1.1dev1 # special case 'dev'
< 1.1.0dev1 # special case 'dev'
== 1.1.dev1 # 0 is inserted before string
< 1.1.0rc1
< 1.1.0

So make sure that you tag your package in such a way that the package name that conda-build spits out will sort the package uploaded with an rc label higher than the package uploaded with the dev label.

How to update your feedstock token?

To reset your feedstock token and fix issues with uploads, follow these steps:

  1. Go to the conda-forge/admin-requests repo and copy examples/example-token-reset.yml to the requests/ folder.
  2. Add the name of your feedstock in the YML file. While adding the name, don't add "-feedstock" to the end of it. For example: for python-feedstock, just add python.

Using arch_rebuild.txt

You can add a feedstock to arch_rebuild.txt if it requires rebuilding with different architectures/platforms (such as ppc64le or aarch64). Check the migration status to see if your package is already in the queue to get migrated. If not, you can add the feedstock to arch_rebuild.txt by opening a PR to the conda-forge-pinning-feedstock repository. Once the PR is merged, the migration bot goes through the list of feedstocks in arch_rebuild.txt and opens a migration PR for any new feedstocks and their dependencies, enabling the aarch64/ppc64le builds.

Migrators and Migrations

When any changes are made in the global pinnings of a package, then the entire stack of the packages that need that package on their host section would need to be updated and rebuilt. Doing it manually can be quite tedious, and that's where migrations come to help. Migrations automate the process of submitting changes to a feedstock and are an integral part of the regro-cf-autotick-bot's duties.

There are several kinds of migrations, which you can read about in Making Migrators. To generate these migrations, you use migrators, which are bots that automatically create pull requests for the affected packages in conda-forge. To propose a migration in one or more pins, the migrator issues a PR into the pinning feedstock using a yaml file expressing the changes to the global pinning file in the migrations folder. Once the PR is merged, the dependency graph is built. After that, the bot walks through the graph, migrates all the nodes (feedstocks) one by one, and issues PRs for those feedstocks.

Usually, the bot generates these migrations automatically. However, when a pin is first made or added, one may need to be added by hand. To do this, you can follow the steps mentioned in Updating package pins.

The way migrations proceed are:

  1. You make a PR into the migrations folder in the conda-forge-pinning-feedstock with a new yaml file representing the migration.
  2. Once the PR is merged, the bot picks it up, builds a migrator graph, and begins the migration process.
  3. A migration PR is issued for a node (a feedstock) only if:
- The node depends on the changed pinnings.
- The node has no dependencies that depend on the new pinnings and have not been migrated.
  1. Process 3 continues until the migration is complete and the change is applied to the global pinning file via a final PR. After this step, we say this migration is closed out.

Sometimes, you might get a migration PR for your package that you don't want to merge. In that case, you should put that PR in draft status but should never close it. If you close the PR, it makes the bot think that another PR implementing the migration is merged instead, letting the migration continue iterating on the graph; however, the downstream dependents fail because the parent (the one we closed the PR of) didn't really get rebuilt. Another reason why it is good to keep the PR open or in draft status is that people might help with it if they want in the future.

In some cases a migration PR may not get opened. Please look for the migration on our status page to see if there are any issues. This may show there are still dependencies needing migration, in which case the best approach is to wait (or if possible offer to help migrate those dependencies). If there is a bot error, there will be a link to the CI job to provide more details about what may have gone wrong. In these cases please raise an issue and include as much information as possible.

It is worth noting that one also has the option to create a migration PR themselves. This can be a good option if the bot errored and that is still being investigated or the migration PR got closed accidentally. To migrate a PR manually:

  1. Fork the feedstock and clone it locally
  2. Create a new branch
  3. Create the directory .ci_support/migrations in the feedstock (if absent)
  4. Copy the migrator from conda-forge-pinning's migrators to .ci_support/migrations and commit it
  5. Rerender the feedstock
  6. Push these changes and open a PR

Security considerations for conda-forge builds

All conda-forge packages are built by strangers on the internet on public cloud infrastructure from source code you likely have not inspected, so you should not use conda-forge packages if you or your team require a high level of security. You are also free to download recipes and rebuild them yourself, if you would like at least that much oversight. However, many people use conda-forge all the time with no issues and here are some things that conda-forge does to help with security in some ways:

  1. Sources (where you specify where the package's source code is coming from) can be pulled from GitHub, PyPI, or other sources and sha256 hashes are always used, so moving of tags or uploading of new sdists can not cause automatic package rebuilds. Also, once packages are accepted and made into feedstocks, only the maintainers of that feedstock have the right to merge PRs made to that feedstock.
  2. Each feedstock can only upload packages for that feedstock. This is enforced by using a cf-staging channel where builds are first sent. A bot then assesses that the submitting feedstock has permission to build the package it has submitted, and only then will it relay the build to the conda-forge channel. This helps mitigate against a bad actor gaining access to an inconspicuous feedstock and then trying to push a build with malicious code into essential infrastructure packages (e.g., OpenSSL or Python).
  3. We have artifact-validation for validating all the conda-forge artifacts uploaded to anaconda.org. This validation scans for various security-related items, such as artifacts that overwrite key pieces of certain packages.
  4. We have a dedicated Security and Systems Sub-Team who works hard towards making sure to secure and maintain appropriate access to the credentials and services/systems used by conda-forge.

If you have found a security-related issue with conda-forge, please check our Security Policy to learn how to report it responsibly.

Significant Changes To Upstream Projects

From time to time, we make changes in upstream projects so that they better integrate into the conda-forge ecosystem. We have listed some, but not all, of those changes here for specific projects along with any associated documentation.

Python

We carry an extensive set of python patches that change some core behaviors around search paths, environment isolation in conda environments, and some operating system limits. See the python feedstock for more details.