Skip to content


Repository files navigation

OpenCL Specification Build Instructions and Notes


The most useful parts of this document are the Introduction, Building The Spec, and notes on installing Software Dependencies.


The default branch of this repository has changed from master to main.


This repository contains the source and tool chain used to generate the formal OpenCL API, OpenCL C, OpenCL Extensions, OpenCL SPIR-V Environment, OpenCL C Specifications, as well as the OpenCL Reference Pages and documentation for the C for OpenCL kernel language.

This file describes the repository structure, tools, and build instructions needed.

Source Code

The OpenCL specifications are maintained by members of the Khronos Group’s OpenCL Working Group, in the OpenCL-Docs Github repository. The documents are all in Asciidoctor format.

Contributions via pull requests on Github are welcome. Pull requests must be provided under the same Creative Commons license as the specification source. You’ll be prompted with a one-time “click-through” Contributor’s License Agreement (CLA) dialog as part of submitting your pull request or other contribution to GitHub.

We intend to maintain a linear history on the GitHub main branch.

Repository Structure




This file


GNU Makefile used to build HTML and PDF spec targets


Support files for asciidoctor build, HTML CSS / Javascript, etc.


KaTeX math renderer for HTML outputs


Main source file for the OpenCL API Specification


Individual sections of the API Specification


Main source file for the OpenCL C Specification


Individual sections of the C Specification


Main source file for the OpenCL C++ Specification


Individual sections of the C++ Specification


Main source file for the OpenCL SPIR-V Environment Specification


Individual sections of the Environment Specification


Main source file for the OpenCL Extensions Specification


Individual sections of the Extensions Specification


Main source file for the C++ for OpenCL Documentation


Individual sections of the C++ for OpenCL Documentation


Shared images, used by all specs


Static reference page sources

Building The Specifications and Reference Pages

If you have installed all required toolchain components, you should be able to build both HTML and PDF outputs for all specifications by:

$ make

Other Makefile targets are available for building the different outputs for each Specification:




HTML outputs for all specifications


PDF outputs for all specifications


HTML and PDF outputs for API Specification


HTML outputs for API Specification


PDF outputs for API Specification


HTML and PDF outputs for C Specification


HTML outputs for C Specification


PDF outputs for C Specification


HTML and PDF outputs for C++ Specification


HTML outputs for C++ Specification


PDF outputs for C++ Specification


HTML and PDF outputs for SPIR-V Environment Specification


HTML outputs for Environment Specification


PDF outputs for Environment Specification


HTML and PDF outputs for Extensions Specification


HTML outputs for Extensions Specification


PDF outputs for Extensions Specification


HTML and PDF outputs for C++ for OpenCL Documentation


HTML outputs for C++ for OpenCL Documentation


PDF outputs for C++ for OpenCL Documentation


HTML outputs for Reference Pages

The specification targets are generated in out/html/ for HTML targets, and out/pdf/ for PDF targets. The reference page target is generated in out/man/html/. If a different output directory is desired, set the Makefile variable OUTDIR on the command line. For example,

make OUTDIR=/tmp apihtml

will create /tmp/html/OpenCL_API.html.

These targets generate a variety of output documents in the directory specified by the Makefile variable $(OUTDIR) (by default, out).

Once you have the basic build working, an appropriate parallelization option to make, such as

make -j 6

may significantly speed up building multiple specs. The asciidoctor HTML build is very fast, even for the whole Specification, but PDF builds may take several minutes.

Reference Page Installation

Most of the reference pages are extracted from the OpenCL API and OpenCL C Specifications, although some are static. While anyone can generate reference page sets for themselves, Khronos publishes them via the main branch of the OpenCL Registry.

When the OpenCL Specification Editor is updating the published reference pages, it is easiest to have local repository clones of this repository (OpenCL-Docs) as well as OpenCL-Registry. Update the pages in the local registry clone via

make -j 6 OUTDIR=path-to-registry-repo/sdk/3.0/docs manhtmlpages

This creates the HTML output pages under sdk/3.0/docs/man/html, and copies the KaTeX package to sdk/3.0/docs/katex. To publish, commit those changes to the registry repository and push it to github.

Our stylesheets

We use a modified version of the Asciidoctor 'colony' theme. This theme is maintained internally to Khronos and the resulting CSS is under config/khronos.css.

Imbedding Equations

Where possible, equations should be written using straight asciidoc markup using the eq role. This covers many common equations and is faster than the alternatives.

For more complex equations, such as multi-case statements, matrices, and complex fractions, equations should be written using the latexmath: inline and block macros. The contents of the latexmath: blocks should be LaTeX math notation. LaTeX math markup delimiters are now inserted by the asciidoctor toolchain.

LaTeX math is passed through unmodified to all HTML output forms, which is subsequently rendered with the KaTeX engine when the html is loaded. A local copy of the KaTeX release is kept in katex/ and copied to the HTML output directory during spec generation. Math is processed into SVGs via asciidoctor-mathematical for PDF output.

The following caveats apply:

  • The special characters < , > , and & can currently be used only in [latexmath] block macros, not in latexmath:[] inline macros. Instead use \lt, \leq, \gt, and \geq for <, , >, and >= respectively. & is an alignment construct for multiline equations, and should only appear in block macros anyway.

  • AMSmath environments (e.g. \begin{equation*}, {align*}, etc.) cannot be used in KaTeX at present, and have been replaced with constructs supported by KaTeX such as {aligned}.

  • Arbitrary LaTeX constructs cannot be used. KaTeX and asciidoctor-mathematical are only equation renderers, not full LaTeX engines. Imbedding LaTeX like \Large or \hbox{\tt\small VK\_FOO} may not work in any of the backends, and should be avoided.

See the “Vulkan Documentation and Extensions” document in the Khronos Vulkan Registry for more details of supported LaTeX math constructs in our toolchain.

Asciidoc Anchors And Xrefs

In asciidoctor, sections can have anchors (labels) applied with the following syntax:

== SPIR-V Intermediate language

In general the anchor should immediately precede the chapter or section title and should use the form '[[chapter-section-label]]'.

Cross-references to those anchors can then be generated with, for example,

See the <<spirv-il>> section for discussion of the SPIR-V intermediate

You can also add anchors on arbitrary paragraphs, using a similar naming scheme.

Software Dependencies

This section describes the software components used by the OpenCL spec toolchain. The specified versions are known to work. Later compatible versions will probably work as well.

Before building the OpenCL specs, you must install the following tools:

  • GNU make (make version: 4.0.8-1; older versions probably OK)

  • Python 3 (python, version: 3.4.2)

  • Ruby (ruby, version: 2.3.3)

    • The Ruby development package (ruby-dev) may also be required in some environments.

  • Git command-line client (git, version: 2.1.4). The build can progress without a git client, but branch/commit information will be omitted from the build. Any version supporting the following operations should work:

    • git symbolic-ref --short HEAD

    • git log -1 --format="%H"

  • ttf Fonts. These are needed the PDF build for latexmath rendering. See Font Dependencies for asciidoctor-mathematical.

The following Ruby Gems and platform package dependencies must also be installed. This process is described in more detail for individual platforms and environment managers below. Please read the remainder of this document (other than platform-specific parts you don’t use) completely before trying to install.

  • Asciidoctor (asciidoctor, version: 2.0.16)

  • Coderay (coderay, version: 1.1.1)

  • hexapdf (version: 0.27.0)

  • rouge (rouge, version 3.19.0)

  • ttfunk (ttfunk, version: 1.5.1)

  • Asciidoctor PDF (asciidoctor-pdf, version: 1.5.0)

  • Asciidoctor Mathematical (asciidoctor-mathematical, version 0.3.5)

  • Dependencies for asciidoctor-mathematical (There are a lot of these!)

  • KaTeX distribution (version 0.7.0 from . This is cached under katex/, and need not be installed from github.

Only the asciidoctor, coderay, and rouge gems are needed if you don’t intend to build PDF versions of the spec and supporting documents.


While it’s easier to install just the toolchain components for HTML builds, people submitting MRs with substantial changes to the Specifications are responsible for verifying that their branches build both html and pdf targets.

Platform-specific toolchain instructions follow:

Windows (General)

Most of the dependencies on Linux packages are light enough that it’s possible to build the spec natively in Windows, but it means bypassing the makefile and calling functions directly. This might be solved in future. For now, there are three options for Windows users: Ubuntu / Windows 10, MinGW, or Cygwin.

Ubuntu / Windows 10

When using the “Ubuntu Subsystem” for Windows 10, most dependencies can be installed via apt-get:

sudo apt-get -qq -y install build-essential python3 git cmake bison flex \
    libffi-dev libgmp-dev libxml2-dev libgdk-pixbuf2.0-dev libcairo2-dev \
    libpango1.0-dev fonts-lyx gtk-doc-tools ghostscript

The default ruby packages on Ubuntu are fairly out of date. Ubuntu only provides ruby and ruby2.0 - the latter is multiple revisions behind the current stable branch, and would require wrangling to get the makefile working with it.

Luckily, there are better options; either rvm or rbenv is recommended to install a more recent version.

  • If you are new to Ruby, you should completely remove (through the package manager, e.g. sudo apt-get remove packagename) all existing Ruby and asciidoctor infrastructure on your machine before trying to use rvm or rbenv for the first time. dpkg -l | egrep 'asciidoctor|ruby|rbenv|rvm' will give you a list of candidate package names to remove.

    • If you already have a favorite Ruby package manager, ignore this advice, and just install the required OS packages and gems.

  • In addition, rvm and rbenv are mutually incompatible. They both rely on inserting shims and $PATH modifications in your bash shell. If you already have one of these installed and are familiar with it, it’s best to stay with that one. One of the editors, who is new to Ruby, found rbenv far more comprehensible than rvm. The other editor likes rvm better.

    • Neither rvm nor rbenv work, out of the box, when invoked from non-Bash shells like tcsh. This can be hacked up by setting the right environment variables and PATH additions based on a bash environment.

  • Most of the tools on Bash for Windows are quite happy with Windows line endings (CR LF), but bash scripts expect Unix line endings (LF). The file .gitattributes at the top of the vulkan tree in the 1.0 branch forces such scripts to be checked out with the proper line endings on non-Linux platforms. If you add new scripts whose names don’t end in .sh, they should be included in .gitattributes as well.

Ubuntu/Windows 10 Using Rbenv

Rbenv is a lighter-weight Ruby environment manager with less functionality than rvm. Its primary task is to manage different Ruby versions, while rvm has additional functionality such as managing “gemsets” that is irrelevant to our needs.

A complete installation script for the toolchain on Ubuntu for Windows, developed on an essentially out-of-the-box environment, follows. If you try this, don’t try to execute the entire thing at once. Do each step separately in case of errors we didn’t encounter.

# Install packages needed by `ruby_build` and by toolchain components.
# See and

sudo apt-get install autoconf bison build-essential libssl-dev \
    libyaml-dev libreadline6-dev zlib1g-dev libncurses5-dev \
    libffi-dev libgdbm3 libgdbm-dev cmake libgmp-dev libxml2 \
    libxml2-dev flex pkg-config libglib2.0-dev \
    libcairo-dev libpango1.0-dev libgdk-pixbuf2.0-dev \

# Install rbenv from
git clone ~/.rbenv

# Set path to shim layers in .bashrc
echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> .bashrc

~/.rbenv/bin/rbenv init

# Set .rbenv environment variables in .bashrc
echo 'eval "$(rbenv init -)"' >> .bashrc

# Restart your shell (e.g. open a new terminal window). Note that
# you do not need to use the `-l` option, since the modifications
# were made to .bashrc rather than .bash_profile. If successful,
# `type rbenv` should print 'rbenv is a function' followed by code.

# Install `ruby_build` plugin from

git clone

# Install Ruby 2.3.3
# This takes in excess of 20 min. to build!
# suggests:
# "You can speed up Ruby installs by avoiding generating ri/RDoc
# documentation for them:
# RUBY_CONFIGURE_OPTS=--disable-install-doc rbenv install 2.3.3
# We have not tried this.

rbenv install 2.3.3

# Configure rbenv globally to always use Ruby 2.3.3.
echo "2.3.3" > ~/.rbenv/version

# Finally, install toolchain components.
# asciidoctor-mathematical also takes in excess of 20 min. to build!
# The same RUBY_CONFIGURE_OPTS advice above may apply here as well.

gem install asciidoctor -v 2.0.16
gem install coderay -v 1.1.1
gem install hexapdf -v 0.27.0
gem install rouge -v 3.19.0
gem install ttfunk -v 1.5.1
gem install asciidoctor-pdf -v 1.5.0
gem install asciidoctor-mathematical -v 0.3.5
Ubuntu/Windows 10 Using RVM

Here are (sparser) instructions for using rvm to setup version 2.3.x:

gpg --keyserver hkp:// --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3
\curl -sSL | bash -s stable --ruby
source ~/.rvm/scripts/rvm
rvm install ruby-2.3
rvm use ruby-2.3
Windows 10 Bash will need to be launched with the "-l" option appended, so that it runs a login shell; otherwise RVM won’t function correctly on future launches.
Ubuntu 16.04 using system Ruby

The Ubuntu 16.04.1 default Ruby install (version 2.3.1) seems to be up-to-date enough to run all the required gems, but also needs the ruby-dev package installed through the package manager.

In addition, the library /var/lib/gems/2.3.0/gems/mathematical-1.6.7/ext/mathematical/lib/ has to be copied or linked into a directory where the loader can find it. This requirement appears to be due to a problem with the asciidoctor-mathematical build process.


MinGW can be obtained here:

Once the installer has run its initial setup, following the instructions on the website, you should install the mingw-developer-tools, mingw-base and msys-base packages. The msys-base package allows you to use a bash terminal from windows with whatever is normally in your path on Windows, as well as the unix tools installed by MinGW.

In the native Windows environment, you should also install the following native packages:

Once this is setup, and the necessary Ruby Gems are installed, launch the msys bash shell, and navigate to the spec Makefile. From there, you’ll need to set PYTHON= to the location of your python executable for version 3.x before your make command - but otherwise everything other than pdf builds should just work.

Building the PDF spec via this path has not yet been tested but may be possible - liblasem is the main issue and it looks like there is now a mingw32 build of it available.


When installing Cygwin, you should install the following packages via setup:

// "curl" is only used to download fonts, can be done in another way
Native versions of some of these packages are usable, but care should be taken for incompatibilities with various parts of cygwin - e.g. paths. Ruby in particular is unable to resolve Windows paths correctly via the native version. Python and Git for Windows can be used, though for Python you’ll need to set the path to it via the PYTHON environment variable, before calling make.

When it comes to installing the mathematical ruby gem, there are two things that will require tweaking to get it working. Firstly, instead of:

gem install asciidoctor-mathematical

You should use

MATHEMATICAL_USE_SYSTEM_LASEM=1 gem install asciidoctor-mathematical

The latter causes it to use the lasem package already installed, rather than trying to build a fresh one.

The mathematical gem also looks for "liblasem" rather than "liblasem0.4" as installed by the lasem0.4-devel package, so it is necessary to add a symlink to your /lib directory using:

ln -s /lib/liblasem-0.4.dll.a /lib/liblasem.dll.a

Ruby Gems are not installed to a location that is in your path normally. Gems are installed to ~/bin/ - you should add this to your path before calling make:

export PATH=~/bin:$PATH

Finally, you’ll need to manually install fonts for lasem via the following commands:

mkdir /usr/share/fonts/truetype cd /usr/share/fonts/truetype
curl -LO \
     -LO \
     -LO \
     -LO \
     -LO \
     -LO \
     -LO \

Mac OS X

Mac OS X should work in the same way as for ubuntu by using the Homebrew package manager, with the exception that you can simply install the ruby package via brew rather than using a ruby-specific version manager.

You’ll likely also need to install additional fonts for the PDF build via mathematical, which you can do with:

cd ~/Library/Fonts
curl -LO \
     -LO \
     -LO \
     -LO \
     -LO \
     -LO \
     -LO \

Then install the required Ruby Gems.

Linux (Debian, Ubuntu, etc.)

The instructions for the Ubuntu / Windows 10 installation are generally applicable to native Linux environments using Debian packages, such as Debian and Ubuntu, although the exact list of packages to install may differ.

Other distributions using different package managers, such as RPM (Fedora) and Yum (SuSE) will have different requirements.

Using rbenv or rvm is neccessary, since the system Ruby packages are often well out of date.

Once the environment manager, Ruby, and ruby_build have been installed, install the required Ruby Gems.

Ruby Gems

The following ruby gems can be installed directly via the gem install command, once the platform is set up:

gem install asciidoctor -v 2.0.16
gem install coderay -v 1.1.1
gem install hexapdf -v 0.27.0
gen install rouge -v 3.19.0
gem install ttfunk -v 1.5.1

# Required only for pdf builds
gem install asciidoctor-pdf -v 1.5.0
gem install asciidoctor-mathematical -v 0.3.5


The specification Asciidoctor source files are licensed under the Creative Commons Attribution 4.0 International License; see

Additional source files used to build the specification are under a mixture of Apache 2.0 and MIT licenses. See the LICENSE file for details.

The official Specification HTML and PDF output documents are posted in the OpenCL Registry. These Specifications are placed under the proprietary Khronos specification copyright and license.

OpenCL and the OpenCL logo are trademarks of Apple Inc. used by permission by Khronos.

Revision History

  • 2023-11-05 - Add hexapdf, remove ghostscript

  • 2020-03-13 - Updated package versions to match Travis build.

  • 2019-06-20 - Add directions for publishing OpenCL 2.2 reference pages, generated from the spec sources in this repository, in the OpenCL-Registry repository.

  • 2018-02-15 - Retarget document from Vulkan repository for OpenCL asciidoctor spec builds.