# JLL packages

BinaryBuilder.jl is designed to produce tarballs that can be used in any environment, but so far their main use has been to provide pre-built libraries and executables to be readily used in Julia packages. This is accomplished by JLL packages (a pun on "Dynamic-Link Library", with the J standing for Julia). They can be installed like any other Julia packages with the Julia package manager in the REPL with

]add NAME_jll

using NAME_jll

However, most users will not ever need to do these steps on their own, JLL packages are usually only used as dependencies of packages wrapping binary libraries or executables.

Most JLL packages live under the JuliaBinaryWrappers organization on GitHub, and the builders to generate them are maintaned in Yggdrasil, the community build tree. BinaryBuilder.jl allows anyone to create their own JLL package and publish them to a GitHub repository of their choice without using Yggdrasil, see the Frequently Asked Questions.

## Anatomy of a JLL package

A somewhat popular misconception is that JLL packages are "special". Instead, they are simple Julia packages with a common structure, as they are generated automatically. This is the typical tree of a JLL package, called in this example NAME_jll.jl:

NAME_jll
├── Artifacts.toml
├── Project.toml
└── src/
├── NAME_jll.jl
└── wrappers/
├── aarch64-linux-gnu.jl
├── aarch64-linux-musl.jl
├── armv7l-linux-gnueabihf.jl
├── armv7l-linux-musleabihf.jl
├── i686-linux-gnu.jl
├── i686-linux-musl.jl
├── i686-w64-mingw32.jl
├── powerpc64le-linux-gnu.jl
├── x86_64-apple-darwin14.jl
├── x86_64-linux-gnu.jl
├── x86_64-linux-musl.jl
├── x86_64-unknown-freebsd11.1.jl
└── x86_64-w64-mingw32.jl

These are the main ingredients of a JLL package:

• LICENSE, a file stating the license of the JLL package. Note that this may differ from the license of the library it wraps, which is instead shipped inside the tarballs;
• a README.md file providing some information about the content of the wrapper, like the list of "products" provided by the package;
• the Artifacts.toml file contains the information about all the available tarballs for the given package. The tarballs are uploaded to GitHub releases;
• the Project.toml file describes the packages dependencies and their compatibilities;
• the main entry point of the package is the file called src/NAME_jll.jl. This is what is executed when you issue the command
using NAME_jll
This file reads the list of tarballs available in Artifacts.toml and choose the platform matching the current platform. Some JLL packages are not built for all supported platforms. If the current platform is one of those platform not supported by the JLL package, this is the end of the package. Instead, if the current platform is supported, the corresponding wrapper in the src/wrappers/ directory will be included;
• the wrappers/ directory contains a file for each of the supported platforms. They are actually mostly identical, with some small differences due to platform-specific details. The wrappers are analyzed in more details in the following section.

## The wrappers

The files in the src/wrappers/ directory are very thin automatically-generated wrappers around the binary package provided by the JLL package. They load all the JLL packages that are dependencies of the current JLL package and export the names of the products listed in the build_tarballs.jl script that produced the current JLL package. Among others, they also define the following unexported variables:

• artifact_dir: the absolute path to where the artifact for the current platform has been installed. This is the "prefix" where the binaries/libraries/files are placed;
• PATH: the value of the PATH environment variable needed to run executables in the current JLL package, if any;
• PATH_list: the list of directories in PATH as a vector of Strings;
• LIBPATH: the value of the environment variable that holds the list of directories in which to search shared libraries. This has the correct value for the libraries provided by the current JLL package;
• LIBPATH_list: the list of directories in LIBPATH as a vector of Strings.

The wrapper files for each platform also define the __init__() function of the JLL package, the code that is executed every time the package is loaded. The __init__() function will populate most of the variables mentioned above and automatically open the shared libraries, if any, listed in the products of the build_tarballs.jl script that generated the JLL package.

The rest of the code in the wrappers is specific to each of the products of the JLL package and detailed below. If you want to see a concrete example of a package providing all the main three products, have a look at Fontconfig_jll.jl.

In addition to the variables defined above by each JLL wrapper, the package JLLWrappers defines an additional unexported variable:

• LIBPATH_env: the name of the environment variable of the search paths of the shared libraries for the current platform. This is equal to LD_LIBRARY_PATH on Linux and FreeBSD, DYLD_FALLBACK_LIBRARY_PATH on macOS, and PATH on Windows.

In what follows, we will use as an example a builder that has these products:

products = [
FileProduct("src/data.txt", :data_txt),
LibraryProduct("libdataproc", :libdataproc),
ExecutableProduct("mungify", :mungify_exe),
]

### LibraryProduct

A LibraryProduct is a shared library that can be ccalled from Julia. Assuming that the product is called libdataproc, the wrapper defines the following variables:

• libdataproc: this is the exported const variable that should be used in ccall:

num_chars = ccall((:count_characters, libdataproc), Cint,
(Cstring, Cint), data_lines[1], length(data_lines[1]))

Roughly speaking, the value of this variable is the basename of the shared library, not its full absolute path;

• libdataproc_splitpath: the path of the shared library, relative to artifact_dir, as returned by splitpath;

• libdataproc_path: the full absolute path of the shared library. Note that this is not const, thus it can't be used in ccall;

• libdataproc_handle: the address in memory of the shared library after it has been loaded at initialization time.

### ExecutableProduct

An ExecutableProduct is a binary executable that can be run on the current platform. If, for example, the ExecutableProduct has been called mungify_exe, the wrapper defines an exported function named mungify_exe that should run by the user in one the following ways: versions:

# Only available in Julia v1.6+
run($(mungify_exe())$arguments)
mungify_exe() do exe
run($exe$arguments)
end

Note that in the latter form exe can be replaced with any name of your choice: with the do-block syntax you are defining the name of the variable that will be used to actually call the binary with run.

The former form is only available when using Julia v1.6, but should be preferred going forward, as it is thread-safe and generally more flexible.

A common point of confusion about ExecutableProducts in JLL packages is why these function wrappers are needed: while in principle you could run the executable directly by using its absolute path in run, these functions ensure that the executable will find all shared libraries it needs while running.

In addition to the function called mungify_exe, for this product there will be the following unexported variables:

• mungify_exe_splitpath: the path of the executable, relative to artifact_dir, as returned by splitpath;
• mungify_exe_path: the full absolute path of the executable;

### FileProduct

A FileProduct is a simple file with no special treatment. If, for example, the FileProduct has been called data_txt, the only variables defined for it are:

• data_txt: this exported variable has the absolute path to the mentioned file:

data_lines = open(data_txt, "r") do io
end
• data_txt_path: this unexported variable is actually equal to data_txt, but is kept for consistency with all other product types.

## Overriding the artifacts in JLL packages

As explained above, JLL packages use the Artifacts system to provide the files. If you wish to override the content of an artifact with their own binaries/libraries/files, you can use the Overrides.toml file.

We detail below a couple of different ways to override the artifact of a JLL package, depending on whether the package is dev'ed or not. The second method is particularly recommended to system administrator who wants to use system libraries in place of the libraries in JLL packages.

### dev'ed JLL packages

In the event that a user wishes to override the content within a dev'ed JLL package, the user may use the dev_jll() method provided by JLL packages to check out a mutable copy of the package to their ~/.julia/dev directory. An override directory will be created within that package directory, providing a convenient location for the user to copy in their own files over the typically artifact-sourced ones. See the segment on "Building and testing JLL packages locally" in the Building Packages section of this documentation for more information on this capability.

### Non-dev'ed JLL packages

As an example, in a Linux system you can override the Fontconfig library provided by Fontconfig_jll.jl and the Bzip2 library provided by Bzip2_jll.jl respectively with /usr/lib/libfontconfig.so and /usr/local/lib/libbz2.so with the following Overrides.toml:

[a3f928ae-7b40-5064-980b-68af3947d34b]
Fontconfig = "/usr"

[6e34b625-4abd-537c-b88f-471c36dfa7a0]
Bzip2 = "/usr/local"

• The UUIDs are those of the JLL packages, a3f928ae-7b40-5064-980b-68af3947d34b for Fontconfig_jll.jl and 6e34b625-4abd-537c-b88f-471c36dfa7a0 for Bzip2_jll.jl. You can either find them in the Project.toml files of the packages (e.g., see the Project.toml file of Fontconfig_jll) or look it up in the registry (e.g., see the entry for Fontconfig_jll in the General registry).
• The artifacts provided by JLL packages have the same name as the packages, without the trailing _jll, Fontconfig and Bzip2 in this case.
• The artifact location is held in the artifact_dir variable mentioned above, which is the "prefix" of the installation of the package. Recall the paths of the products in the JLL package is relative to artifact_dir and the files you want to use to override the products of the JLL package must have the same tree structure as the artifact. In our example we need to use /usr to override Fontconfig and /usr/local for Bzip2.

### Overriding specific products

Instead of overriding the entire artifact, you can override a particular product (library, executable, or file) within a JLL using Preferences.jl.

Compat

This section requires Julia 1.6 or later.

For example, to override our libbz2 example:

using Preferences
set_preferences!(
"LocalPreferences.toml",
"Bzip2_jll",
"libbzip2_path" => "/usr/local/lib/libbz2.so",
)

Note that the product name is libbzip2, but we use libbzip2_path.

Warning

There are two common cases where this will not work:

1. The JLL is part of the Julia stdlib, for example Zlib_jll
2. The JLL has not been compiled with JLLWrappers.jl as a dependency. In this case, it means that the last build of the JLL pre-dates the introduction of the JLLWrappers package and needs a fresh build. Please open an issue on Yggdrasil requesting a new build, or make a pull request to update the relevant build_tarballs.jl script.