# How to use a custom binary

Many solvers are not written in Julia, but instead in languages like C or C++. JuMP interacts with these solvers through binary dependencies.

For many open-source solvers, we automatically install the appropriate binary when you run Pkg.add("Solver"). For example, Pkg.add("ECOS") will also install the ECOS binary.

This page explains how this installation works, and how you can use a custom binary.

Compat

These instructions require Julia 1.6 or later.

## Background

Each solver that JuMP supports is structured as a Julia package. For example, the interface for the ECOS solver is provided by the ECOS.jl package.

Tip

This page uses the example of ECOS.jl because it is simple to compile. Other solvers follow similar conventions. For example, the interface to the Clp solver is provided by Clp.jl.

The ECOS.jl package provides an interface between the C API of ECOS and MathOptInterface. However, it does not handle the installation of the solver binary; that is the job for a JLL package.

A JLL is a Julia package that wraps a pre-compiled binary. Binaries are built using Yggdrasil (for example, ECOS) and hosted in the JuliaBinaryWrappers GitHub repository (for example, ECOS_jll.jl).

JLL packages contain little code. Their only job is to dlopen a dynamic library, along with any dependencies.

JLL packages manage their binary dependencies using Julia's artifact system. Each JLL package has an Artifacts.toml file which describes where to find each binary artifact for each different platform that it might be installed on. Here is the Artifacts.toml file for ECOS_jll.jl.

The binaries installed by the JLL package should be sufficient for most users. In rare cases, however, you may require a custom binary. The two main reasons to use a custom binary are:

• You want a binary with custom compilation settings (for example, debugging)
• You want a binary with a set of dependencies that are not available on Yggdrasil (for example, a commerial solver like Gurobi or CPLEX).

The following sections explain how to replace the binaries provided by a JLL package with the custom ones you have compiled. As a reminder, we use ECOS as an example for simplicity, but the steps are the same for other solvers.

## Explore the JLL you want to override

The first step is to explore the structure and filenames of the JLL package we want to override.

Find the location of the files using .artifact_dir:

julia> using ECOS_jll

julia> ECOS_jll.artifact_dir
"/Users/oscar/.julia/artifacts/2addb75332eff5a1657b46bb6bf30d2410bc7ecf"
Tip

This path may be different on other machines.

Here is what it contains:

julia> readdir(ECOS_jll.artifact_dir)
4-element Vector{String}:
"include"
"lib"
"logs"
"share"

1-element Vector{String}:
"libecos.dylib"

Other solvers may have a bin directory containing executables. To use a custom binary of ECOS, we need to replace /lib/libecos.dylib with our custom binary.

## Compile a custom binary

The next step is to compile a custom binary. Because ECOS is written in C with no dependencies, this is easy to do if you have a C compiler:

oscar@Oscars-MBP jll_example % git clone https://github.com/embotech/ecos.git
[... lines omitted ...]
oscar@Oscars-MBP jll_example % cd ecos
oscar@Oscars-MBP ecos % make shared
[... many lines omitted...]
oscar@Oscars-MBP ecos % mkdir lib
oscar@Oscars-MBP ecos % cp libecos.dylib lib
Warning

Compiling custom solver binaries is an advanced operation. Due to the complexities of compiling various solvers, the JuMP community is unable to help you diagnose and fix compilation issues.

After this compilation step, we now have a folder /tmp/jll_example/ecos that contains lib and include directories with the same files as ECOS_jll:

julia> readdir(joinpath("ecos", "lib"))
1-element Vector{String}:
"libecos.dylib"

## Overriding a single library

To override the libecos library, we need to know what ECOS_jll calls it. (In most cases, it will also be libecos, but not always.)

There are two ways you can check.

1. Check the bottom of the JLL's GitHub README. For example, ECOS_jll has a single LibraryProduct called libecos.
2. Type ECOS_jll. and the press the [TAB] key twice to auto-complete available options:
julia> ECOS_jll.
LIBPATH           PATH_list          best_wrapper       get_libecos_path   libecos_handle
LIBPATH_list      __init__           dev_jll            is_available       libecos_path
PATH              artifact_dir       find_artifact_dir  libecos
Here you can see there is libecos, and more usefully for us, libecos_path.

Once you know the name of the variable to override (the one that ends in _path), use Preferences.jl to specify a new path:

using Preferences
set_preferences!(
"LocalPreferences.toml",
"ECOS_jll",
"libecos_path" => "/tmp/jll_example/ecos/lib/libecos"
)

This will create a file in your current directory called LocalPreferences.toml with the contents:

[ECOS_jll]
libecos_path = "/tmp/jll_example/ecos/lib/libecos"

Now if you restart Julia, you will see:

julia> using ECOS_jll

julia> ECOS_jll.libecos
"/tmp/jll_example/ecos/lib/libecos"

To go back to using the default library, just delete the LocalPreferences.toml file.

## Overriding an entire artifact

Sometimes a solver may provide a number of libraries and executables, and specifying the path for each of the becomes tedious. In this case, we can use Julia's Override.toml to replace an entire artifact.

Overriding an entire artifact requires you to replicate the structure and contents of the JLL package that we explored above.

In most cases you need only reproduce the include, lib, and bin directories (if they exist). You can safely ignore any logs or share directories. Take careful note of what files each directory contains and what they are called.

For our ECOS example, we already reproduced the structure when we compiled ECOS.

So, now we need to tell Julia to use our custom installation instead of the default. We can do this by making an override file at ~/.julia/artifacts/Overrides.toml.

Overrides.toml has the following content:

# Override for ECOS_jll
2addb75332eff5a1657b46bb6bf30d2410bc7ecf = "/tmp/jll_example/ecos"

where 2addb75332eff5a1657b46bb6bf30d2410bc7ecf is the folder from the original ECOS_jll.artifact_dir and "/tmp/jll_example/ecos" is the location of our new installation. Replace these as appropriate for your system.

If you restart Julia after creating the override file, you will see:

julia> using ECOS_jll

julia> ECOS_jll.artifact_dir
"/tmp/jll_example/ecos"

Now when we use ECOS it will use our custom binary.

## Using Cbc with a custom binary

As a second example, we demonstrate how to use Cbc.jl with a custom binary.

### Explore the JLL you want to override

First, let's check where Cbc_jll is installed:

julia> using Cbc_jll

julia> Cbc_jll.artifact_dir
"/Users/oscar/.julia/artifacts/e481bc81db5e229ba1f52b2b4bd57484204b1b06"

5-element Vector{String}:
"bin"
"include"
"lib"
"logs"
"share"

1-element Vector{String}:
"cbc"

10-element Vector{String}:
"libCbc.3.10.5.dylib"
"libCbc.3.dylib"
"libCbc.dylib"
"libCbcSolver.3.10.5.dylib"
"libCbcSolver.3.dylib"
"libCbcSolver.dylib"
"libOsiCbc.3.10.5.dylib"
"libOsiCbc.3.dylib"
"libOsiCbc.dylib"
"pkgconfig"

### Compile a custom binary

Next, we need to compile Cbc. Cbc can be difficult to compile (it has a lot of dependencies), but for macOS users there is a homebrew recipe:

(base) oscar@Oscars-MBP jll_example % brew install cbc
[ ... lines omitted ... ]
(base) oscar@Oscars-MBP jll_example % brew list cbc
/usr/local/Cellar/cbc/2.10.5/bin/cbc
/usr/local/Cellar/cbc/2.10.5/include/cbc/ (76 files)
/usr/local/Cellar/cbc/2.10.5/lib/libCbc.3.10.5.dylib
/usr/local/Cellar/cbc/2.10.5/lib/libCbcSolver.3.10.5.dylib
/usr/local/Cellar/cbc/2.10.5/lib/libOsiCbc.3.10.5.dylib
/usr/local/Cellar/cbc/2.10.5/lib/pkgconfig/ (2 files)
/usr/local/Cellar/cbc/2.10.5/lib/ (6 other files)
/usr/local/Cellar/cbc/2.10.5/share/cbc/ (59 files)
/usr/local/Cellar/cbc/2.10.5/share/coin/ (4 files)

### Override single libraries

To use Preferences.jl to override specific libraries we first check the names of each library in Cbc_jll:

julia> Cbc_jll.
LIBPATH               cbc                    get_libcbcsolver_path  libOsiCbc_path
LIBPATH_list          cbc_path               is_available           libcbcsolver
PATH                  dev_jll                libCbc                 libcbcsolver_handle
PATH_list             find_artifact_dir      libCbc_handle          libcbcsolver_path
__init__              get_cbc_path           libCbc_path
artifact_dir          get_libCbc_path        libOsiCbc
best_wrapper          get_libOsiCbc_path     libOsiCbc_handle

Then we add the following to LocalPreferences.toml:

[Cbc_jll]
cbc_path = "/usr/local/Cellar/cbc/2.10.5/bin/cbc"
libCbc_path = "/usr/local/Cellar/cbc/2.10.5/lib/libCbc.3.10.5"
libOsiCbc_path = "/usr/local/Cellar/cbc/2.10.5/lib/libOsiCbc.3.10.5"
libcbcsolver_path = "/usr/local/Cellar/cbc/2.10.5/lib/libCbcSolver.3.10.5"
Info

Note that capitalization matters, so libcbcsolver_path corresponds to libCbcSolver.3.10.5.

### Override entire artifact

To use the homebrew install as our custom binary we add the following to ~/.julia/artifacts/Overrides.toml:

# Override for Cbc_jll
e481bc81db5e229ba1f52b2b4bd57484204b1b06 = "/usr/local/Cellar/cbc/2.10.5"