Skip to content

Static Linking

Crystal supports static linking, i.e. it can link a binary with static libraries so that these libraries don't need to be available as runtime dependencies. This improves portability at the cost of larger binaries.

Static linking can be enabled using the --static compiler flag. See the usage instructions in the language reference.

When --static is given, linking static libraries is enabled, but it's not exclusive. The produced binary won't be fully static linked if the dynamic version of a library is higher in the compiler's library lookup chain than the static variant (or if the static library is entirely missing). In order to build a static binary you need to make sure that static versions of the linked libraries are available and the compiler can find them.

The compiler uses the CRYSTAL_LIBRARY_PATH environment variable as a first lookup destination for static and dynamic libraries that are to be linked. This can be used to provide static versions of libraries that are also available as dynamic libraries.

Not all libraries work well with being statically linked, so there may be some issues. openssl for example is known for complications, as well as glibc (see Fully Static Linking).

Some package managers provide specific packages for static libraries, where foo provides the dynamic library and foo-static for example provides the static library. Sometimes static libraries are also included in development packages.

Fully Static Linking

A fully statically linked program has no dynamic library dependencies at all. This is useful for delivering portable, pre-compiled binaries. Prominent examples of fully statically linked Crystal programs are the crystal and shards binaries from the official distribution packages.

In order to link a program fully statically, all dependencies need to be available as static libraries at compile time. This can be tricky sometimes, especially with common libc libraries.



glibc is the most common libc implementation on Linux systems. Unfortunately, it doesn't play nicely with static linking and it's highly discouraged.

Instead, static linking against musl-libc is the recommended option on Linux. Since it's statically linked, a binary linked against musl-libc will also run on a glibc system. That's the entire point of it.

It is however completely fine to statically link other libraries besides a dynamically linked glibc.


musl-libc is a clean, efficient libc implementation with excellent static linking support.

The recommended way to build a statically linked Crystal program is Alpine Linux, a minimal Linux distribution based on musl-libc.

Official Docker Images based on Alpine Linux are available on Docker Hub at crystallang/crystal. The latest release is tagged as crystallang/crystal:latest-alpine. The Dockerfile source is available at crystal-lang/distribution-scripts.

With pre-installed crystal compiler, shards, and static libraries of all of stdlib's dependencies these Docker images allow to easily build static Crystal binaries even from glibc-based systems. The official Crystal compiler builds for Linux are created using these images.

Here's an example how the Docker image can be used to build a statically linked Hello World program:

$ echo 'puts "Hello World!"' >
$ docker run --rm -it -v $(pwd):/workspace -w /workspace crystallang/crystal:latest-alpine \
    crystal build --static
$ ./hello-world
Hello World!
$ ldd hello-world
        statically linked

Alpine’s package manager APK is also easy to work with to install static libraries. Available packages can be found at


macOS doesn't officially support fully static linking because the required system libraries are not available as static libraries.


Windows doesn't support fully static linking because the Win32 libraries are not available as static libraries.

Currently, static linking is the default mode of linking on Windows, and dynamic linking can be opted in via the -Dpreview_dll compile-time flag. In order to distinguish static libraries from DLL import libraries, when the compiler searches for a library foo.lib in a given directory, foo-static.lib will be attempted first while linking statically, and foo-dynamic.lib will be attempted first while linking dynamically. The official Windows packages are distributed with both static and DLL import libraries for all third-party dependencies, except for LLVM.

Static linking implies using the static version of Microsoft's C runtime library (/MT), and dynamic linking implies the dynamic version (/MD); extra C libraries should be built with this in mind to avoid linker warnings about mixing CRT versions. There is currently no way to use the dynamic CRT while linking statically.

Identifying Static Dependencies

If you want to statically link dependencies, you need to have their static libraries available. Most systems don't install static libraries by default, so you need to install them explicitly. First you have to know which libraries your program links against.


Static libraries have the file extension .a on POSIX and .lib on Windows. DLL import libraries on Windows also have the .lib extension. Dynamic libraries have .so on Linux and most other POSIX platforms, .dylib on macOS and .dll on Windows.

On most POSIX systems the tool ldd shows which dynamic libraries an executable links to. The equivalent on macOS is otool -L and the equivalent on Windows is dumpbin /dependents.

The following example shows the output of ldd for a simple Hello World program built with Crystal 0.36.1 and LLVM 10.0 on Ubuntu 18.04 LTS (in the crystallang/crystal:0.36.1 docker image). The result varies on other systems and versions.

$ ldd hello-world_glibc (0x00007ffeaf990000) => /lib/x86_64-linux-gnu/ (0x00007fc393624000) => /lib/x86_64-linux-gnu/ (0x00007fc393286000) => /lib/x86_64-linux-gnu/ (0x00007fc393067000) => /usr/lib/x86_64-linux-gnu/ (0x00007fc392e16000) => /lib/x86_64-linux-gnu/ (0x00007fc392c12000) => /lib/x86_64-linux-gnu/ (0x00007fc3929fa000) => /lib/x86_64-linux-gnu/ (0x00007fc392609000)
    /lib64/ (0x00007fc393dde000)

These libraries are the minimal dependencies of Crystal's standard library. Even an empty program requires these libraries for setting up the Crystal runtime.

This looks like a lot, but most of these libraries are actually part of the libc distribution.

On Alpine Linux the list is much smaller because musl includes more symbols directly into a single binary. The following example shows the output of the same program built with Crystal 0.36.1 and LLVM 10.0 on Alpine Linux 3.12 (in the crystallang/crystal:0.36.1-alpine docker image).

$ ldd hello-world_musl
    /lib/ (0x7fe14b05b000) => /usr/lib/ (0x7fe14af1d000) => /usr/lib/ (0x7fe14aead000) => /usr/lib/ (0x7fe14ae99000) => /lib/ (0x7fe14b05b000)

The individual libraries are libpcre, libgc and the rest is musl (libc). The same libraries are used in the Ubuntu example.

In order to link this program statically, we need static versions of these three libraries.


The *-alpine docker images ship with static versions of all libraries used by the standard library. If your program links no other libraries then adding the --static flag to the build command is all you need to link fully statically.

Dynamic library lookup

The lookup paths of dynamic libraries at runtime can be controlled by the CRYSTAL_LIBRARY_RPATH environment variable during compilation. Currently this is supported on Linux and Windows.


If CRYSTAL_LIBRARY_RPATH is defined during compilation, it is passed unmodified to the linker via an -Wl,rpath option. The exact behavior depends on the linker; usually, this is appended to the ELF executable's DT_RUNPATH or DT_RPATH dynamic tag entry. The special $ORIGIN / $LIB / $PLATFORM variables might not be supported on all platforms.


The standard library supports experimental DLL delay loading, and may alter the search order of DLLs by using delay loading.

If a /DELAYLOAD linker flag is passed for a given DLL, then the first time an executable loads a symbol from that DLL, it will attempt the semicolon-separated paths in the executable's CRYSTAL_LIBRARY_RPATH first, in the order they are declared, before the default lookup order. $ORIGIN inside CRYSTAL_LIBRARY_RPATH expands into the path of the running executable itself. For example, if CRYSTAL_LIBRARY_RPATH=$ORIGIN\mylibs;C:\bar during compilation, --link-flags=/DELAYLOAD:calc.dll is supplied, and the executable is located at C:\foo\test.exe, then the executable searches for C:\foo\mylibs\calc.dll, then C:\bar\calc.dll, and uses the default order afterwards.

Non-delay-loaded DLLs are loaded immediately upon program startup, and do not respect CRYSTAL_LIBRARY_RPATH.

By default, no DLLs are delay-loaded. However, if the -Dpreview_win32_delay_load compile-time flag is specified at compilation time, the compiler will detect all DLL dependencies from their import libraries, inserting a /DELAYLOAD linker flag per DLL during compilation.