Zig, shared libraries and Musl

I'm currently working on upgrading the build script for zig-sqlite and learned something about shared libraries and musl.

For context, zig-sqlite provides a Zig wrapper for SQLite which facilitates using SQLite in Zig.

One of the things you can do with it is building a SQLite loadable extension (I've written an entire blog post about this). These extensions are shared libraries that can be loaded in various ways by SQLite and as you can imagine the shared library is operating system and architecture dependant (it uses dlopen and equivalent under the hood).

Well, one thing I didn't realize until today was that shared libraries built with musl are not loadable by glibc and vice-versa (in most cases; see the musl FAQ).

This incompatibility hit me today because I've been testing my changes with the native-linux target which defaults to using musl instead of glibc, and while the build is successful the shared library doesn't work with glibc. You can see this with ldd:

$ ldd zig-out/lib/libzigcrypto.so
zig-out/lib/libzigcrypto.so: error while loading shared libraries: /lib64/libc.so: invalid ELF header

In my case, I saw this error too when trying to load the extension with sqlite3:

$ sqlite3
SQLite version 3.46.1 2024-08-13 09:16:08
sqlite> .load ./zig-out/lib/libzigcrypto
Error: /lib64/libc.so: invalid ELF header

I had no idea what this meant at first but this comment on the Zig repository is where I learned that:

• there are, in fact, two different dynamic linkers for musl and glibc
• ldd only works with glibc-based binaries or libraries

Fortunately, solving this is trivial: I just had to build with the native-linux-gnu target.