Discovery

A solver is a dynamically loadable library installed into a fznso directory of its own, rather than onto the normal library path. Applications find one by name, never a path, and the directories are searched in this order:

1. $FZNSO_SOLVER_PATH, split on the platform’s path separator (:, or ; on Windows);
2. the per-user directory: $XDG_DATA_HOME/fznso (falling back to ~/.local/share/fznso), ~/Library/Application Support/fznso on macOS, or %LOCALAPPDATA%\fznso on Windows;
3. the system directories: /usr/local/lib/fznso and /usr/lib/fznso, or a fznso directory beside the running executable on Windows.

Earlier directories win outright, so a solver dropped in $FZNSO_SOLVER_PATH or the per-user directory overrides a system one even if the system one is newer; the newest version is chosen only among candidates of equal precedence.

A candidate whose fznso_<name>_abi_version does not match the application’s is skipped, and the search continues with the next candidate.

Why a dedicated directory

Because the names would collide otherwise. An FZnSO solver is usually a shim named after the library it wraps, and plenty of systems already have a libgecode on the library path.

A directory of their own is also what lets a loader find a solver by name, from a directory listing. The alternative is opening every library on the system to see which ones export the right symbols — loading a great many libraries that will never be used.

Nothing ever links against a solver, which is what frees the version placement from platform convention: no soname is ever resolved.