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

The dedicated directory is there for three reasons:

• Names would otherwise collide. An FZnSO solver is usually a shim named after the library it wraps, so libgecode.so would sit on top of real Gecode.
• Enumeration stays cheap. Listing the installed solvers is a single directory listing, rather than a scan of every system library.
• Nothing loads one by accident. Solvers stay off the default linker search path, where nothing should find them.

The third point is also what frees the version placement from platform convention: nothing ever links against a solver, so no soname is ever resolved.