The FFI is normally used by extracting functions and other objects from shared objects (a.k.a. shared libraries or dynamically loaded libraries). The ffi-lib function loads a shared object.
path is a path without a version or suffix (i.e., without ".dll", ".so", or ".dylib"); and
version is a list of versions to try in order with #f (i.e., no version) as the last element of the list; for example, '("2" #f) indicates version 2 with a fallback to a versionless library.
A string or #f version is equivalent to a list containing just the string or #f, and an empty string (by itself or in a list) is equivalent to #f.
Beware of relying on versionless library names. On some platforms, versionless library names are provided only by development packages. At the same time, other platforms may require a versionless fallback. A list of version strings followed by #f is typically best for version.
Assuming that path is not #f, the result from ffi-lib represents the library found by the following search process:
If path is not an absolute path, look in each directory reported by get-lib-dirs. In each directory, try path with the first version in version, adding a suitable suffix if path does not already end in the suffix, then try the second version in version, etc. (If version is an empty list, no paths are tried in this step.)
Try the same filenames again, but without converting the path to an absolute path, which allows the operating system to use its own search paths. (If version is an empty list, no paths are tried in this step.)
Try path without adding any version or suffix, and without converting to an absolute path.
Try the version-adjusted filenames again, but relative to the current directory. (If version is an empty list, no paths are tried in this step.)
Try path without adding any version or suffix, but converted to an absolute path relative to the current directory.
If none of the paths succeed and fail is a function, then fail is called in tail position. If fail is #f, an error is reported from trying the first path from the second bullet above or (if version is an empty list) from the third bullet above. A library file may exist but fail to load for some reason; the eventual error message will unfortunately name the fallback from the second or third bullet, since some operating systems offer no way to determine why a given library path failed.
If path is #f, then the resulting foreign-library value represents all libraries loaded in the current process, including libraries previously opened with ffi-lib. In particular, use #f to access C-level functionality exported by the run-time system (as described in Inside: Racket C API). The version argument is ignored when path is #f.
Due to the way the operating system performs dynamic binding, loaded libraries are associated with Racket (or DrRacket) for the duration of the process. Re-evaluating ffi-lib (or hitting the Run button in DrRacket) will not force a re-load of the corresponding library.
If the object is not found, and failure-thunk is provided, it is used to produce a return value. For example, a failure thunk can be provided to report a specific error if an object is not found:
(define foo (get-ffi-obj "foo" foolib (_fun _int -> _int) (lambda () (error 'foolib "installed foolib does not provide \"foo\""))))
The default (also when failure-thunk is provided as #f) is to raise an exception.
A parameter-like function is useful in case Racket code and library code interact through a library value. Although make-c-parameter can be used with any time, it is not recommended to use this for foreign functions, since each reference through the parameter will construct the low-level interface before the actual call.
(define-c id lib-expr type-expr)