2 Loading Foreign Libraries

▼ |  | The Racket Foreign Interface

1 | Overview
2 | Loading Foreign Libraries
3 | C Types
4 | Pointer Functions
5 | Derived Utilities
6 | Miscellaneous Support
7 | Unexported Primitive Functions
| Bibliography
| Index

On this page:

ffi-lib?
ffi-lib
get-ffi-obj
set-ffi-obj!
make-c-parameter
define-c
ffi-obj-ref

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### 2`` Loading Foreign Libraries

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.

> procedure (ffi-lib? v) → boolean?
> v : any/c

Returns #t if v is a foreign-library value, #f otherwise.

> > procedure
> > (ffi-lib |  | path |  |  |  |
> > | [ | version |  |  |  |
> > |  | #:get-lib-dirs get-lib-dirs |  |  |  |
> > |  | #:fail fail |  |  |  |
> > |  | #:global? global?]) |  | → |  | any
> path : (or/c path-string? #f)
> version : (or/c string? (listof (or/c string? #f)) #f) = #f
> get-lib-dirs : (-> (listof path?)) = get-lib-search-dirs
> fail : (or/c #f (-> any)) = #f
> global? : any/c = (eq? 'global (system-type 'so-mode))

Returns a foreign-library value or the result of fail. Normally,

- 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.

If path is not #f, global? is true, and the operating system supports opening a library in “global” mode so that the library’s symbols are used for resolving references from libraries that are loaded later, then global mode is used to open the library. Otherwise, the library is opened in “local” mode, where the library’s symbols are not made available for future resolution. This local-versus-global choice does not affect whether the library’s symbols are available via (ffi-lib #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.

> procedure (get-ffi-obj objname lib type [failure-thunk]) → any
> objname : (or/c string? bytes? symbol?)
> lib : (or/c ffi-lib? path-string? #f)
> type : ctype?
> failure-thunk : (or/c (-> any) #f) = #f

Looks for objname in lib library. If lib is not a foreign-library value it is converted to one by calling ffi-lib. If objname is found in lib, it is converted to Racket using the given type. Types are described in C Types; in particular the get-ffi-obj procedure is most often used with function types created with _fun.

Keep in mind that get-ffi-obj is an unsafe procedure; see Overview for details.

If the name 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 name 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.

> procedure (set-ffi-obj! objname lib type new) → void?
> objname : (or/c string? bytes? symbol?)
> lib : (or/c ffi-lib? path-string? #f)
> type : ctype?
> new : any/c

Looks for objname in lib similarly to get-ffi-obj, but then it stores the given new value into the library, converting it to a C value. This can be used for setting library customization variables that are part of its interface, including Racket callbacks.

> > procedure
> > (make-c-parameter objname lib type) |  | → |  |
> > (and/c (-> any)
> > (any/c -> void?))
> objname : (or/c string? bytes? symbol?)
> lib : (or/c ffi-lib? path-string? #f)
> type : ctype?

Returns a parameter-like procedure that can either references the specified foreign value, or set it. The arguments are handled as in get-ffi-obj.

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.

> syntax (define-c id lib-expr type-expr)

Defines id behave like a Racket binding, but id is actually redirected through a parameter-like procedure created by make-c-parameter. The id is used both for the Racket binding and for the foreign name.

> procedure (ffi-obj-ref objname lib [failure-thunk]) → any
> objname : (or/c string? bytes? symbol?)
> lib : (or/c ffi-lib? path-string? #f)
> failure-thunk : (or/c (-> any) #f) = #f

Returns a pointer for the specified foreign name, calls failure-thunk if the name is not found, or raises an exception if failure-thunk is #f.

Normally, get-ffi-obj should be used, instead.

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