This manual documents FFI 1.0.
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Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License.”
This FFI provides Scheme syntax for calling C functions and accessing C data. The functions and data structures are declared in a case sensitive .cdecl file, which is used by a shim generator to produce callout and callback trampoline functions. The trampolines are compiled and linked to the C toolkit, producing a shared object that Scheme can dynamically load.
Examples of the new syntax:
(malloc (C-sizeof "GdkEvent")) => #[alien 42 0x081afc60]
(C->= #@42 "GdkEvent any type" 14)
(C-> #@42 "GdkEvent any type") => 14
(C-enum "GdkEventType" 14) => |GDK_MAP|
(C-enum "GDK_MAP") => 14
(C-sizeof "GdkColor") => 12
(C-offset "GdkColor blue") => 8
(C-array-loc #@43 "GdkColor" (C-enum "GTK_STATE_NORMAL")) => #[alien 44 0x081afc60] ; New alien.
(C-array-loc! #@43 "GdkColor" (C-enum "GTK_STATE_PRELIGHT")) => #[alien 43 0x081afc78] ; Modified alien.
(C-call "gtk_window_new" retval args …) => #!unspecific
(C-callback "delete_event") => #[alien-function 44 Scm_delete_event]
(C-callback (lambda (window event) …)) => 13 ; A fixnum registration ID.
A Scheme-like declaration of a toolkit’s C functions, constants, and data types is given in a case sensitive .cdecl file. The C declarations look like this:
(extern (* GtkWidget) gtk_window_new (type GtkWindowType)) (typedef GtkWindowType (enum (GTK_WINDOW_TOPLEVEL) (GTK_WINDOW_POPUP)))
c-generate procedure reads these declarations and
writes three files: library-types.bin (a fasdump of the
parsed declarations), library-const.c (a C program that
prints C constants and struct offsets), and library-shim.c
(trampoline functions adapting Scheme procedure application to C
function call). The -const.c program generates a
-const.scm file, which can be expanded into a
(load-option 'FFI) (c-generate "prhello" "#include <gtk/gtk.h>")
The -types.bin and -const.bin files together provide
the information needed to expand
C-... syntax, and are only
needed at syntax time. The compiled -shim.so file is used at
run time, dynamically loaded into the Scheme machine. See Compiling and Linking, which describes these files in more detail, and shows
how they might be built and installed.
C-include syntax loads the -types.bin and
-const.bin files at syntax time. It should appear
at the top level of any file containing
C-... syntax, or be
expanded in the syntax environment of such code.
C-call syntax arranges to invoke a callout
trampoline. Arguments to the trampoline can be integers, floats,
strings (or bytevectors) or aliens (non-heap pointers, to C data
structures, see Alien Data). If a string argument might contain
non-ASCII characters (code points U+0080 and larger), it should be
converted to a bytevector e.g. by
string->utf8, else an error
could be signaled.
(let ((alien (make-alien '|GtkWidget|))) (C-call "gtk_window_new" alien type) (if (alien-null? alien) (error "could not open new window")) alien)
C-callback syntax is used when registering a
Scheme callback trampoline. The two forms of the syntax provide two
arguments for the registration function: the callback trampoline’s
address, and a “user data” argument. When the toolkit calls the
trampoline, it must provide the fixnum-sized user data as an argument.
(C-call "g_signal_connect" window "delete_event" (C-callback "delete_event") ; e.g. &Scm_delete_event (C-callback ; e.g. 314 (lambda (window event) (C-call "gtk_widget_destroy" window) 0)))
The first use of
C-callback (above) expands into a callback
trampoline address — an alien function. The second use evaluates to
a fixnum, which is associated with the given Scheme procedure.
C->= syntaxes peek and
poke values into alien data structures. They take an alien and a
constant string specifying the alien data type and the member to be
accessed (if any).
(C-> alien "GdkRectangle y") → (#[primitive c-peek-int] alien 4)
(C->= alien "GdkRectangle width" 0) → (#[primitive c-poke-int] alien 8 0)
(C-> alien "GdkEvent any type") → (#[primitive c-peek-int] alien 0)
(C-> alien "gfloat") → (#[primitive c-peek-float] alien 0)
A three argument form of the syntax provides an alien to receive a peeked pointer. This avoids consing a new alien.
(C-> alien "GtkWidget style" alien)
The above syntax is understood to say “The data at this
address is a GtkWidget. Load its
style member into
alien’s old address).”
C-offset syntaxes all
transform into integer constants. The last two transform into a padded
byte size and a byte offset respectively.
(C-enum "GTK_WINDOW_POPUP") → 1
(C-sizeof "GdkColor") → 12
(C-offset "GdkColor blue") → 8
The two element form of the
C-enum syntax can be used to find
the name of a constant given its runtime value. It expects the name
of an enum type in a constant string. If the runtime (second)
argument is not one of the constants declared by that type, the
returned value is
(C-enum "GdkEventType" (C-> #@42 "GdkEvent any type")) => |GDK_MAP|
syntaxes compute the locations of C array elements. They can be used
to advance a scan pointer or locate an element by its index. The
examples in the synopsis might expand as shown here.
(C-array-loc #@43 "GdkColor" (C-enum "GTK_STATE_NORMAL")) → (alien-byte-increment #@43 (* (C-sizeof "GdkColor") (C-enum "GTK_STATE_NORMAL"))) → (alien-byte-increment #@43 (* 12 0)) => #@44 (C-array-loc! #@43 "GdkColor" (C-enum "GTK_STATE_PRELIGHT")) → (alien-byte-increment! #@43 (* (C-sizeof "GdkColor") (C-enum "GTK_STATE_PRELIGHT"))) → (alien-byte-increment! #@43 (* 12 2)) => #@43
A simple scan of characters in the wide string
look like this.
(let ((len (C-> alien "toolkit_string_type int_member")) (scan (C-> alien "toolkit_string_type array_member"))) (let loop ((n 0)) (if (< n len) (let ((wchar (C-> scan "wchar"))) (process wchar) (C-array-loc! scan "wchar" 1) (loop (1+ n))))))
That is a quick look at the facilities. The next section describes the C declaration language, and the following sections examine the FFI’s syntax and runtime facilities in detail. Final sections provide an example program and show how its dynamically loaded shim is built.
A shim between Scheme and a C toolkit is specified by a case sensitive .cdecl file containing Scheme-like declarations of all relevant toolkit types, constants, and functions. Callback functions to be passed to the toolkit are also specified here.
Each top-level form in the C declaration file must look like one of these:
(include "filename") (typedef Name any) (struct Name (Member type) …) (union Name (Member type) …) (enum Name (Member) …) (extern function-type Name (param1 arg-type) …) (callback callback-type Name (param1 callback-arg-type) …)
include expression includes another .cdecl file in
the current .cdecl file. The string argument is interpreted
relative to the current file’s directory.
any can be a type or the word
arg-type can be any type except anonymous structs and unions.
function-type can be any arg-type or
callback-arg-type can be any type except struct and union types.
callback-type can be any callback-arg-type or
type can look like any of these:
Name basics (* any) (enum Name) (enum Name (Member) …) (struct Name) (struct Name (Member type) …) (union Name) (union Name (Member type) …)
Name should be defined via a
typedef form somewhere in
the (included) file(s). It does not have to be defined before it is
referenced. It does not have to be defined at all if it is
only the target of a pointer type.
basics can be any of the words:
double (all lowercase).
While the informal grammar above allows anonymous structs to be member
types, they are useless outside a named type declaration. The peek
and poke (
C->=) syntaxes require a type name
"struct _GdkEventAny") before
any member names.
C-include syntax takes a library name and loads
the corresponding -types and -const files at syntax
time. This makes the C types and constants available to the other
C-... syntax expanders. The form binds
the syntax environment unless it is already defined there. Thus a
(C-include "library") form can be placed at the top of every
C-... syntax, or loaded into the syntax-time
environment of those files.
A C data structure is represented by an alien containing the data
structure’s memory address. “Peek” primitives are available to read
pointers and the basic C types (e.g. ints, floats) at small (fixnum)
offsets from an alien’s address. They return to Scheme an alien
address, integer or flonum as appropriate. “Poke” primitives
do the reverse, storing pointers, integers or floats at fixnum offsets
from alien addresses.
Other procedures on aliens are
c-peek-cstring. Refer to
ffi.pkg in The Source for a complete list.
C->= syntaxes apply the peek and poke
primitives to constant offsets. They expect their first argument
subform to be a constant string — space-separated words naming a C
type and any member to be accessed. A member within a struct or union
member is specified by appending its name. For example
_GdkEvent any window" would specify a peek at the
member of the
any member of the
struct _GdkEvent data at
some alien address. Note that the final member’s type must be a basic
C type, pointer type, or enum type. Otherwise, an error is signaled
at syntax time.
(C-> alien "struct _GdkEvent any window" window-alien) → (#[primitive c-peek-pointer] alien 0 window-alien) => #[alien 44 (* GdkWindow) 0x081afc60]
Note that in the example above, the final member has a pointer type. In this case an extra alien argument can be provided to receive the peeked pointer. Otherwise a new alien is created and returned.
malloc procedure returns an alien that will automatically
free the malloced memory when it is garbage collected.
It can also be explicitly freed with the
free procedure. The alien
address can be incremented to scan the malloced memory, then freed
(without returning it to the original, malloced address). A band
restore marks all malloced aliens as though they have been freed.
(free (malloc '|GdkRectangle|))
C-call syntax produces code that applies
to an alien function structure — a cache for the callout
trampoline’s entry address.
(C-call "gtk_button_new" (make-alien '(* |GtkWidget|))) → (call-alien '#[alien-function gtk_button_new] (make-alien …))
The alien function contains all the information needed to load the
callout trampoline on demand (i.e. its name and library). Once the
alien function has cached the entry address,
invoke the trampoline (via
#[primitive c-call]). The
trampoline gets its arguments off the Scheme stack, converts them to C
values, calls the C function, conses a result, and returns it to
A function returning a pointer type is treated specially.
Its trampoline expects an extra (first) argument.
If the argument is
#f, the return value is ignored.
If the argument is an alien, the function’s return value clobbers the
alien’s address. This makes it easy to grab pointers to toolkit
resources without dropping them, and to avoid unnecessary consing of
A function returning a struct or union type is treated similarly.
Its trampoline expects an extra (first) argument.
If the argument is
#f, the return value is ignored.
If the argument is an alien, the returned struct or union is copied to
Struct and union type parameters of a function are treated similarly. The function’s trampoline expects an alien argument for each such parameter and copies the struct or union from the argument address into a local variable. Callbacks currently cannot receive struct or union type arguments, though they can receive pointer type arguments (consing an alien for each).
alien-function structures are fasdumpable. The caching
mechanism invalidates the cache when a band is restored, or a
fasdumped object is fasloaded. The alien function will lookup the
trampoline entry point again on demand.
A callback declaration must include a parameter named “ID”. The ID
argument will be used to find the Scheme callback procedure. It must
be the same “user data” value provided to the toolkit when the
callback was registered. For example, a callback trampoline named
Scm_delete_event might be declared like this:
(callback gint delete_event (window (* GtkWidget)) (event (* GdkEventAny)) (ID gpointer))
The callback might be registered with the toolkit like this:
(C-call "g_signal_connect" window "delete_event" (C-callback "delete_event") ; e.g. &Scm_delete_event (C-callback ; e.g. 314 (lambda (window event) (C-call "gtk_widget_destroy" window) 0)))
The toolkit’s registration function,
g_signal_connect, would be
declared like this:
(extern void g_signal_connect (object (* GtkObject)) (name (* gchar)) (CALLBACK GtkSignalFunc) (ID gpointer))
This function should have parameters named
ID. The callout trampoline will convert the callback argument
from a Scheme alien function to an entry address. The
will be converted to a C integer and then cast to its declared type
(in this example,
Note that the registered callback procedures are effectively pinned. They cannot be garbage collected. They are “on call” to handle callbacks from the toolkit until they are explicitly de-registered. A band restore automatically de-registers all callbacks.
Callback procedures are executed with thread preemption suspended. Thus Scheme will not switch to another thread, especially not one preempted in an earlier callback. Such a thread could finish its callback and return from the later callback, not to its original caller.
Scheme will not preempt a callback, but if the callback calls
suspend-thread it will switch to a running thread. If the
callback does IO (and blocks), suspends, yields, sleeps, or grabs
(waits for) a mutex, the runtime system will switch to another thread,
possibly a thread that blocked for IO during an earlier callback but
is now recently unblocked and determined to finish and return to the
wrong caller. Thus callback procedures should be written as if they
were interrupt handlers. They should be short and simple because they
must not wait.
outf-error procedure is provided for debugging purposes.
It writes one or more argument strings (and
non-strings) to the Unix “stderr” channel, atomically, via a machine
primitive, bypassing the runtime’s IO buffering and thread switching.
Thus trace messages from multiple threads will appear on stderr
c-generate procedure takes a library name and an
optional preamble. It reads the library.cdecl file and
writes two .c files. The preamble is included at the top of
both. It typically contains
#include C pre-processor
directives required by the C library, but could include additional
shim code. Here is a short script that generates a shim for the
example “Hello, World!” program.
(load-option 'FFI) (c-generate "prhello" "#include <gtk/gtk.h>")
This script will produce three files:
This file contains the trampoline functions — one for each declared
C extern or callback. It includes the mit-scheme.h header
file, found in the
AUXDIR directory —
This file contains a C program that creates prhello-const.scm. It is compiled and linked as normal for programs using the toolkit, and does not depend on the Scheme machine. It does not actually call any toolkit functions. It just collects information from the compiler about the declared C types and constants.
This file is a fasdumped
c-includes structure containing all of
the types, constants and functions declared in the .cdecl file.
The following Makefile rules describe the process of building and installing a shim for the example “Hello, World!” program.
AUXDIR=/usr/local/lib/mit-scheme-i386 install: build install -m 644 prhello-types.bin $(AUXDIR) install -m 644 prhello-const.bin $(AUXDIR) install -m 644 prhello-shim.so $(AUXDIR) uninstall: rm $(AUXDIR)/prhello-* clean: rm prhello-const* prhello-types* prhello-shim* build: prhello-shim.so prhello-types.bin prhello-const.bin prhello-shim.so: prhello-shim.o $(CC) -shared -fPIC -o $@ $^ `pkg-config --libs gtk+-3.0` prhello-shim.o: prhello-shim.c $(CC) -I$(AUXDIR) -Wall -fPIC `pkg-config --cflags gtk+-3.0` -o $@ -c $< prhello-shim.c prhello-const.c prhello-types.bin: prhello.cdecl echo '(generate-shim "prhello" "#include <gtk/gtk.h>")' \ | mit-scheme --batch-mode prhello-const.bin: prhello-const.scm echo '(sf "prhello-const")' | mit-scheme --batch-mode prhello-const.scm: prhello-const ./prhello-const prhello-const: prhello-const.o $(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS) `pkg-config --libs gtk+-3.0` prhello-const.o: prhello-const.c $(CC) `pkg-config --cflags gtk+-3.0` $(CFLAGS) -o $@ -c $<
The FFI also supports libraries created by GNU automake (libtool). The source distribution includes several simple plugins. Each uses a portable Makefile.am to build and install its shared object.
includes the C declarations and Scheme code required to implement
Havoc Pennington’s Hello World example from
GGAD. For an extra,
Schemely treat, its
delete_event callback is a Scheme procedure
closed over a binding of
counter that is used to implement some
#| -*-Scheme-*- This is Havoc Pennington's Hello World example from GGAD, in the raw FFI. Note that no arrangements have been made to de-register the callbacks. |# (declare (usual-integrations)) (C-include "prhello") (define (hello) (C-call "gtk_init" 0 null-alien) (let ((window (let ((alien (make-alien '|GtkWidget|))) (C-call "gtk_window_new" alien (C-enum "GTK_WINDOW_TOPLEVEL")) (if (alien-null? alien) (error "Could not create window.")) alien)) (button (let ((alien (make-alien '|GtkWidget|))) (C-call "gtk_button_new" alien) (if (alien-null? alien) (error "Could not create button.")) alien)) (label (let ((alien (make-alien '|GtkWidget|))) (C-call "gtk_label_new" alien "Hello, World!") (if (alien-null? alien) (error "Could not create label.")) alien))) (C-call "gtk_container_add" button label) (C-call "gtk_container_add" window button) (C-call "gtk_window_set_title" window "Hello") (C-call "gtk_container_set_border_width" button 10) (let ((counter 0)) (C-call "g_signal_connect" window "delete_event" (C-callback "delete_event") ;trampoline (C-callback ;callback ID (lambda (w e) (outf-error ";Delete me "(- 2 counter)" times.\n") (set! counter (1+ counter)) ;; Three or more is the charm. (if (> counter 2) (begin (C-call "gtk_main_quit") 0) 1)))) (C-call "g_signal_connect" button "clicked" (C-callback "clicked") ;trampoline (C-callback ;callback ID (lambda (w) (let ((gstring (make-alien '(* |gchar|)))) (C-call "gtk_label_get_text" gstring label) (let ((text (c-peek-cstring gstring))) (C-call "gtk_label_set_text" label (list->string (reverse! (string->list text)))))) unspecific)))) (C-call "gtk_widget_show_all" window) (C-call "gtk_main") window))
Here are the C declarations.
#| -*-Scheme-*- C declarations for prhello.scm. |# (typedef gint int) (typedef guint uint) (typedef gchar char) (typedef gboolean gint) (typedef gpointer (* mumble)) (extern void gtk_init (argc (* int)) (argv (* (* (* char))))) (extern (* GtkWidget) gtk_window_new (type GtkWindowType)) (typedef GtkWindowType (enum (GTK_WINDOW_TOPLEVEL) (GTK_WINDOW_POPUP))) (extern (* GtkWidget) gtk_button_new) (extern (* GtkWidget) gtk_label_new (str (* (const char)))) (extern void gtk_container_add (container (* GtkContainer)) (widget (* GtkWidget))) (extern void gtk_window_set_title (window (* GtkWindow)) (title (* (const gchar)))) (extern void gtk_container_set_border_width (container (* GtkContainer)) (border_width guint)) (extern void gtk_widget_show_all (widget (* GtkWidget))) (extern void g_signal_connect (instance gpointer) (name (* gchar)) (CALLBACK GCallback) (ID gpointer)) (typedef GCallback (* mumble)) (callback gboolean delete_event (window (* GtkWidget)) (event (* GdkEventAny)) (ID gpointer)) (callback void clicked (widget (* GtkWidget)) (ID gpointer)) (extern void gtk_widget_destroy (widget (* GtkWidget))) (extern (* (const gchar)) gtk_label_get_text (label (* GtkLabel))) (extern void gtk_label_set_text (label (* GtkLabel)) (str (* (const char)))) (extern void gtk_main) (extern void gtk_main_quit)
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The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warrany Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:
with the Invariant Sections being list their titles, with the Front-Cover Texts being list, and with the Back-Cover Texts being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.