1. Module (chicken foreign)
  2. Accessing external objects
    1. foreign-code
    2. foreign-value
    3. foreign-declare
    4. define-foreign-type
    5. foreign-type-size
    6. define-foreign-variable
    7. foreign-lambda
    8. foreign-lambda*
    9. foreign-safe-lambda
    10. foreign-safe-lambda*
    11. foreign-primitive
  3. Callbacks
    1. define-external
    2. C_callback
    3. C_callback_adjust_stack
  4. Locations
    1. define-location
    2. let-location
    3. location

Module (chicken foreign)

This module provides a foreign function interface to access externally defined functions and variables in C-compatible languages, as well as operating-system specific functionality.

Note that this interface is source-based, since CHICKEN translates Scheme code to C. It is not binary, as in many other Scheme implementations.

Several special forms of this interface refer to foreign type specifies, consult the Foreign type specifies of this manual for more information.

Accessing external objects

foreign-code

[syntax] (foreign-code STRING ...)

Executes the embedded C/C++ code STRING ..., which should be a sequence of C statements, which are executed and return an unspecified result.

(foreign-code "doSomeInitStuff();")     =>  #<unspecified>

Code wrapped inside foreign-code may not invoke callbacks into Scheme.

foreign-value

[syntax] (foreign-value CODE TYPE)

Evaluates the embedded C/C++ expression CODE (which may be a string or symbol), returning a value of type given in the foreign-type specifier TYPE.

(print (foreign-value "my_version_string" c-string))

foreign-declare

[syntax] (foreign-declare STRING ...)

Include given strings verbatim into header of generated file.

define-foreign-type

[syntax] (define-foreign-type NAME TYPE [ARGCONVERT [RETCONVERT]])

Defines an alias for TYPE with the name NAME (a symbol). TYPE may be a type-specifier or a string naming a C type. The namespace of foreign type specifiers is separate from the normal Scheme namespace. The optional arguments ARGCONVERT and RETCONVERT should evaluate to procedures that map argument- and result-values to a value that can be transformed to TYPE:

(define-foreign-type char-vector 
  nonnull-c-string
  (compose list->string vector->list)
  (compose list->vector string->list) )

(define strlen
  (foreign-lambda int "strlen" char-vector) )

(strlen '#(#\a #\b #\c))                      ==> 3

(define memset
  (foreign-lambda char-vector "memset" char-vector char int) )

(memset '#(#_ #_ #_) #\X 3)                ==> #(#\X #\X #\X)

Foreign type-definitions are only visible in the compilation-unit in which they are defined, so use include to use the same definitions in multiple files.

foreign-type-size

[syntax] (foreign-type-size TYPE)

Returns the size of the storage required to hold values of the given foreign type TYPE. This is basically equivalent to

(foreign-value "sizeof(TYPE)" size_t)

but also handles user-defined types and allows "TYPE" to be a string, which will be given literally to the sizeof operator.

define-foreign-variable

[syntax] (define-foreign-variable NAME TYPE [STRING])

Defines a foreign variable of name NAME (a symbol). STRING should be the real name of a foreign variable or parameterless macro. If STRING is not given, then the variable name NAME will be converted to a string and used instead. All references and assignments (via set!) are modified to correctly convert values between Scheme and C representation. This foreign variable can only be accessed in the current compilation unit, but the name can be lexically shadowed. Note that STRING can name an arbitrary C expression. If no assignments are performed, then STRING doesn't even have to specify an lvalue. See that define-foreign-variable will not generate C declarations or memory allocation code; use it to include references to variables in external C code. To actually create Scheme variables visible from C, use define-external (see the Manual section on Callbacks). For example, the following code:

(import foreign)
(define-foreign-variable x double "var_x")
(print x)

will not work, because a reference to var_x will be inserted in the C code, but no declaration will be included (this can be easily verified by translating the program into C with csc -t program.scm). Changing the second line to (define-external x double 0.5) will work (and the value 0.5 will be printed).

foreign-lambda

[syntax] (foreign-lambda RETURNTYPE NAME ARGTYPE ...)

Represents a binding to an external routine. This form can be used in the position of an ordinary lambda expression. NAME specifies the name of the external procedure and should be a string or a symbol.

foreign-lambda*

[syntax] (foreign-lambda* RETURNTYPE ((ARGTYPE VARIABLE) ...) STRING ...)

Similar to foreign-lambda, but instead of generating code to call an external function, the body of the C procedure is directly given in STRING ...:

(define my-strlen
  (foreign-lambda* int ((c-string str))
    "int n = 0;
     while(*(str++)) ++n;
     C_return(n);") )

(my-strlen "one two three")             ==> 13

For obscure technical reasons you should use the C_return macro instead of the normal return statement to return a result from the foreign lambda body as some cleanup code has to be run before execution commences in the calling code.

foreign-safe-lambda

[syntax] (foreign-safe-lambda RETURNTYPE NAME ARGTYPE ...)

This is similar to foreign-lambda, but also allows the called function to call Scheme functions. See Callbacks.

foreign-safe-lambda*

[syntax] (foreign-safe-lambda* RETURNTYPE ((ARGTYPE VARIABLE)...) STRING ...)

This is similar to foreign-lambda*, but also allows the called function to call Scheme functions and allocate Scheme data-objects. See Callbacks.

foreign-primitive

[syntax] (foreign-primitive [RETURNTYPE] ((ARGTYPE VARIABLE) ...) STRING ...)

This is also similar to foreign-lambda* but the code will be executed in a primitive CPS context, which means it will not actually return, but call its continuation on exit. This means that code inside this form may allocate Scheme data on the C stack (the nursery) with C_alloc (see below). You can return multiple values inside the body of the foreign-primitive form by using the following C code:

C_word av[N + 2] = { C_SCHEME_UNDEFINED, C_k, X1, ... };
C_values(N + 2, av);

where N is the number of values to be returned, and X1, ... are the results, which should be Scheme data objects. When returning multiple values, the return-type should be omitted. Of course, if you have to dynamically compute the values, you do not have to use C's array initialization syntax, but you can just assign them one by one.

Returning just a single value can still be done via the C_return(...) macro.

Callbacks

To enable an external C function to call back to Scheme, the form foreign-safe-lambda (or foreign-safe-lambda*) has to be used. This generates special code to save and restore important state information during execution of C code. There are two ways of calling Scheme procedures from C: the first is to invoke the runtime function C_callback with the closure to be called and the number of arguments. The second is to define an externally visible wrapper function around a Scheme procedure with the define-external form.

Note: the names of all functions, variables and macros exported by the CHICKEN runtime system start with C_. It is advisable to use a different naming scheme for your own code to avoid name clashes. Callbacks (defined by define-external) do not capture the lexical environment.

Non-local exits leaving the scope of the invocation of a callback from Scheme into C will not remove the C call-frame from the stack (and will result in a memory leak). Note: The same applies to SRFI-18 threading, which is implemented with call/cc; additionally, if you enter one callback, switch threads and then exit a different callback, your program is likely to crash.

define-external

[syntax] (define-external [QUALIFIERS] (NAME (ARGUMENTTYPE1 VARIABLE1) ...) RETURNTYPE BODY ...)
[syntax] (define-external NAME TYPE [INIT])

The first form defines an externally callable Scheme procedure. NAME should be a symbol, which, when converted to a string, represents a legal C identifier. ARGUMENTTYPE1 ... and RETURNTYPE are foreign type specifiers for the argument variables VAR1 ... and the result, respectively. QUALIFIERS is an optional qualifier for the foreign procedure definition, like __stdcall.

(define-external (foo (c-string x)) int (string-length x))

The second form of define-external can be used to define variables that are accessible from foreign code. It declares a global variable named by the symbol NAME that has the type TYPE. INIT can be an arbitrary expression that is used to initialize the variable. NAME is accessible from Scheme just like any other foreign variable defined by define-foreign-variable.

(define-external foo int 42)
((foreign-lambda* int ()
  "C_return(foo);"))           ==> 42

Note: don't be tempted to assign strings or bytevectors to external variables. Garbage collection moves those objects around, so it is a very bad idea to assign pointers to heap-data. If you have to do so, then copy the data object into statically allocated memory (for example by using object-evict).

Results of type scheme-object returned by define-external are always allocated in the secondary heap, that is, not in the stack.

C_callback

[C function] C_word C_callback (C_word closure, int argc)

This function can be used to invoke the Scheme procedure closure. argc should contain the number of arguments that are passed to the procedure on the temporary stack. Values are put onto the temporary stack with the C_save macro.

C_callback_adjust_stack

[C function] void C_callback_adjust_stack (C_word *ptr, int size)

The runtime-system uses the stack as a special allocation area and internally holds pointers to estimated limits to distinguish between Scheme data objects inside the stack from objects outside of it. If you invoke callbacks at wildly differing stack-levels, these limits may shift from invocation to invocation. Callbacks defined with define-external will perform appropriate adjustments automatically, but if you invoke C_callback manually, you should perform a C_callback_adjust_stack to make sure the internal limits are set properly. ptr should point to some data object on the stack and size is the number of words contained in the data object (or some estimate). The call will make sure the limits are adjusted so that the value pointed to by ptr is located in the stack.

Locations

It is also possible to define variables containing unboxed C data, so called locations. It should be noted that locations may only contain simple data, that is: everything that fits into a machine word, and double-precision floating point values.

define-location

[syntax] (define-location NAME TYPE [INIT])

Identical to (define-external NAME TYPE [INIT]), but the variable is not accessible from outside of the current compilation unit (it is declared static).

let-location

[syntax] (let-location ((NAME TYPE [INIT]) ...) BODY ...)

Defines a lexically bound location.

location

[syntax] (location NAME)
[syntax] (location X)
[read] #$

This form returns a pointer object that contains the address of the variable NAME. If the argument to location is not a location defined by define-location, define-external or let-location, then

(location X)

is essentially equivalent to

(make-locative X)

(See the manual section on locatives for more information about locatives)

Note that (location X) may be abbreviated as #$X.

(define-external foo int)
((foreign-lambda* void (((c-pointer int) ip)) "*ip = 123;") 
  (location foo))
foo                                                                    ==> 123

This facility is especially useful in situations, where a C function returns more than one result value:

#>
#include <math.h>
<#

(define modf
  (foreign-lambda double "modf" double (c-pointer double)) )

(let-location ([i double])
  (let ([f (modf 1.99 (location i))])
    (print "i=" i ", f=" f) ) )

See location and c-string* for a tip on returning a c-string* type.

location returns a value of type c-pointer, when given the name of a callback-procedure defined with define-external.


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