1. 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
  2. Returning large objects or chunks of memory to Scheme

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.

Returning large objects or chunks of memory to Scheme

When you call a C function which needs to return quantities of data, several issues arise:

So some would advise you to just return a pointer to Scheme, use memcpy or any other function(s) which you need to get the data into CHICKEN-managed memory and into the desired kind of data structure, then free the C data. For this example, we are trying to return an array of doubles into an f64vector; we can accomplish that by adding a specialized copy function to the C library being integrated:

void CopyResults(double* vector) {
    memcpy(vector, bezierBuffer, totalOutputPoints * sizeof(double));
}

// The original C function which takes an array of doubles, 
// does some sort of transmogrification,
// retains a new malloc'd array of the results
// and returns the count
int GenerateResults(double* vector, int count) {
    ... 
}

and the "egg" which calls the C functions can be implemented like this:

(module memcpy-demo (input->output)
    (import chicken scheme foreign)
    (use srfi-4)

    (define CopyResults (foreign-lambda void "CopyResults" f64vector))

    (define GenerateResults (foreign-lambda integer "GenerateResults" f64vector integer))

    (define (input->output input)
        (let* ([size (GenerateResults input (f64vector-length input))] 
               [vect (make-f64vector size)])
            (printf "returned size ~a~%" size)
            (CopyResults vect)
            vect)))

The foreign-lambda takes care of the details in this case so that an f64vector allocated in the nursery can be treated as a plain old array of doubles in C (assuming your C compiler uses 64-bit values for double).

Various eggs provide other examples, and some of them do it more efficiently too, but this method is relatively clean and compact.


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