FFI in Chez Scheme for C functions with variadic arguments (varargs)

4
votes

I want to write a FFI for printf function of C in Chez Scheme using foreign-procedure. But I cannot figure out what I should put as the signature, since the last argument in printf function is a variadic argument. Here's my code:

(import (chezscheme))

(define (print-format)
    (foreign-procedure "printf" 
        (string void*) int)) ;; <-- Here, the type format is "(arg arg ...) ret"

(print-format "Hello, %s!" "Ryan")

I have tried this as well to no avail:

(define (print-format . args)
    (foreign-procedure "printf" 
        (string args) int))

This is not working either:

(define (print-format)
    (foreign-procedure "printf" 
        (string ...) int))

How do you specify variadic arguments in function signature for foreign-procedure?

1
cprintfschemevariadicchez-scheme

1 Answers

3
votes

Although it is not the ultimate solution, you could use macros to accommodate variable number of parameters to a system call.

create-list is used to supply proper number of parameters to the foreign-procedure system call.

For example, macro call

(print-format "Hello %s and %s" "Ryan" "Greg")

is expanded as

((foreign-procedure "printf" (string string string) int) "Hello %s and %s" "Ryan" "Greg")

(define create-list
  (lambda (element n)
    "create a list by replicating element n times"
    (letrec ((helper
           (lambda (lst element n)
             (cond ((zero? n) lst)
                   (else
                    (helper
                     (cons element lst) element (- n 1)))))))
      (helper '() element n))))

(define-syntax print-format
  (lambda (x)
    (syntax-case x ()
      ((_ cmd ...)
       (with-syntax
        ((system-call-spec
          (syntax
           (create-list 'string
                        (length (syntax (cmd ...)))))))
        (with-syntax
         ((proc (syntax
                 (eval
                  `(foreign-procedure "printf"
                                      (,@system-call-spec) int)))))
         (syntax
          (proc cmd ...))))))))

(print-format "Hello %s!" "Ryan")
(print-format "Hello %s and %s" "Ryan" "Greg")