Question

我想使用printf在Chez Scheme中为C的foreign-procedure函数编写一个FFI。但由于printf函数中的最后一个参数是可变参量，因此我无法弄清楚应作为签名的内容。这是我的代码：

(import (chezscheme))

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

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

我也尝试这样做，但无济于事：

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

这也不起作用：

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

如何在foreign-procedure的函数签名中指定可变参数？

Answer 1

尽管这不是最终的解决方案，您可以使用宏来容纳可变数字系统调用的参数设置。

create-list用于为foreign-procedure提供适当数量的参数系统调用。

例如，宏调用

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

扩展为

((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")

Chez方案中带有可变参数的C函数的FFI

1 个答案: