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Calling_C_Functions

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Ken Haase edited this page Jun 26, 2019 · 1 revision

Kno has a simple foreign function interface to access C functions from either Kno itself or external libraries. The current implementation only handles scalar types, including integers (various sizes), floating point numbers (ditto), strings, KNO's native lisp pointers, and opaque pointer values.

The procedure FFI/PROC creates an FFI procedure and has the form:

`(ffi/proc *external_name* *library file (or #f)* *return_type* *arg1type* *arg2type* ...)`

For example, we can do:

(define strlen (ffi/proc "strlen" #f 'size_t 'string))
#|kno>|# (strlen "foo")
3
(define unixtime (ffi/proc "time" #f time_t #[basetype ptr nullable #t]))
#|kno>|# (strlen "foo")
#T2019-06-12T15:41:56Z
(define unix-getenv (ffi/proc "getenv" #f 'strcpy 'string))
#|kno>|# (unix-getenv "USER")
"haase"

KNO's foreign function interface does not provide destructuring of structured objects or pointers to structured objects, but it does functions can return raw pointer values which are wrapped by KNO and can be passed to other foreign functions.

Type codes

The base type codes are symbols, including:

  • void is a 'non value'; this only makes sense for return types
  • float is a low precision floating point number
  • double is a higher precision float point number
  • uint is an unsigned integer
  • int is an integer
  • ushort is an unsigned short integer
  • short is a short integer
  • ulong is an unsigned long integer
  • long is a long integer
  • uchar is an unsigned byte
  • byte is an unsigned byte
  • ssize_t is an integer ssize_t value
  • char is a signed byte
  • string is a utf8-string
  • packet is a null terminated set of bytes
  • ptr is some kind of pointer value
  • cons is a raw KNO cons pointer
  • lisp is a KNO lisp pointer, which has already been incref'd
  • lispref is a KNO lisp pointer, which needs to be incref'd
  • time_t is a unix time value
  • null is any object but is always passed as a NULL pointer value

Tagged results

KNO's FFI interface allows the annotation of values, including pointer values, with **typetag*s. This can be used to have the foreign function interface do type checking on arguments. For example,a simple wrapper of the UNIX filename access could be:

(define fopen
  (ffi/proc "fopen" #f #[basetype ptr typetag file] 'string 'string))
(define fclose
  (ffi/proc "fclose" #f 'int #[basetype ptr typetag file]))
(define fgetc
  (ffi/proc "fgetc" #f 'int #[basetype ptr typetag file]))
(define fputc
  (ffi/proc "fgetc" #f 'int 'int #[basetype ptr typetag file]))

In this example, fopen returns a raw pointer tagged with file and the other functions will automatically check for this tagged value when they are invoked.

Light wrappers

For more complicated libraries, one approach for connecting with them is to define a lightweight C wrapper which provides the basic API in terms of the core FFI types and then linking to that wrapper using the FFI interface.

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