> From: cra...@gmx.net
> Date: Sat, 01 Aug 2015 20:22:48 +0200
>
> [Description of cairo_surface_write_to_png_stream...]
>
> I'd like to make this accessible from Scheme, preferably by just calling
> it in C, and returning all the data it writes as a vector8 (so a
> string).
>
> I've not been successful in declaring the callback,
(callback cairo_status_t
write_to_png
(ID (* void))
(data (* (const uchar)))
(length uint))
> and if I just return a char* from a wrapper function, it will be
> truncated at the first #\nul.
You used c-peek-cstring? Try c-peek-bytes.
> How can I either implement the callback, so that I get at the data and
> length in a Scheme function, or so that I can read all the incoming data
> in C, and then pass it all back to Scheme correctly?
To do it in Scheme, you would need to do something like this:
(define (cairo-surface-write-to-png-string surface)
(let ((accumulator '())
(callback-id))
(dynamic-wind
(lambda ()
(set! callback-id
(C-callback
(named-lambda (write-to-png data len)
(let ((str (make-string len)))
(c-peek-bytes data 0 len str 0)
(set! accumulator (cons str accumulator))
(C-enum "CAIRO_STATUS_SUCCESS"))))))
(lambda ()
(let ((status
(C-call "cairo_surface_write_to_png_stream"
surface (C-callback "write_to_png") callback-id)))
(if (not (eq? status (C-enum "CAIRO_STATUS_SUCCESS")))
(error "Cairo write-to-png unsuccessful:" status))))
(lambda ()
(de-register-c-callback callback-id)))
(apply string-append accumulator)))
So you see why I used cairo_surface_write_to_png and a temp. file to
put up my tellurion.
http://birchwood-abbey.net/~matt/Scheme/#Tellurion
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