[racket] Foray into ffi/unsafe, nudge needed

Dear list,

I thought it would be an interesting exercise to learn something about
Racket and FFI by wrapping libgeos (http://trac.osgeo.org/geos/) similar to
what the R people have done (
http://cran.r-project.org/web/packages/rgeos/index.html).

Right out of the gate, I seem to be stuck, and am hoping for a nudge
forward. The C library requires initialization before all else, and in turn
that initialization function takes two pointers to two callback logging
functions (from geos_c.h):


/* what a message handler looks like */
typedef void (*GEOSMessageHandler)(const char *fmt, ...);

/* what the call to initGEOS() looks like */
extern void GEOS_DLL initGEOS(GEOSMessageHandler notice_function,
GEOSMessageHandler error_function);


I found examples in Python and in C (included in the R version) where they
had created these functions and plumbed them into the logging
infrastructure:

# Python
NOTICEFUNC = CFUNCTYPE(None, c_char_p, c_char_p)
def notice_h(fmt, lst):
    fmt, lst = fmt.decode(), lst.decode()
    try:
        warn_msg = fmt % lst
    except:
        warn_msg = fmt
    logger.warning('GEOS_NOTICE: %s\n' % warn_msg)
notice_h = NOTICEFUNC(notice_h)


/* C - RGeos */
static void __warningHandler(const char *fmt, ...) {

    char buf[BUFSIZ], *p;
    va_list(ap);
    va_start(ap, fmt);
    vsprintf(buf, fmt, ap);
    va_end(ap);
    p = buf + strlen(buf) - 1;
    if(strlen(buf) > 0 && *p == '\n') *p = '\0';

    warning(buf);

    return;
}



I've found _cprocedure, which says it can be used for callbacks to racket
functions. After reading through that section in the docs a couple of times
I'm still a bit murky on what that would look like. It would seem I need to
create a c-callable function that:
1. receives one or more 'char'
2. converts the char to a racket string
3. pass the string to 'log-FOO' depending on the severity

Thank you for any tips/pointers/opinions/examples you can provide. I'm
several dimensions of novice here, and hope someone can help point me in
the right direction.

Cheers,
Peter
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