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List:       racket-users
Subject:    [racket] Foray into ffi/unsafe, nudge needed
From:       clements () brinckerhoff ! org (John Clements)
Date:       2013-08-30 18:23:09
Message-ID: 082CB432-98DF-4AC1-8014-DD27A794FBA5 () brinckerhoff ! org
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On Aug 30, 2013, at 9:50 AM, Peter Schmiedeskamp wrote:

> 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

You may want to take a look at my portaudio package; it uses callbacks in (what I \
believe to be) the way you describe.

http://planet.racket-lang.org/display.ss?package=portaudio.plt&owner=clements

or just

https://github.com/jbclements/rack-portaudio


John


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