Hi Jon, thanks a lot for your "find-out-yourself" code - not only will I remember this method for application to future problems, but also did it really solve my problem!
In fact the result was _int64 for the seconds and _int32 for the microseconds, which left me wondering why my code didn't work, as I had experimented with _int32 for μsecs already... until I realized the error must be with the first field then! It turns out I'd read the documentation sloppily, and had always thought that _int was turned into either 64 or 32 bit depending on the architecture - now I see it's _long that is... And having defined the secs as an _int, with my 64 bit Racket executable, I've "left over" a portion of the secs for the μsecs (which also explains the "too long" like 9359042975779072 that I obtained when having μsecs as _long). (Just out of curiosity, I also ran my original code on a manually compiled 32-bit version of DrRacket I have, and there it indeed worked fine :-;) So, thanks again! Sigrid Am 16.07.2011 um 02:40 schrieb Jon Zeppieri: > On Fri, Jul 15, 2011 at 4:28 PM, keyd...@gmx.de <keyd...@gmx.de> wrote: > > [snip] > >>> >>> There is, however, a PLaneT package (planet dherman/c:4:0) providing the >>> facilities to parse header files and extract structure layout >>> information like that required here using the system's C compiler. >> >> >> Thanks for the pointer (pun intended :-;), I will have a look... But in fact >> I hope that other than experimenting with calls like these, that is, when >> working with a "real", user-friendly library, I will not have too great a >> need to use it :-) >> > > If you want the code to be portable, you're going to have to do > something a bit nasty, since the types in question simply don't have a > standard size. > > If you're only interested in running the code on your machine (or on > some particular set of systems), you can just support those. > > Have you tried looking at the output of > (current-inexact-milliseconds)? If your system supports millisecond > resolution (and if racket knows how to get it), milliseconds should be > given in the fractional portion of the result. > >>> >>> The zero microseconds value could mean that you were actually "lucky" >>> with the call, that your system simply doesn't support time values with >>> microsecond precision or that your system's implementation of >>> gettimeofday doesn't support that. >> >> >> Well, I think I should at least get the milliseconds, because Racket has >> them in (current-milliseconds), so in that case I'd expect to see something >> like 123000 in the microsecond field... > > (current-milliseconds) does not necessarily (and simply does not, > period, on my machine) return milliseconds since the epoch. The docs > say: "Returns the current “time” in fixnum milliseconds (possibly > negative). This time is based on a platform-specific starting date or > on the machine’s start-up time." > > You might be able to determine the right size at runtime... > > The following works by starting out with 64-bit ints and narrowing, > first the seconds field, then the microseconds. I *think* it's fairly > safe; it should always err on the side of passing in too large a chunk > of memory, so it shouldn't start stomping on random addresses. But, > you know, make sure you save whatever you have before trying it. > > > #lang racket > > (require ffi/unsafe) > > (define sizes (list _int64 _int32 _int16 _int8)) > (define num-iterations 5) > > > (define (make-gettimeofday stype utype) > (get-ffi-obj 'gettimeofday #f > (_fun [timeval : (_ptr o (_list-struct stype utype))] > (_intptr = 0) > -> _int > -> timeval))) > > (define (find-seconds-type) > (for/first ([s (in-list sizes)] > #:when (let* ([fn (make-gettimeofday s _int64)] > [timeval (fn)]) > (= (current-seconds) (first timeval)))) > s)) > > (define (find-micro-type stype) > (for/first ([s (in-list sizes)] > #:when (for/and ([i (in-range num-iterations)]) > (let* ([fn (make-gettimeofday stype s)] > [timeval (fn)] > [millis (current-inexact-milliseconds)] > [micros (remainder (* millis 1000) 1000000)]) > (and (>= (second timeval) 0) > (>= micros (second timeval)) > (< (- micros (second timeval)) 100))))) > s)) > > > (define gettimeofday > (let* ([stype (find-seconds-type)] > [utype (and stype (find-micro-type stype))]) > (cond [utype > (printf "(~a, ~a)\n" stype utype) > (make-gettimeofday stype utype)] > [else > (error "Unable to determine type for gettimeofday")]))) _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
