Hi,
I am kind of stuck trying to get the current microsecond from gettimeofday...
and I wonder how best to systematically approach the question of what racket
datatype to choose when defining a FFI function.
From the man page I see that timeval is defined as
struct timeval {
time_t tv_sec; /* seconds since Jan. 1, 1970 */
suseconds_t tv_usec; /* and microseconds */
};
but the search for suseconds_t - in case of time_t a first attempt at just
using _int in racket worked fine - left me searching through a bunch of header
files, following one typedef after the other...
In fact I'd assume that using _int for suseconds_t should work fine, too, given
that man even tells us that
"The values in timeval are opaque types whose length may vary on different
machines; depending on
them to have any given length may lead to errors."
So, I wonder if there is a different kind of error in my code:
(define libsys (ffi-lib "libsystem"))
(define-cstruct _timeval ((secs _int) (μsecs _int)))
(define-cstruct _timezone ((minswest _int) (dsttime _int)))
(define gettimeofday (get-ffi-obj "gettimeofday" libsys (_fun (tm : (_ptr o
_timeval)) (tz : (_ptr o _timezone)) -> _int -> (values (timeval-secs tm)
(timeval-μsecs tm) (timezone-minswest tz) (timezone-dsttime tz)))))
(gettimeofday)
leading to the output
1310672543
0
-60
1
with a 0 in the microsecond field...? I'd be grateful for any hints!
Also, I am confused by the coexistence of _xxx.h and xxx.h header files (like
sys/types.h and _types.h), and I wonder what is the signification of single and
double underscores in variables defined in the header files - could anyone
perhaps point me to a resource documenting these conventions?
Many thanks in advance!
Sigrid
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