keyd...@gmx.de wrote: > [...] > Well in this case in fact, I was happy with the result, the 1 looked fine > given we're having daylight saving time :-; > [...]
Hello Sigrid, sure the one is fine, but I think this field really never contains anything else than 1 when DST is in effect or 0 when it's not, so it has a boolean meaning. > [...] > If tp is NULL and tzp is non-NULL, gettimeofday() will populate the timezone > struct in tzp. If tp is non-NULL and tzp is NULL, then only the timeval > struct in tp is populated. If both tp and tzp are NULL, nothing is returned. > > I was wondering whether this really meant the caller had to specify a > different kind of pointer (null or non null) in a variable designed for > OUTPUT/return... this sounded illogical somehow, but in any case, using > Racket (_ptr o _timeval) seems to have worked - meaning that Racket does NOT > provide a null pointer here I guess? > [...] Correct, the caller has to provide a storage location for the output she's interested in and may pass NULL to indicate that no storage has been allocated because she's not interested in a return value. This is a common convention for C functions. Of course there can also be functions that always expect an output parameter to point to a valid storage location and that always write to the address indicated. In C++ output parameters are often declared using reference types, which are a special kind of pointer types that is always implicitly dereferenced when used in expressions and cannot be set to NULL explicitly (at least not if the expression used to do so is simple enough for the compiler to realize what you're up to ;-) If you declare a (_ptr o _something) type, racket always allocates space for a something, passes the pointer to the foreign function and then dereferences it to extract the _something after the foreign function returns. Racket should indeed never pass a NULL pointer here. Ciao, Thomas -- When C++ is your hammer, every problem looks like your thumb. _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
