I always feel better when I convert void * to char * but that's probably because C++ doesn't allow pointer arithmetic on void *'s. The argument being that you don't know the size of what's being pointed to with a void * and therefore can't know how far to seek the the pointer to get to the next valid address. I think C takes a more low-level approach and says "void * is just an address void * + 1 means the next valid address". Anyway... it just seems to help when porting code between C/C++ to use char *... my $0.02. Dave On Thursday, July 10, 2003, at 11:40AM, Luigi Rizzo <rizzo_at_icir.org> wrote: >On Thu, Jul 10, 2003 at 03:42:04AM -0700, Terry Lambert wrote: >> Luigi Rizzo wrote: >> > in several places in ipfw2.c i have to move pointers across >> > structures of variable length (lists of ipfw2 instructions >> > returned by the getsockopt()), and i use the following type of code: >> > >> > void *next; >> > foo *p; >> > next = (void *)p + len; >> > foo = (foo *)p + len; > ^^^^^^^^^^^^^^ > >sorry i meant p = (void *)p + len; > >... >> I don't understand the second one. The first one blows up because >> you aren't parenthesizing, e.g.: >> >> next = (void *)(p + len); >> >> The compiler is complaining because it doesn't know sizeof(*((void *)0)) > >ok, it actually evaluates to 1 and i thought it was some standard, probably >it is not so i guess i have to cast to (char *) instead > > thanks > luigi > >> (pointer arithmatic is coerced to the type of the lvalue, in most >> cases of casts). >> >> Unless you are referencing them as array elements (in which case, >> packing becomes a problem for you, when referencing them as arrays >> of foo's, since you don't know how foo's are packed in an array), >> you should probably cast them to char for the arithmatic, and add >> them with byte counts. >> >> -- Terry >_______________________________________________ >freebsd-current_at_freebsd.org mailing list >http://lists.freebsd.org/mailman/listinfo/freebsd-current >To unsubscribe, send any mail to "freebsd-current-unsubscribe_at_freebsd.org" > >Received on Thu Jul 10 2003 - 08:38:44 UTC
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