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;
# >
# > When using WARNS=5, the compiler in -current flags them with 'Warning
# > void * arithmetic'.
# >
# > What is the best way to do the above given that i do need to use
# > these variable-size structures ?
#
# I don't understand the second one. The first one blows up because
# you aren't parenthesizing, e.g.:
#
# next = (void *)(p + len);
Huh? next has type pointer-to-void so a cast is never necessary
to assign to it. This is the raison d'�tre for void pointers in C.
In standardese void is an incomplete type that can not be completed.
Incomplete types have no size. Therefore, any arithmetic on
pointer-to-void is undefined. One needs to cast to a non-void pointer
before doing arithmetic. Whether this is a char* or other depends on
what you want to do.
Some compilers by default assume sizeof(void) = 1 (e.g. the same as
char in all its qualifications) for simplicity, but this is far from
portable.
Regards,
Jens
--
Jens Schweikhardt http://www.schweikhardt.net/
SIGSIG -- signature too long (core dumped)
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