On Thu, Jul 14, 2016 at 09:28:34PM +0200, Tobias Stoeckmann wrote:
> Casting the value to be checked to size_t renders the check useless.
> If st_size is SIZE_MAX+1, it will be truncated to 0 and the check
> succeeds. In fact, this check can never be false because every value
> stored in a size_t is smaller or equal to SIZE_MAX.
>
> I think this adjustment was meant to fix a compiler warning for 64 bit
yes
> systems for which sizeof(off_t) is sizeof(size_t), but the signedness
> differs. One possibility is to do a "binary and" operation with the
> value SIZE_MAX. If the original value and the and-operated value differ,
> it means that a higher bit was set and therefore the file was too large.
>
> Signed-off-by: Tobias Stoeckmann <tobias@xxxxxxxxxxxxxx>
> ---
> text-utils/tailf.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/text-utils/tailf.c b/text-utils/tailf.c
> index 6219aa2..5ddbee9 100644
> --- a/text-utils/tailf.c
> +++ b/text-utils/tailf.c
> @@ -284,7 +284,7 @@ int main(int argc, char **argv)
> errx(EXIT_FAILURE, _("%s: is not a file"), filename);
>
> /* mmap is based on size_t */
> - if (st.st_size && (size_t) st.st_size <= SIZE_MAX)
> + if (st.st_size && st.st_size == (st.st_size & SIZE_MAX))
but compiler still don't like it, because "& SIZE_MAX" result is
unsigned number.
text-utils/tailf.c: In function ‘main’:
text-utils/tailf.c:287:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
I guess you need to cast:
if (st.st_size > 0 && st.st_size == (off_t) (st.st_size & SIZE_MAX))
Maybe it would be nice to have a macros for this purpose in include/c.h,
something like:
#define is_between(x, mi, ma) (x >= mi && x == (__typeof__(x)) (x & ma))
#define is_sizet_convertible(x) is_between(x, 0, SIZE_MAX)
#define is_uint32_convertible(x) is_between(x, 0, UINT32_MAX)
...
if (st.st_size && is_sizet_convertible(st.st_size))
tailf(filename, lines, &st);
Correct? or maybe I need a coffee? ;-)
Karel
--
Karel Zak <kzak@xxxxxxxxxx>
http://karelzak.blogspot.com
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