Messages in this thread Patch in this message | | | Subject | Re: [PATCH 1/2] lib: test_scanf: Fix incorrect use of type_min() with unsigned types | From | Rasmus Villemoes <> | Date | Tue, 25 May 2021 11:55:34 +0200 |
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On 24/05/2021 17.59, Richard Fitzgerald wrote: > sparse was producing warnings of the form: > > sparse: cast truncates bits from constant value (ffff0001 becomes 1) > > The problem was that value_representable_in_type() compared unsigned types > against type_min(). But type_min() is only valid for signed types because > it is calculating the value -type_max() - 1.
... and casts that to (T), so it does produce 0 as it should. E.g. for T==unsigned char, we get
#define type_min(T) ((T)((T)-type_max(T)-(T)1)) (T)((T)-255 - (T)1) (T)(-256)
which is 0 of type unsigned char.
The minimum value of an > unsigned is obviously 0, so only type_max() need be tested.
That part is true.
But type_min and type_max have been carefully created to produce values of the appropriate type that actually represent the minimum/maximum representable in that type, without invoking UB. If this program doesn't produce the expected results for you, I'd be very interested in knowing your compiler version:
#include <stdio.h>
#define is_signed_type(type) (((type)(-1)) < (type)1) #define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type))) #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T))) #define type_min(T) ((T)((T)-type_max(T)-(T)1))
int main(int argc, char *argv[]) { #define p(T, PT, fmt) do { \ PT vmin = type_min(T); \ PT vmax = type_max(T); \ printf("min(%s) = "fmt", max(%s) = "fmt"\n",#T, vmin, #T, vmax); \ } while (0)
p(_Bool, int, "%d"); p(unsigned char, int, "%d"); p(signed char, int, "%d"); p(unsigned int, unsigned int, "%u"); p(unsigned long long, unsigned long long, "%llu"); p(signed long long, signed long long, "%lld"); return 0; }
> lib/test_scanf.c | 13 ++++++------- > 1 file changed, 6 insertions(+), 7 deletions(-) > > diff --git a/lib/test_scanf.c b/lib/test_scanf.c > index 8d577aec6c28..48ff5747a4da 100644 > --- a/lib/test_scanf.c > +++ b/lib/test_scanf.c > @@ -187,8 +187,8 @@ static const unsigned long long numbers[] __initconst = { > #define value_representable_in_type(T, val) \ > (is_signed_type(T) \ > ? ((long long)(val) >= type_min(T)) && ((long long)(val) <= type_max(T)) \ > - : ((unsigned long long)(val) >= type_min(T)) && \ > - ((unsigned long long)(val) <= type_max(T))) > + : ((unsigned long long)(val) <= type_max(T)))
With or without this, these tests are tautological when T is "long long" or "unsigned long long". I don't know if that is intended. But it won't, say, exclude ~0ULL if that is in the numbers[] array from being treated as fitting in a "long long".
Rasmus
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