On Tue, 7 Jan 2025 at 14:39, Frank Mehnert <frank.mehn...@kernkonzept.com> wrote: > > Hi Kjetil, > > On Dienstag, 7. Januar 2025 14:00:16 MEZ Kjetil Oftedal wrote: > > How can the compiler in this case assume that endptr and str do not > > point to the same array object? > > > > ISO C99 6.5,8 Relational operators > > 5) > > ... > > "If the expression P points to an element of an array object and the > > expression Q points to the last element of the same array object, > > the pointer expression Q+1 compares greater than P. In all other > > cases, the behavior is undefined" > > > > How does the compiler at compile time know that endptr which is > > equivalent is to str[maxlen] is not within the > > range of string declared by str? > > Or in the terms of the above standard, how does it know that endptr is not > > Q+1? > > > > [...] > > The compiler does not need to know! > > Let's consider the pointer 'x', the offset 'offs' and > > y = x + offset > > with y < x (because offset is huge). > > So if y < (x + offset) then y points to a different object by definition. > Hence, the compiler is allowed to remove this test. > > Frank > -- > Dr.-Ing. Frank Mehnert, frank.mehn...@kernkonzept.com, +49-351-41 883 224 > > Kernkonzept GmbH. Sitz: Dresden. Amtsgericht Dresden, HRB 31129. > Geschäftsführer: Dr.-Ing. Michael Hohmuth > >
Correspondingly y = x + offset with y < x (Offset is small E.g 1). So if y < (x + offset) and offset is 1, then the Q+1 rule holds, and the compiler is not allowed to remove this test as it is well defined. This cannot be assumed at compile time, thus the test needs to be done to respect the requirements of the C standards. Best regards, Kjetil Oftedal _______________________________________________ devel mailing list -- devel@uclibc-ng.org To unsubscribe send an email to devel-le...@uclibc-ng.org
