karl@haddock.ima.isc.com (Karl Heuer) (02/23/89)
In article <830@atanasoff.cs.iastate.edu> hascall@atanasoff.cs.iastate.edu (John Hascall) writes: >In article <8943@alice.UUCP> ark@alice.UUCP (Andrew Koenig) writes: >>For that reason it's hard to see how a C implementation could possibly >>do anything but put [an array] in contiguous memory. > >How about: Assume int's are (say) 2 bytes. Assume further that ... all >accesses must be on an 8-byte boundary. Then sizeof(int) is 8, and the elements of the array consists of contiguous 8-byte units, of which only two bytes are significant. This sounds much like a Cray-2, in fact. Question for comp.std.c (to which I've redirected followups): I've been told that the Cray-2 has sizeof(int) == 8, yet INT_MAX == 0x7FFFFFFF (i.e. the arithmetic is only accurate to 4 bytes when using int). Is this legal in a conforming implementation? I think I can prove that UINT_MAX must be 2*^64-1, but I'm less sure about INT_MAX. Section 3.1.2.5 has a restriction to binary architectures, which by the definition in the footnote seems to require every bit except the highest to represent a power of two; should this be interpreted as a requirement that 2*^63-1 must be representable in an 8-byte int? Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint (I've implicitly assumed 8-bit bytes above, simply because it would be too cumbersome to type the more correct expressions involving CHAR_BIT.)
gwyn@smoke.BRL.MIL (Doug Gwyn ) (02/23/89)
In article <11838@haddock.ima.isc.com> karl@haddock.ima.isc.com (Karl Heuer) writes: >Question for comp.std.c (to which I've redirected followups): I've been told >that the Cray-2 has sizeof(int) == 8, yet INT_MAX == 0x7FFFFFFF (i.e. the >arithmetic is only accurate to 4 bytes when using int). Is this legal in a >conforming implementation? I think I can prove that UINT_MAX must be 2*^64-1, >but I'm less sure about INT_MAX. Section 3.1.2.5 has a restriction to binary >architectures, which by the definition in the footnote seems to require every >bit except the highest to represent a power of two; should this be interpreted >as a requirement that 2*^63-1 must be representable in an 8-byte int? I think an implementation such as you describe is legal. What is required is that integers be represented in a binary numeration system, that a nonnegative signed integer of a given size have the same representation as the corresponding unsigned integer with the same value, that all integers in the ranges given by INT_MAX etc. be representable, and that unsigned arithmetic be performed modulo UINT_MAX+1 (similarly for unsigned long, etc.). I don't see any way these requirements can be combined to "prove" that every bit pattern contained in the "sizeof" soace has to be interpretable as a valid integral value. In fact I'm pretty sure we didn't want to require that, since (as you note) some architectures really don't support it.