Re: size of long long int on amd 64

["Nelson H. F. Beebe" <beebe at math dot utah dot edu>]

Vincent Torri <Vincent.Torri@iecn.u-nancy.fr> asks on Mon, 28 Nov 2005
14:24:11 +0100 (CET) about my explanation of the ISO C Standard's
minimum requirements on integer type sizes:

>> Is there some differences about the size of intrinsic types in C++,
>> compared to C ?

The 1998 ISO C++ Standard says that its integer types are constrained
by limits in <climit>, and that "The contents are the same as the
Standard C library header <limits.h>."  The 2003 ISO C++ Standard has
identical wording.  I still have many C++ compilers that haven't
reached the 1998 level, and none that claim 2003 conformance.

Because the size constraints are minimum requirements, it would be
legal for a C++ compiler to make the sizes of its integer types
different from those in C compilers on the same system, but doing so
would destroy language interoperability, which is far too important:
no serious C++ compiler would ever do that.

Recall that most, and very possibly, all, C++ compilers still use a
large portion of the underlying C libraries (-lc and -lm) at link
time.

Since I mentioned Fortran briefly in my earlier posting, it is worth
remarking that Java, which shares a good deal of syntax, and heritage,
with C, is based on an underlying precisely-specified virtual
machine. It consequently has exact host-platform-independent range
requirements on its byte, short, int, and long data types (8, 16, 32,
and 64 bits, respectively), and requires that char be an unsigned
16-bit type.  See section 4.2 of

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  LCCN =         "QA76.73.J38 G68 2000",
  bibdate =      "Tue Feb 20 18:39:03 MST 2001",
  series =       "Java series",
  URL =          "http://java.sun.com/people/jag/";,
  acknowledgement = ack-nhfb,
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}

Because of the platform-dependence of the sizes of the C integer data
types, the 1999 ISO C Standard introduced new types that provide
guarantees on minimum sizes:

>> ...
>>      7.18.1.1 Exact-width integer types
>>
>> 1    The typedef name intN_t designates a signed integer type with width
>>      N , no padding bits, and a two's complement representation. Thus,
>>      int8_t denotes a signed integer type with a width of exactly 8
>>      bits.
>>
>> 2    The typedef name uintN_t designates an unsigned integer type with
>>      width N . Thus, uint24_t denotes an unsigned integer type with a
>>      width of exactly 24 bits.
>>
>> 3    These types are optional. However, if an implementation provides
>>      integer types with widths of 8, 16, 32, or 64 bits, it shall
>>      define the corresponding typedef names.
>>
>>      7.18.1.2 Minimum-width integer types
>>
>> 1    The typedef name int_leastN_t designates a signed integer type with
>>      a width of at least N , such that no signed integer type with
>>      lesser size has at least the specified width.  Thus,
>>      int_least32_t denotes a signed integer type with a width of at
>>      least 32 bits.
>>
>> 2    The typedef name uint_leastN_t designates an unsigned integer type
>>      with a width of at least N , such that no unsigned integer type
>>      with lesser size has at least the specified width. Thus,
>>      uint_least16_t denotes an unsigned integer type with a width of
>>      at least 16 bits.
>>
>> 3    The following types are required:
>>
>> 	      int_least8_t			uint_least8_t
>> 	      int_least16_t			uint_least16_t
>> 	      int_least32_t			uint_least32_t
>> 	      int_least64_t			uint_least64_t
>>
>>      All other types of this form are optional.
>> ...

C99 also provides the intmax_t and uintmax_t types, corresponding to
the largest size.

Since there are still plenty of C compilers that conform only to the
earlier ISO 1989 C Standard, it is not yet portable to use these new
datatypes, at least not without a fallback that supplies suitable type
definitions when they are not provided by the compiler.

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- Nelson H. F. Beebe                    Tel: +1 801 581 5254                  -
- University of Utah                    FAX: +1 801 581 4148                  -
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