Vincent Torri <Vincent.Torri@xxxxxxxxxxxxxxx> 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",
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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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