Re: size of long long int on amd 64

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

John Love-Jensen <eljay@adobe.com> writes on Mon, 28 Nov 2005 05:42:54 -0700 (MST):

>> ...
>> I'd have thought that on a 64-bit word-size architecture, int would be
>> 64-bits big, long would be 128-bits big, and long long would be
>> 256-bits big.
>> ...

No, the only requirement on C's integer data types is that

	sizeof(char) <= sizeof(short) <= sizeof(int) <= sizeof(long) <= sizeof(long long)

subject to minimum size requirements on each of them that are
specified in the ISO C Standard.  It is perfectly legal for all of
these to have the same size, as long as they are big enough: on a
64-bit architecture, they all could be 64 bits long.  See section 5.1
of this book, which deserves to be on every serious C programmer's
shelf:

@String{pub-PH                  = "Pren{\-}tice-Hall"}
@String{pub-PH:adr              = "Upper Saddle River, NJ 07458, USA"}

@Book{Harbison:2002:CRM,
  author =       "Samuel P. {Harbison III} and Guy L. {Steele Jr.}",
  title =        "{C} --- {A} Reference Manual",
  publisher =    pub-PH,
  address =      pub-PH:adr,
  edition =      "Fifth",
  pages =        "xviii + 533",
  year =         "2002",
  ISBN =         "0-13-089592-X",
  ISBN-13 =      "978-0-13-089592-9",
  LCCN =         "QA76.73.C15 H38 2002",
  bibdate =      "Sat Mar 30 08:29:26 2002",
  price =        "US\$45.00",
  URL =          "http://www.CAReferenceManual.com/;
                 http://www.phptr.com/ptrbooks/ptr_013089592X.html";,
  acknowledgement = ack-nhfb,
}

It is common on many Unix systems for int and long to have the same
size, and on IBM PC DOS, some compilers made short and int the same
size.

Here is what the 1999 ISO C Standard has to say (notice the
requirement phrase: ``Their ... values shall be equal or greater in
magnitude...'') [I've replaced superscripts in inline comments by
Fortran's power operator **]:

>> ...
>>      5.2.4.2.1	Sizes of integer types <limits.h>
>>
>> 1    The values given below shall be replaced by constant expressions
>>      suitable for use in #if preprocessing directives. Moreover,
>>      except for CHAR_BIT and MB_LEN_MAX, the following shall be
>>      replaced by expressions that have the same type as would an
>>      expression that is an object of the corresponding type converted
>>      according to the integer promotions.  Their
>>      implementation-defined values shall be equal or greater in
>>      magnitude (absolute value) to those shown, with the same sign.
>>
>>      -- number of bits for smallest object that is not a bit-field (byte)
>> 	CHAR_BIT 8
>>
>>      -- minimum value for an object of type signed char
>> 	SCHAR_MIN -127 // -(2**{7} - 1)
>>
>>      -- maximum value for an object of type signed char
>> 	SCHAR_MAX +127 // 2**7 - 1
>>
>>      -- maximum value for an object of type unsigned char
>> 	UCHAR_MAX 255 // 2**8 - 1
>>
>>      -- minimum value for an object of type char
>> 	CHAR_MIN				     see below
>>
>>      -- maximum value for an object of type char
>> 	CHAR_MAX				     see below
>>
>>      -- maximum number of bytes in a multibyte character, for any supported locale
>> 	MB_LEN_MAX 1
>>
>>      -- minimum value for an object of type short int
>> 	SHRT_MIN -32767 // -(2**15 - 1)
>>
>>      -- maximum value for an object of type short int
>> 	SHRT_MAX +32767 // 2**15 - 1
>>
>>      -- maximum value for an object of type unsigned short int
>> 	USHRT_MAX 65535 // 2**16 - 1
>>
>>      -- minimum value for an object of type int
>> 	INT_MIN -32767 // -(2**15 - 1)
>>
>>      -- maximum value for an object of type int
>> 	INT_MAX +32767 // 2**15 - 1
>>
>>      -- maximum value for an object of type unsigned int
>> 	UINT_MAX 65535 // 2**16 - 1
>>
>>      -- minimum value for an object of type long int
>> 	LONG_MIN -2147483647 // -(2**31 - 1)
>>
>>      -- maximum value for an object of type long int
>> 	LONG_MAX +2147483647 // 2**31 - 1
>>
>>      -- maximum value for an object of type unsigned long int
>> 	ULONG_MAX 4294967295 // 2**32 - 1
>>
>>      -- minimum value for an object of type long long int
>> 	LLONG_MIN -9223372036854775807 // -(2**63 - 1)
>>
>>      -- maximum value for an object of type long long int
>> 	 LLONG_MAX +9223372036854775807 // 2**63 - 1
>>
>>      -- maximum value for an object of type unsigned long long int
>> 	 ULLONG_MAX 18446744073709551615 // 2**64 - 1
>>
>> 2    If the value of an object of type char is treated as a signed
>>      integer when used in an expression, the value of CHAR_MIN shall
>>      be the same as that of SCHAR_MIN and the value of CHAR_MAX shall
>>      be the same as that of SCHAR_MAX. Otherwise, the value of
>>      CHAR_MIN shall be 0 and the value of CHAR_MAX shall be the same
>>      as that of UCHAR_MAX.15) The value UCHAR_MAX shall equal
>>      2CHAR_BIT - 1.
>> ...

There are similar constraints on float, double, and long double: they
can all be of identical size, provided that they satisfy minimum range
and precision specified in the ISO C Standard.  I have used systems
where this was the case, and also, where a char was nine bits (four
then fitting in a 36-bit word).

C's constraints on the sizes of floating-point types are different
from Fortran, where the size of DOUBLE PRECISION is required to be
twice that of REAL: that is essential knowledge for proper storage
alignment in EQUIVALENCE statements and COMMON blocks.

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