C/C++ Programmer's Guide (G06.25+)

Compiler Pragmas
HP C/C++ Programmer’s Guide for NonStop Systems—429301-008
13-47
IEEE_FLOAT
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In addition to the differences shown in Table 13-4, note the following:
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For the float type in IEEE floating-point format, the smallest positive nonzero
number is approximately 1.40129846E-45. This is a denormalized number;
Tandem floating-point format does not have denormalized numbers.
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For the double type in IEEE floating-point format, the smallest positive
nonzero number is approximately 4.9406564584124654E-324. This is a
denormalized number.
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IEEE floating-point format has special values for Infinity and Not a Number
(NaN). In addition, for every value except NaN values, there is a positive value
and a negative value. This includes infinities (+Infinity and -Infinity) and zeroes
(+0 and -0).
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When converting from fixed-point to floating-point format or from a floating-point
number to a narrower floating-point number, IEEE floating point typically rounds to
the nearest value according to the current IEEE floating-point rounding mode.
When converting from floating-point to fixed-point formats, IEEE floating-point
normally truncates the nearest representable value, as specified by the ANSI C
standard.
The IEEE floating-point standard requires a way to convert from floating point to
fixed point with rounding. To make this conversion, use the rint() function and
then cast the result to an integer type.
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A new header file (ieeefp.h) has been introduced which contains interfaces that
apply only to IEEE floating point and are callable only when compiling using the
IEEE_FLOAT pragma. Calling the functions in ieeefp.h when generating TNS
floating-point code causes a syntax error because of incompatible floating-point
calling conventions. If you want to use any of the resources defined by ieeefp.h,
you must include it in your source.
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For more information about compiling and linking programs that use IEEE floating-
point format, see Compiling and Linking Floating-Point Programs on page 16-4.