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(gcc-300.info)Hex Floats

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Hex Floats
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   ISO C99 supports floating-point numbers written not only in the usual
decimal notation, such as `1.55e1', but also numbers such as `0x1.fp3'
written in hexadecimal format.  As a GNU extension, GCC supports this
in C89 mode (except in some cases when strictly conforming) and in C++.
In that format the `0x' hex introducer and the `p' or `P' exponent
field are mandatory.  The exponent is a decimal number that indicates
the power of 2 by which the significant part will be multiplied.  Thus
`0x1.f' is 1 15/16, `p3' multiplies it by 8, and the value of `0x1.fp3'
is the same as `1.55e1'.

   Unlike for floating-point numbers in the decimal notation the
exponent is always required in the hexadecimal notation.  Otherwise the
compiler would not be able to resolve the ambiguity of, e.g., `0x1.f'.
This could mean `1.0f' or `1.9375' since `f' is also the extension for
floating-point constants of type `float'.