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 C90 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 is multiplied. Thus `0x1.f` is

$1 {15over16}$,

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`.