ISO C99 and ISO C++17 support 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++98, C++11 and C++14 modes. 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 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`

.