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 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
.