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### 5.10 Hex Floats

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`.