printf and long double

Yes -- for long double, you need to use %Lf (i.e., upper-case 'L').

From the printf manpage:

l (ell) A following integer conversion corresponds to a long int or unsigned long int argument, or a following n conversion corresponds to a pointer to a long int argument, or a following c conversion corresponds to a wint_t argument, or a following s conversion corresponds to a pointer to wchar_t argument.

and

L A following a, A, e, E, f, F, g, or G conversion corresponds to a long double argument. (C99 allows %LF, but SUSv2 does not.)

So, you want %Le , not %le

Edit: Some further investigation seems to indicate that Mingw uses the MSVC/win32 runtime(for stuff like printf) - which maps long double to double. So mixing a compiler (like gcc) that provides a native long double with a runtime that does not seems to .. be a mess.

If you are using MinGW, the problem is that by default, MinGW uses the I/O resp. formatting functions from the Microsoft C runtime, which doesn't support 80 bit floating point numbers (long double == double in Microsoft land).

However, MinGW also comes with a set of alternative implementations that do properly support long doubles. To use them, prefix the function names with __mingw_ (e.g. __mingw_printf). Depending on the nature of your project, you might also want to globally #define printf __mingw_printf or use -D__USE_MINGW_ANSI_STDIO (which enables the MinGW versions of all the printf-family functions).

In addition to the wrong modifier, which port of gcc to Windows? mingw uses the Microsoft C library and I seem to remember that this library has no support for 80bits long double (microsoft C compiler use 64 bits long double for various reasons).