Conversion specifier of long double in C

Question

The long double data type can have these conversion specifiers in C: %Le,%LE,%Lf,%Lg,%LG (reference).

I wrote a small program to test :

#include <stdio.h>
int main(void) {
  long double d = 656546.67894L;
  printf("%.0Le\n",d);
  printf("%.0LE\n",d);
  printf("%.0Lf\n",d);
  printf("%.0Lg\n",d);
  printf("%.0LG\n",d);
  return 0; 
}

Output:

-0

-4E-153

-0

-4e-153

-4E-153

But none is giving the desired output, which is 656547 (as you may easily understand). What is the reason?

The compiler used is gcc version 3.4.2 (mingw-special).

Answer 1

From an old mingw wiki:

mingw uses the Microsoft C run-time libraries and their implementation of printf does not support the 'long double' type. As a work-around, you could cast to 'double' and pass that to printf instead. For example:

printf("value = %g\n", (double) my_long_double_value);

Note that a similar problem exists for 'long long' type. Use the 'I64' (eye sixty-four) length modifier instead of gcc's 'll' (ell ell). For example:

printf("value = %I64d\n", my_long_long_value);

Edit (6 years later): Also see the comment below from Keith Thompson for a workaround:

#define __USE_MINGW_ANSI_STDIO 1 in the source file or change the command line to gcc -D__USE_MINGW_ANSI_STDIO=1

Answer 2

The MinGW C library is provided by MSVCRT.DLL, which is shipped with Windows and is in fact the old VC++ 6.0 library.

MinGW does however use the GNU C++ library, and although that relies on the underlying C library, it does support long double for output using iostreams. Even if you do not wish to use C++ generally, it may be worth using just enough to support this capability.