printf and long double

Question

I am using the latest gcc with Netbeans on Windows  Why doesn t long double work  Is the printf specifier  lf wrong   Code    include  lt stdio h gt   int main void        float aboat   32000 0      double abet   5 32e-5      long double dip   5 32e-5       printf   f can be written  e n   aboat  aboat       printf   f can be written  e n   abet  abet       printf   lf can be written  le n   dip  dip        return 0      Output   32000 000000 can be written 3 200000e 004 0 000053 can be written 5 320000e-005 -1950228512509697500000000000000000000000000000000000000000000000000000000000000 000000000000000000000000000000000000000000000000000000000000000000000000 000000 can be written 2 725000e 002 Press  Enter  to close the terminal

User · Answer

Was having this issue testing long doubles, and alas, I came across a fix! You have to compile your project with -D__USE_MINGW_ANSI_STDIO:

Jason Huntley@centurian /home/developer/dependencies/Python-2.7.3/test $ gcc main.c

Jason Huntley@centurian /home/developer/dependencies/Python-2.7.3/test $ a.exe c=0.000000

Jason Huntley@centurian /home/developer/dependencies/Python-2.7.3/test $ gcc main.c -D__USE_MINGW_ANSI_STDIO

Jason Huntley@centurian /home/developer/dependencies/Python-2.7.3/test $ a.exe c=42.000000

Code:

Jason Huntley@centurian /home/developer/dependencies/Python-2.7.3/test
$ cat main.c
#include <stdio.h>

int main(int argc, char **argv)
{
   long double c=42;

   c/3;

   printf("c=%Lf\n",c);

   return 0;
}

User · Answer

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

User · Answer

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

User · Answer

As has been said in other answers  the correct conversion specifier is   Lf    You might want to turn on the format warning by using -Wformat  or -Wall  which includes -Wformat  in the gcc invocation     gcc source c   gcc -Wall source c source c  In function  main   source c 5  warning  format   lf  expects type  double   but argument 2 has type  long double  source c 5  warning  format   le  expects type  double   but argument 3 has type  long double

User · Answer

printf and scanf function in C C   uses Microsoft C library and this library has no support for 10 byte long double  So when you are using printf and scanf function in your C C   code to print a long double as output and to take some input as a long double  it will always give you wrong result   If you want to use long double then you have to use     mingw printf   and     mingw scanf   function instead of printf and scanf  It has support for 10 byte long double   Or you can define two macro like this      define printf   mingw printf   and    define scanf   mingw scanf    Use standard format for long double    Lf

User · Answer

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

User · Answer

In C99 the length modifier for long double seems to be L and not l  man fprintf  or equivalent for windows  should tell you for your particular platform

User · Answer

Yes -- for long double  you need to use  Lf  i e   upper-case  L

[c] printf and long double

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