printf and long double
CGccPrintfLong DoubleC Problem Overview
I am using the latest gcc with Netbeans on Windows. Why doesn't long double
work? Is the printf
specifier %lf
wrong?
Code:
#include <stdio.h>
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 ...
C Solutions
Solution 1 - C
Yes -- for long double
, you need to use %Lf
(i.e., upper-case 'L').
Solution 2 - C
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.
Solution 3 - C
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).
Solution 4 - C
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).
Solution 5 - C
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;
}
Solution 6 - C
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.
Solution 7 - C
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 $ 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
$
:
source.c:5: warning: format "%lf" expects type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
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
$
, but argument 2 has type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
source.c:5: warning: format "%lf" expects type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
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
$
, but argument 2 has type long double
source.c:5: warning: format "%le" expects type double, but argument 3 has type long double
$
source.c:5: warning: format "%le" expects type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
source.c:5: warning: format "%lf" expects type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
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
$
, but argument 2 has type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
source.c:5: warning: format "%lf" expects type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
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
$
, but argument 2 has type long double
source.c:5: warning: format "%le" expects type double, but argument 3 has type long double
$
source.c:5: warning: format "%le" expects type double, but argument 3 has type long double
$
, but argument 3 has type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
source.c:5: warning: format "%lf" expects type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
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
$
, but argument 2 has type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
source.c:5: warning: format "%lf" expects type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
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
$
, but argument 2 has type long double
source.c:5: warning: format "%le" expects type double, but argument 3 has type long double
$
source.c:5: warning: format "%le" expects type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
source.c:5: warning: format "%lf" expects type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
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
$
, but argument 2 has type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
source.c:5: warning: format "%lf" expects type $ gcc source.c
$ gcc -Wall source.c
source.c: In function $ 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
$
:
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
$
, but argument 2 has type long double
source.c:5: warning: format "%le" expects type double, but argument 3 has type long double
$
source.c:5: warning: format "%le" expects type double, but argument 3 has type long double
$
, but argument 3 has type long double
$
$
Solution 8 - C
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