stupid bug with fscanf - comma instead of point - c++

Question

I have a stupid bug in one of my c++ source for a project. I do in this part of the source I/O operations. I have a stupid bug where I print the fscanf read value. Below in this part :
Firstly, I don't read the good value and when I print a float value, I get a decimal value with a comma ',' instead of a point '.' between the integer part and the floating part.
FILE* file3;
file3=fopen("test.dat","r");
float test1;
fscanf(file3," %f ",&test1);
printf("here %f\n",test1);
float test3 = 1.2345;
printf("here %f\n",test3);
fclose(file3);
where test.dat file contains "1.1234" and I get at the execution :
here 1,000000
here 1,234500
So, I did a simple test C program compiled with g++ :
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE* file3;
float test3;
file3=fopen("test.dat","r");
fscanf(file3,"%f",&test3);
printf("here %f\n",test3);
fclose(file3);
}
and it gives :
here 1.123400
This is the first time I have a bug like this. Anyone could see what's wrong ?

Answer 1

Is your C++ locale somehow set to use a European convention? They use commas where we use points and points for thousand's separators.
Have a look at settings of environment variables
LANG
LC_CTYPE
LC_ALL
try setting en_GB or en_US. Having established that it is a locale problem, next decide what behaviour makes sense. Is diplaying 1224,45 a bug at all? The user has locale set for a reason.

Answer 2

You're using code that using the locale set for the programs environment. In some locale's such as in French-speaking locale's, the comma is a the decimal separator. So this code is doing what its locale is presumably telling it to.
In your simple code, you have not initialise the locale support, so this does not happen.
Assuming a Unix-like environment, what is the value of the environment variable LANG, and the various LC_* environment variables?
env | grep -e ^LANG -e ^LC_
For some background reading, try some of the GNU Libc manual (Locales and Internationalisation)
http://www.gnu.org/software/libc/manual/html_node/Locales.html#Locales

Answer 3

My guess is that the application is setting the locale to the
users preference, with std::locale::global( std::locale( "" )
). Which is what console applications should
do, always; they should also imbue std::cin, std::cout and
std::cerr with this locale. Most linguistic communities use
the comma for the decimal, rather than the point. (For demon
processes, it is often more appropriate to use the "C" locale,
or the "Posix" locale under Unix, regardless of where the
server is actually running. Since the "C" locale is the
default, doing nothing is generally fine for demons and
servers.)
The global locale affects all C style input and output, which
is yet another reason for using C++'s iostream instead. With
std::ifstream, just imbue the stream with the locale in which
the file was written before doing your first input. For
machine generated files, the "C" locale is the usual default,
so your code would become something like:
std::ifstream file3( "test.dat" );
if ( ! file3.is_open() ) {
// error handling...
}
file3.imbue( std::locale( "C" ) );
float test1;
file3 >> test1;
// ...
For output to the terminal, expect the locale conventions to be
followed. And set the environment variables to specify the
locale you want to see.