float, stream and toolchain

Question

I'm working with a raspberry 2 running Arch

Linux a4h-rpi2-arch 4.1.12-1-ARCH #1 SMP PREEMPT Tue Oct 27 19:16:16 MDT 2015 armv7l GNU/Linux

I got some problem to display float on cout using c++ streams. Here is a simple main :

#include <iostream>

using namespace std;



int main() {

float f;
f = 9.87654321f;
cout << f << endl;
f = 987.654321f;
cout << f << endl;
f = 987654.321f;
cout << f << endl;
f = 9876543.21f;
cout << f << endl;
f = 0.0000987654321f;
cout << f << endl;
return 0;
}

i cross compiled it with raspberry toolchain :

arm-bcm2708-linux-gnueabi-g++ -o /tmp/test_stream /tmp/test.cpp

Here is result :

test_stream
0
0
0
0
0

Now if I compile it with

arm-bcm2708hardfp-linux-gnueabi-g++ -o /tmp/test_stream /tmp/test.cpp

I get correct results :

test_stream
9.87654
987.654
987654
9.87654e+06
9.87654e-05

In first case I'm only using software to perform float calculations. I shouldn't have any differences in result between 2 toolchain? Only compute time should be better using hard float hw unit?

lah

Answer 1

In first case I'm only using software to perform float calculations.

No; since the executable is dynamically linked to the system library on a platform with hardware floating point support, there is no facility to perform such calculations with software.

At least, that's my guess. I think it will turn out differently if you compile a static binary.

Put another way, this:

arm-bcm2708-linux-gnueabi-g++

Is evidently the wrong compiler for the platform.