Thread: unexpected result

Hi everybody
i tried to run a simple code to find the mean value of an array using gsl library with eclipse. but i got "0" as a result. i would appreciate any help or advices.
the code is below with header file.

The code is

#include <iostream>
#include <cstdlib>
#include <ctime> // used for other parts of code
#include <cmath> // used fr other part of code
#include <gsl/gsl_statistics_double.h>
using namespace std;

double foo(double *data)
{
size_t data_size = sizeof(data)/sizeof(data[0]);
return gsl_stats_mean(data, 1, data_size);
}

int main()
{
double k;
double mod_data[] = {4.88,3.56,3.81,4.01,2.5,3.71,4.8,5.2};
k=foo(mod_data);
cout<<k<<endl;
system("pause");
return 0;
}