gsl_histogram_find gives segmentation fault (c++, gsl)

Question

I'm trying to write a simple program code which reads in a histogram via GSL and then finds me the corresponding bin index for a specific point on the x-axis. The code looks like this:

#include <iostream>
#include <string>
#include <cmath>
#include <fstream>
#include <sstream>

#include <gsl/gsl_histogram.h>

int main() {

        gsl_histogram* h_transform;
        size_t h_Bins = 3;
        h_transform = gsl_histogram_alloc(h_Bins);
        double range[4] = { 1.0, 10.0, 100.0, 1000.0 };
        double bins[3] = {7.0, 0.0011, 9e-02};
        gsl_histogram_set_ranges(h_transform, range, 4);
        for(int i=0; i<h_Bins; i++) {
                h_transform->bin[i] = bins[i];
        }

        for (size_t i=0; i<h_Bins; i++) {
                std::cout << "range[" << i << "] = " << h_transform->range[i] << std::endl;
                std::cout << "bin[" << i << "] = " << h_transform->bin[i] << std::endl;
        }
                std::cout << "range[" << h_range_size << "] = " << h_transform->range[h_range_size] << std::endl;

        size_t* h_index;
        double x = 1.1;
        std::cout << "before find" << std::endl;
        gsl_histogram_find(h_transform, x, h_index);
        std::cout << "after h_find" << std::endl;
        std::cout << "h_index = " << *h_index << std::endl;
        std::cout << "get = " << gsl_histogram_get(h_transform, *h_index) << std::endl;

        return 0;
}

When I then compile this code with

g++ -o gslTest gslTest.cpp -lgsl -lgslcblas -lm

and run it with ./gslTest I get the following output:

range[0] = 1
bin[0] = 7
range[1] = 10
bin[1] = 0.0011
range[2] = 100
bin[2] = 0.09
range[3] = 1000
before find
Segmentation fault (core dumped)

Right now I can't wrap my head around this. I got the same error a few days ago, fixed it and now it reappeared and I can't remember the fix...

Hopefully, some of you might be better at solving this than me! Thanks in advance!

Answer 1

One glaring issue is this:

size_t* h_index;
double x = 1.1;
std::cout << "before find" << std::endl;
gsl_histogram_find(h_transform, x, h_index);

Assuming that gsl_histogram_find last parameter is a pointer, then you are sending an uninitialized pointer to that function. There is absolutely nothing gsl_histogram_find can do with that pointer except check if it is NULL, or dereference it, use it, and thus cause havoc.

What you probably should be doing is this:

size_t h_index;
double x = 1.1;
std::cout << "before find" << std::endl;
gsl_histogram_find(h_transform, x, &h_index);

When a function wants a pointer, it almost always means that the function wants the address of an existing entity. Please see this answer with respect to passing pointers to functions and what it really means.