Segmentation fault

Question

Hi,

This is probably a silly question, but I can't see what I am doing wrong here. I have the class:

#include <sys/time.h>
#include <gsl/gsl_cdf.h>
#include <gsl/gsl_randist.h>
#include <cmath>
#include "randomnumbergenerator.h"

class RandomNumberGenerator
{
    gsl_rng * rn;
public:
    RandomNumberGenerator();
    ~RandomNumberGenerator();
    double univariate();
    void bivariateGaussian(double rho, double &x, double &y);
};

long currentMicroseconds()
{
    struct timeval now;
    gettimeofday(&now, NULL);
    return now.tv_usec;
}

RandomNumberGenerator::RandomNumberGenerator()
{
    const gsl_rng_type * T;


    gsl_rng_env_setup();

    //T = gsl_rng_default;
    T = gsl_rng_mt19937;
    rn = gsl_rng_alloc (T);
    gsl_rng_set(rn,currentMicroseconds());
}

double RandomNumberGenerator::univariate()
{
    return gsl_rng_uniform(rn);
}

void RandomNumberGenerator::bivariateGaussian(double rho, double &x, double &y)
{
    gsl_ran_bivariate_gaussian (rn, 1.0, 1.0, rho, &x, &y);
}

RandomNumberGenerator::~RandomNumberGenerator()
{
    gsl_rng_free (rn);
}

Which I call from here:

double x;
double y;
rng.bivariateGaussian(rho, x, y);

but I get a segmentation fault on gsl_ran_bivariate_gaussian (rn, 1.0, 1.0, rho, &x, &y);

Any idea?

Answer 1

Which compiler? I assume that rn is a member variable of RandomNumberGenerator. Do you initialize it to 0 in the constructor? You don't seem to be checking for an error return from gsl_rng_alloc, you probably should be because the only thing I can see right off that may be causing a problem is if rn isn't pointing to anything valid at the call that's segfaulting.

Looking at the manual for gsl_rng_alloc you can check to see if it returns NULL or 0 and then throw an exception if it doesn't. For example:

#include <stdexcept>

RandomNumberGenerator::RandomNumberGenerator()
{
    const gsl_rng_type * T;


    gsl_rng_env_setup();

    //T = gsl_rng_default;
    T = gsl_rng_mt19937;
    rn = gsl_rng_alloc (T);
    if (rn == 0) {
        throw ::std::runtime_error("Failed to allocation a random number generator.");
    }
    gsl_rng_set(rn,currentMicroseconds());
}

Also, have you tried compiling with -O0 to turn of all optimization?

Answer 2

check to see if rn has really been allocated. It is probably the only thing that can cause segmentation fault.

i tested your code on my computer, it runs okay as far as they can tell. May be check installation of GSL, they have a test suite you can use

Answer 3

In:

double x;
double y;
rng.bivariateGaussian(rho, x, y);

are x and y perhaps supposed to be arrays rather than single variables? I'd expect a distribution to produce N values rather than one (or two).

Answer 4

I don't program c++, but C. Hopefully this will someway also apply to you. But on C I sometimes use a debugger like GDB or the debugger in Eclipse. I also use valgrind(I really like this tool a lot) to fix memory leaks/segmentation faults. I advice you to like at this tutorial to get a better understanding of what valgrind can do for you. Valgrind can do a lot more so I would advice you to read about valgrind/helgrind.

Answer 5

Hi, thanks everyone for your answers. The bug was in the piece of code that I didn't paste :( I was passing an instance of RandomNumberGenerator as a normal parameter. When I changed it to passing as reference it started to work magically.