Hello.

I apologize if this topic has been covered somewhere else, but I thought that it might be best to post my code. I was struggling with pointer types earlier, and this page http://cboard.cprogramming.com/756650-post4.html helped me to find a solution to that. But I am still getting a segmentation fault, and I don't know what's causing it. I am sort of new to C (I knew some C++, but I wasn't great at that either), and the whole double pointer thing is a bit confusing. I know that I'm trying to get to memory that I don't have, but I don't know where.

Thanks

This is the program, in its entirety:

Code:

/* This is a single body test simulation */
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#define G 6.7*pow(10,-11)
#define m_s 2*pow(10,5) /* Mass of the sun, in units of 10^25 kg */
#define t_max pow(10,8) /* stop time */
#define h 1 /* stepsize */
double xdot(double **arrayptr, double delta, short j, unsigned int i); /* an easy reference for the first order equations */
void RungeKutta4(double**, unsigned int i); /* iteration for forth order (4 step) method */
int main()
{
const unsigned int n = t_max/h; /* array size */
double** data; /* this holds the datas */
unsigned int i; /* loop index */
FILE *fp;
data = malloc(4*sizeof(double*));
for(i=0; i<n; ++i)
data[i] = malloc( n*sizeof(*data[i]) );
data[0][0] = 0; /* initial earth x-position */
data[1][0] = 1.5 * pow(10,10); /* initial earth y-position */
data[2][0] = 3 * pow(10,-6); /* initial earth x-velocity */
data[3][0] = 0; /* initial earth y-velocity */
for(i=0; i<n; ++i)
RungeKutta4(data, i);
fp = fopen("single_body.txt", "w+");
fwrite(data, sizeof(double), 4*n*sizeof(double)/sizeof(double), fp);
return 0;
}
double xdot(double **data0, double delta, short j, unsigned int i) /* returns a first order ODE from the system of ++j equations */
{
if(j<2)
return (data0[j+2][i] + delta);
else /* |r|^3 is calculated twice, but it's a bit messy if you don't */
return G * m_s * (-(data0[j-2][i] + delta) )/pow(sqrt( pow(-(data0[j][i] + delta),2) +
pow(-(data0[j][i] + delta), 2) ), 3);
}
void RungeKutta4(double **datai, unsigned int i) /* a single 'four'-step iteration */
{
short j;
double s1, s2, s3, s4;
for(j = 0; j<4; ++j)
{
s1 = xdot(datai, 0, j, i);
s2 = xdot(datai, h*s1/2, j, i);
s3 = xdot(datai, h*s2/2, j, i);
s4 = xdot(datai, h*s3, j, i);
datai[j][i+1] = datai[j][i] + h*(s1 + 2*s2 + 2*s3 +s4)/6;
}
}