I'm writing a program for an assignment I have at school to model the oscillations of a coupled harmonic oscillator (a block connected to a wall by a spring, and another block connected to the first block by a spring). With this my extension I have chosen is to model n blocks connected by n springs! Up to a reasonable amount, I started by trying 3.
When I attempt to compile it it compiles fine and I can run the program for the first part which only requires input. After that it just crashes, giving a bus error. Here is my code, sorry it might seem like a wall!! It begins by taking a number of values from the command like for the positions of the blocks.
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
/* Set variables for the width of the blocks and rest length of the springs*/
#define w 1.0
#define R 1.0
/*Define our external function for finding the runge kutta constants*/
double rkutta(double *xpos, double *omeg, double *j, int delta, int n, int N);
int main(int argc, char *argv[])
{
FILE* output;
int tmp, N;
double *x, *v, *m, *k, *omega, mtmp, ktmp, t, dt, *xstep, *vstep;
/*Find value of number of masses from the command line*/
N = argc - 1;
dt = 0.001;
/*Allocate array memory for each variable required*/
x = malloc(N*sizeof(double));
v = malloc(N*sizeof(double));
m = malloc(N*sizeof(double));
k = malloc(N*sizeof(double));
omega = malloc((2*N - 1)*sizeof(double));
/*Read values for the x position of each mass from command line*/
if(x != NULL && v != NULL && m != NULL && k != NULL && omega !=NULL)
{
for(tmp=0; tmp<N; tmp++)
{
sscanf(argv[tmp+1], "%lf", &x[tmp]);
}
}
else
{
printf("***************************\n");
printf("**Error allocating arrays**\n");
printf("***************************\n");
}
/*Check there are an appropriate amount of masses,
if so take values for other quantities from user*/
if(N <= 1)
{
printf("************************************\n");
printf("**There must be at least 2 masses!**\n");
printf("************************************\n");
}
else if(N == 2)
{
for(tmp=0; tmp<N; tmp++)
{
printf("Input a value for the velocity of Block %d\n", tmp+1);
scanf("%lf", &v[tmp]);
}
for(tmp=0; tmp<N; tmp++)
{
printf("Input a value for the mass of Block %d\n", tmp+1);
scanf("%lf", &m[tmp]);
}
for(tmp=0; tmp<N; tmp++)
{
printf("Input a value for the spring constant of Spring %d\n", tmp+1);
scanf("%lf", &k[tmp]);
}
}
else
{
for(tmp=0; tmp<N; tmp++)
{
printf("Input a value for the velocity of Mass %d\n", tmp+1);
scanf("%lf", &v[tmp]);
}
printf("Input a value for the mass of each Block\n");
scanf("%lf", &mtmp);
for(tmp=0; tmp<N; tmp++)
{
m[tmp] = mtmp;
}
printf("Input a value for the spring constant of each Spring\n");
scanf("%lf", &ktmp);
for(tmp=0; tmp<N; tmp++)
{
k[tmp] = ktmp;
}
}
/*Compute values of each omega*/
for(tmp=0; tmp<(2*N-1); tmp++)
{
if(tmp % 2)
{
omega[tmp] = k[(tmp+1)/2] / m[(tmp-1)/2];
}
else
{
omega[tmp] = k[tmp/2] / m[tmp/2];
}
}
/*Define arrays for runge kutta constants*/
double *a, *b, *c, *d;
/*Calculate the values of the runge kutta constants*/
for(tmp=0; tmp<(2*N); tmp++)
{
if(tmp < N)
{
a[tmp] = v[tmp];
}
else
{
a[tmp] = rkutta(x, omega, 0, 0, (tmp-N), N);
}
}
for(tmp=0; tmp<(2*N); tmp++)
{
if(tmp < N)
{
b[tmp] = v[tmp] + 0.5*dt*a[tmp+2];
}
else
{
b[tmp] = rkutta(x, omega, a, (0.5*dt), (tmp-N), N);
}
}
for(tmp=0; tmp<(2*N); tmp++)
{
if(tmp < N)
{
c[tmp] = v[tmp] + 0.5*dt*b[tmp+2];
}
else
{
c[tmp] = rkutta(x, omega, b, (0.5*dt), (tmp-N), N);
}
}
for(tmp=0; tmp<(2*N); tmp++)
{
if(tmp < N)
{
d[tmp] = v[tmp] + dt*c[tmp+2];
}
else
{
d[tmp] = rkutta(x, omega, c, dt, (tmp-N), N);
}
}
/*Open file to output data*/
output = fopen("1209937_proj1.out", "w");
for(t=0; t<=0.1; t=t+dt)
{
if(output != (FILE*)NULL)
{
fprintf(output, "%lf ", t);
for(tmp=0; tmp<N; tmp++)
{
fprintf(output, "%lf ", x[tmp]);
}
for(tmp=0; tmp<N; tmp++)
{
if(tmp<N-1)
{
fprintf(output, "%lf ", v[tmp]);
}
else
{
fprintf(output, "%lf\n", v[tmp]);
}
}
}
else
{
printf("*********************************\n");
printf("**Error outputting data to file**\n");
printf("*********************************\n");
return(EXIT_FAILURE);
}
/*Use runge kutta to find the next value of v and x*/
for(tmp=0; tmp<N; tmp++)
{
xstep[tmp] = x[tmp] + (dt/6)*(a[tmp]+2*b[tmp]+2*c[tmp]+d[tmp]);
vstep[tmp] = v[tmp] + (dt/6)*(a[tmp+2]+2*b[tmp+2]);
vstep[tmp] = vstep[tmp] + (dt/6)*(2*c[tmp+2]+d[tmp+2]);
x[tmp] = xstep[tmp];
v[tmp] = vstep[tmp];
}
}
free(x);
free(v);
free(m);
free(k);
free(omega);
fclose(output);
return(EXIT_SUCCESS);
}
/*Given various quantities find runge kutta values*/
double rkutta(double *xpos, double *omeg, double *j, int delta, int n, int N)
{
int temp;
double result;
result = 0;
for(temp=0; temp<N; temp++)
{
xpos[temp] = xpos[temp] + delta*j[temp];
}
if(n=0)
{
result = -omeg[n]*(xpos[n]-R) + omeg[n+1]*(xpos[n+1]-xpos[n]-w-R);
}
else if(n < N-1)
{
result = -omeg[2*n]*(xpos[n]-xpos[n-1]-w-R);
result = result + omeg[(2*n+1)]*(xpos[n+1]-xpos[n]-w-R);
}
else
{
result = -omeg[(2*n-1)]*(xpos[n]-xpos[n-1]-w-R);
}
return(result);
}
I haven't really done much programming so I'm sorry if it's something obvious. I just want to fix it. The program runs up until after all the data has been put in by the user.
Running in the debugger it fails here for me:
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400cfb in main (argc=3, argv=0x7fffffffe508) at t.c:116
116 a[tmp] = v[tmp];
Looking at a we find that its un-initialized.
(gdb) p a
$1 = (double *) 0xc2
Looking at your program, we see that you define:
/*Define arrays for runge kutta constants*/
double *a, *b, *c, *d;
But you have not defined an array, you've defined an empty pointer and not set it to anything.
Change this to
double a[100], b[100], c[100], d[100] ;
And then the compiler will create memory for those (at least for N up to 50).