Segmentation fault while assigning array value - c

Question

void particle_initialize()
{
int i;
for (i = 0; i<particlenumber; i++)
{
Pu[i]=0;
}
return; }
void forceon_particle()
{
for (i = 0; i<particlenumber; i++)
{
FDx[i]=(U[i]-Pu[i])/taup;
FDy[i]=(V[i]-Pv[i])/taup;
}
return; }
void particle_motion()
{
int i;
for (i = 0; i<particlenumber; i++)
{
Pu[i]=Pu[i]+FDx[i]*tp;
Px[i]=Px[i]+Pu[i]*tp;}
return;
}
Hi Everyone, I'm unable to understand why I'm getting segmentation error in the function particle_motion.
The error comes only when I try assigning Px[i]=Px[i]+Pu[i]*tp; Py[i]=Py[i]+Pv[i]*tp;. Kindly help.
/*Developing flow in a Two Dimensional channel*/
#include<stdio.h>
#include<math.h>
#include<stdlib.h>
#define NX 101
#define NY 101
#define Q 9
#define T 300001
#define u_in 0.01
#define omega 1
#define kvisc 1/6
#define W0 4.0/9.0
#define W1 1.0/9.0
#define W2 1.0/36.0
#define rho0 1.0
#define particlenumber 10000
#define dp 0.01
#define rhop 2
#define tp 1/omega
#define pi 3.14
int cx [Q]={0,1,0,-1,0,1,-1,-1,1};
int cy [Q]={0,0,1,0,-1,1,1,-1,-1};
double t [Q]={W0,W1,W1,W1,W1,W2,W2,W2,W2};
int next_x [NX][Q];
int next_y [NY][Q];
double f1 [NX][NY][Q];
double f2 [NX][NY][Q];
double ux [NX][NY];
double uy [NX][NY];
double uxeq [NX][NY];//eq velocity for effect of particle
double uyeq [NX][NY];
double ustar [NX][NY];//forcing term added to ueq
double vstar [NX][NY];
double rho [NX][NY];
double Px [particlenumber];//location of particles
double Py [particlenumber];
double Pu [particlenumber];//velocity of particles
double Pv [particlenumber];
double FDx [particlenumber];// drag force
double FDy [particlenumber];
double U [particlenumber];//velocity of particle
double V [particlenumber];
double FPx [particlenumber];//force on fluid due to particle
double FPy [particlenumber];
double uforcing [particlenumber];
double rhoatpar [particlenumber];
double vforcing [particlenumber];
void tables();
void initialise();
void cal_obs();
void streaming();
void collision();
void density_data();
void particle_initialize();
void forceon_particle();
void intrapolation_velocity();
void particle_motion();
void forceon_fluid();
void intrapolation_density();
void reverse_intrapolation();
int main()
{
int count=0, k;
tables();
initialise();
particle_initialize();
for(k=0;k<T;k++)
{
//printf("k=%d\t",k);
cal_obs();
collision();
streaming();
if (k % 10000 ==0)
{
density_data(count, k);
}
intrapolation_velocity();
forceon_particle();
intrapolation_density();
forceon_fluid();
reverse_intrapolation();
particle_motion();
}
return 0;
}
void tables()
{
int x,y,i;
for(x=0; x<NX; x++)
{
for (y=0; y<NY; y++)
{
for (i=0; i<Q; i++)
{
next_x[x][i] = x + cx[i];
if (next_x[x][i]>NX-1)
next_x[x][i] = 0;
if (next_x[x][i]<0)
next_x[x][i] = NX-1;
next_y[y][i] = y + cy[i];
if (next_y[y][i]>NY-1)
next_y[y][i] = 0;
if (next_y[y][i]<0)
next_y[y][i] = NY-1;
}
}
}
return;
}
void initialise()
{
int x,y,i;
for(x=0; x<NX; x++)
{
for (y=0; y<NY; y++)
{
ustar[x][y]=0;//initialize forcing term zero
vstar[x][y]=0;
for (i=0; i<Q; i++)
{
f1[x][y][i]=rho0*t[i];
}
}
}
return;
}
void cal_obs()
{
int x,y,i;
double dense;
for(x=0; x<NX; x++)
{
for (y=0; y<NY; y++)
{
dense=ux[x][y]=uy[x][y]=0.0;
for (i=0; i<Q; i++)
{
dense += f1[x][y][i];
ux[x][y] += f1[x][y][i]*cx[i];
uy[x][y] += f1[x][y][i]*cy[i];
}
if(dense != 0.0)
{
rho[x][y] = dense;
ux[x][y] = ux[x][y]/rho[x][y];
uy[x][y] = uy[x][y]/rho[x][y];
}
else
{
rho[x][y] = 0.0;
ux[x][y] = 0.0;
uy[x][y] = 0.0;
}
}
}
for(x=0; x<NX; x++)
{
ux[x][0]=u_in;
ux[x][NY-1]=-u_in;
}
return;
}
void collision()
{
int x,y,i;
double udotc,u2,feq;
for(x=0; x<NX; x++)
{
for (y=0; y<NY; y++)
{
for (i=0; i<Q; i++)
{
uxeq[x][y]=ustar[x][y]+ux[x][y];
uyeq[x][y]=vstar[x][y]+uy[x][y];
udotc = uxeq[x][y]*cx[i]+uyeq[x][y]*cy[i];
u2=uxeq[x][y]*uxeq[x][y] + uyeq[x][y]*uyeq[x][y];
feq = t[i]*rho[x][y]*(1+3*udotc+4.5*udotc*udotc-1.5*u2);
f1[x][y][i]=f1[x][y][i]*(1-omega)+feq*omega;
}
}
}
return;
}
void streaming()
{
int x,y,i,newx,newy;
double rho_w;
for(x=0; x<NX; x++)
{
for (y=0; y<NY; y++)
{
for (i=0; i<Q; i++)
{
newx=next_x [x][i];
newy=next_y[y][i];
f2[newx][newy][i] = f1[x][y][i];
}
}
}
for(x=0; x<NX; x++)
{
for (y=0; y<NY; y++)
{
for (i=0; i<Q; i++)
{
f1[x][y][i]=f2[x][y][i];
}
}
}
return;
}
void density_data(int data_counter, int id)
{
int x, y;
FILE *f3;
char phase [100];
/* Data for Tecplot*/
sprintf ( phase,"phaseVelocity2D_%d.csv",id);
f3 = fopen(phase, "w");
fprintf(f3," X,Y,Rho,U,V\n");
for (y =0; y < NY; y++)
{ for (x = 0; x < NX; x++)
fprintf(f3,"%d%c%d%c%lf%c%lf%c%lf\n",x,',',y,',',rho[x][y],',',ux[x][y],',',uy[x][y]);
}
fflush(f3);
fclose(f3);
int i;
FILE *f4;
char particle [100];
sprintf ( particle,"particle_%d.csv",id);
f4 = fopen(particle, "w");
fprintf(f4," Id,X,Y\n");
for (i =0; i < particlenumber; i++)
{
fprintf(f4,"%d%c%lf%c%lf\n",i,',',Px[i],',',Py[i]);
}
fflush(f4);
fclose(f4);
return;
}
void particle_initialize()
{
int i;
srand(time(0));
float randomx;
for (i = 0; i<particlenumber; i++)
{
randomx=(float)rand()/RAND_MAX;
Px[i]=randomx*NX;
Pu[i]=0;
}
float randomy;
for (i = 0; i<particlenumber; i++)
{
randomy=(float)rand()/RAND_MAX;
Py[i]=randomy*NY;
Pv[i]=0;
}
return;
}
void forceon_particle()
{
int i;
double taup;
taup=(rhop*dp*dp)/(18*kvisc*rho0);
for (i = 0; i<particlenumber; i++)
{
FDx[i]=(U[i]-Pu[i])/taup;
FDy[i]=(V[i]-Pv[i])/taup;
}
return;
}
void intrapolation_velocity()
{
int i;
int node1x, node1y, node2x, node2y,node3x, node3y,node4x, node4y;
double w1,w2,w3,w4;
for (i = 0; i<particlenumber; i++)
{
node1x=trunc(Px[i]);
node1y=trunc(Py[i]);
node2x=node1x;
node2y=node1y+1;
node3x=node1x+1;
node3y=node1y+1;
node4x=node1x+1;
node4y=node1y;
w1=(Px[i]-node1x)*(Px[i]-node1x)+(Py[i]-node1y)*(Py[i]-node1y);
w1=sqrt(w1);
w2=(Px[i]-node2x)*(Px[i]-node2x)+(Py[i]-node2y)*(Py[i]-node2y);
w2=sqrt(w2);
w3=(Px[i]-node3x)*(Px[i]-node3x)+(Py[i]-node3y)*(Py[i]-node3y);
w3=sqrt(w3);
w4=(Px[i]-node4x)*(Px[i]-node4x)+(Py[i]-node4y)*(Py[i]-node4y);
w4=sqrt(w4);
U[i]=w1*ux[node1x][node1y]+w2*ux[node2x][node2y]+w3*ux[node3x][node3y]+w4*ux[node4x][node4y];
U[i]=U[i]/(w1+w2+w3+w4);
V[i]=w1*uy[node1x][node1y]+w2*uy[node2x][node2y]+w3*uy[node3x][node3y]+w4*uy[node4x][node4y];
V[i]=V[i]/(w1+w2+w3+w4);
}
return;
}
void particle_motion()
{
int i;
double A,B;
for (i = 0; i<particlenumber; i++)
{
Pu[i]=Pu[i]+FDx[i]*tp;//Pu[i] added so that it adds previous velocity
Pv[i]=Pv[i]+FDy[i]*tp;
}
for (i = 0; i<particlenumber; i++)
{
Px[i]=Px[i]+Pu[i]*tp;
Py[i]=Py[i]+Pv[i]*tp;
//if particle goes out of bound we r introducing periodic BC
if(Px[i]<0||Px[i]>(NX-1))
{
if(Px[i]<0)
{Px[i]=Px[i]+NX-1;}
else if(Px[i]>(NX-1))
{Px[i]=Px[i]-NX+1;}
}
if(Py[i]<0||Py[i]>(NY-1))
{
if(Py[i]<0)
{Py[i]=Py[i]+NY-1;}
else if(Py[i]>(NY-1))
{Py[i]=Py[i]-NY+1;}
}
}
return;
}
void forceon_fluid()
{
int i;
double taup;
long double mp;
mp=(1/6)*(pi)*rhop*dp*dp*dp;
taup=(rhop*dp*dp)/(18*kvisc*rho0);
for (i = 0; i<particlenumber; i++)
{
FPx[i]=mp*(U[i]-Pu[i]);
FPx[i]=FPx[i]/taup;
FPy[i]=mp*(V[i]-Pv[i]);
FPy[i]=FPy[i]/taup;
uforcing[i]=(FPx[i])/(rhoatpar[i]*omega);
vforcing[i]=(FPy[i])/(rhoatpar[i]*omega);
}
return;
}
void intrapolation_density()
{
int i;
int node1x, node1y, node2x, node2y,node3x, node3y,node4x, node4y;
double w1,w2,w3,w4;
for (i = 0; i<particlenumber; i++)
{
node1x=trunc(Px[i]);
node1y=trunc(Py[i]);
node2x=node1x;
node2y=node1y+1;
node3x=node1x+1;
node3y=node1y+1;
node4x=node1x+1;
node4y=node1y;
w1=(Px[i]-node1x)*(Px[i]-node1x)+(Py[i]-node1y)*(Py[i]-node1y);
w1=sqrt(w1);
w2=(Px[i]-node2x)*(Px[i]-node2x)+(Py[i]-node2y)*(Py[i]-node2y);
w2=sqrt(w2);
w3=(Px[i]-node3x)*(Px[i]-node3x)+(Py[i]-node3y)*(Py[i]-node3y);
w3=sqrt(w3);
w4=(Px[i]-node4x)*(Px[i]-node4x)+(Py[i]-node4y)*(Py[i]-node4y);
w4=sqrt(w4);
rhoatpar[i]=w1*rho[node1x][node1y]+w2*rho[node2x][node2y]+w3*rho[node3x][node3y]+w4*rho[node4x][node4y];
rhoatpar[i]=rhoatpar[i]/(w1+w2+w3+w4);
}
return;
}
void reverse_intrapolation()
{
int i;
int node1x, node1y, node2x, node2y,node3x, node3y,node4x, node4y;
double w1,w2,w3,w4;
int x,y;
for(x=0; x<NX; x++)
{
for (y=0; y<NY; y++)
{
ustar[x][y]=0;//initialize forcing term zero
vstar[x][y]=0;
}
}
for (i = 0; i<particlenumber; i++)
{
node1x=trunc(Px[i]);
node1y=trunc(Py[i]);
node2x=node1x;
node2y=node1y+1;
node3x=node1x+1;
node3y=node1y+1;
node4x=node1x+1;
node4y=node1y;
w1=(Px[i]-node1x)*(Px[i]-node1x)+(Py[i]-node1y)*(Py[i]-node1y);
w1=sqrt(w1);
w2=(Px[i]-node2x)*(Px[i]-node2x)+(Py[i]-node2y)*(Py[i]-node2y);
w2=sqrt(w2);
w3=(Px[i]-node3x)*(Px[i]-node3x)+(Py[i]-node3y)*(Py[i]-node3y);
w3=sqrt(w3);
w4=(Px[i]-node4x)*(Px[i]-node4x)+(Py[i]-node4y)*(Py[i]-node4y);
w4=sqrt(w4);
ustar[node1x][node1y]=ustar[node1x][node1y]+(w1*uforcing[i])/(w1+w2+w3+w4);
vstar[node1x][node1y]=vstar[node1x][node1y]+(w1*vforcing[i])/(w1+w2+w3+w4);
ustar[node2x][node2y]=ustar[node2x][node2y]+(w1*uforcing[i])/(w1+w2+w3+w4);
vstar[node2x][node2y]=vstar[node2x][node2y]+(w1*vforcing[i])/(w1+w2+w3+w4);
ustar[node3x][node3y]=ustar[node3x][node3y]+(w1*uforcing[i])/(w1+w2+w3+w4);
vstar[node3x][node3y]=vstar[node3x][node3y]+(w1*vforcing[i])/(w1+w2+w3+w4);
ustar[node4x][node4y]=ustar[node4x][node4y]+(w1*uforcing[i])/(w1+w2+w3+w4);
vstar[node4x][node4y]=vstar[node4x][node4y]+(w1*vforcing[i])/(w1+w2+w3+w4);
}
return;
}
I have added the whole code as the error may be due to other functions which might also access the arrays.

Answer 1

The issue is at this statement.
V[i] = w1* uy[node1x][node1y] + w2* uy[node2x][node2y] + w3 * uy[node3x][node3y] + w4*uy[node4x][node4y];
Here,the arrays are uy[101][101], ux[101][101]. Look at the values of node1x and node1y:
node1x=trunc(Px[i]);
node1y=trunc(Py[i]);
See the values of Px[i] and Py[i] :
Px[i]=randomx*NX; // NX=101. randomx ranges from 0 to 1. randomx=(float)rand()/RAND_MAX
Py[i]=randomy*NY; // NY=101. randomy ranges from 0 to 1. randomy=(float)rand()/RAND_MAX
I think at some point of time randomx = 1, then Px[i] = 101, then node1x = 101 then you are accessing uy[101][node1y]. This causes segmentation fault because the array is uy[101][101] , and you cannot access uy[101][node1y]element.