multidimensional very large array - arraylist

Question

Hi i want to use multidimensional very large array. I tried following code. It compiles but when i execute it it gives me segmentation fault error.
'int NT = 35; int NX = 25; int NY = 25; int NZ = 25;
double dt = 0.1; double dx = 0.5; double dy = 0.5; double dz = 0.5;
double PosT[NT];
double PosX[NX]; double PosY[NY]; double PosZ[NZ];
for(int i=0;i<NT;i++)
PosT[i] = i*dt+dt;
for(int i=0; i<NX;i++)
PosX[i] = dx*i;
for(int i=0; i<NY;i++)
PosY[i] = dy*i;
for(int i=0; i<NZ;i++)
PosZ[i] = dz*i;
double* b_x=(double*)malloc(NX*NY*NZ*sizeof(double));
double* b_y=(double*)malloc(NX*NY*NZ*sizeof(double));
double** B=(double**)malloc(NX*NY*NZ*NT*sizeof(double*));
if(b_x==NULL||b_y==NULL){
cout<<"Malloc space error!"<<endl;
return 0;
}
for(int ix=0;ix<NX;ix++){
for(int iy=0;iy<NY;iy++){
for(int iz=0;iz<NZ;iz++){
int position=ix*NY*NZ+iy*NZ+iz;
b_x[position] =0.;
b_y[position] =0.;
}
}
}'
but when i work in below part then i got segmentation error, my codes next part is following lines which include 2d arrays. and this 2d array is very large ,
perhaps due to this i am getting segmentation error
'if(B==NULL){
cout<<"Malloc space error!"<<endl;
return 0;
}
cout<<"work"<<endl;
for(int ix=0;ix<NX;ix++){
for(int iy=0;iy<NY;iy++){
for(int iz=0;iz<NZ;iz++){
int position=ix*NY*NZ+iy*NZ+iz;
for(int it=0;it<NT;it++){
B[position][it]=0.;
}
}
}
}
cout<<"not working"<<endl;'
so code between work and not working has problem which causes segmentation error. Any solutions for this.

Answer 1

int NT = 35; int NX = 25; int NY = 25; int NZ = 25;
For simplicity, let's change all of these to NT=NX=NY=NZ=2. This line:
double** B=(double**)malloc(NX*NY*NZ*NT*sizeof(double*));
would then allocate space for 16 pointers. On the first iteration through the loops, this line:
B[position][it]=0.;
would be equivalent to:
double *tmp = B[0]; // Load uninitialized pointer from B[0]
tmp[0] = 0.0; // Dereference uninitialized pointer to store something.
It shouldn't be at all surprising that this code results in a SIGSEGV.
What you probably meant:
double *B = malloc(NX*NY*NZ*NT*sizeof(double));
for(int ix = 0; ix < NX; ix++) {
for(int iy = 0; iy < NY; iy++) {
for(int iz = 0; iz < NZ; iz++) {
for(int it = 0; it < NT; it++) {
int position = NT * (NZ * (NY * ix + iy) + iz) + it;
B[position] = 0.0;
}
}
}
}