Creating a histogram from a large file - c

Question

I have two large files containing float-type data. I want to create a histogram using the two files. However, to access the 256^3 data values, I tried using a nested for-loop in C which failed with a segmentation fault.
Is there an alternative method that I can use to access the data values that would not result in a segmentation fault?
Here is my code:
int main () {
int i,j,k,n_grid;
float pc,mass_g,spat_res,vol_pix,max_vel,min_vel;
float velocity,density,max;
long start,index,index1,index2,counter,lSize,dim;
FILE *p,*q;
pc = 3.085677e-18; //1 parsec in cm
n_grid = 256; //dimension of observed portion of input grid
mass_g = 0.0;
spat_res = pc*0.71/512; //spatial resolution of grid in cm
vol_pix = pow(spat_res,3); //volume of each voxel in cm^3
p = fopen("../dens_104/rho_052.dat","rb");
q = fopen("../dens_104/velz.dat","rb");
if (!p){
printf("Unable to open dens file!");
return 0;
}
else if (!q){
printf("Unable to open the velx file!");
return 0;
}
//obtain file size(s)
fseek(p,0,SEEK_END);
lSize = ftell(p);
rewind(p);
dim = pow(lSize/sizeof(float),1.0/3.0); //actual size of input grid
max = 0;
counter = 0;
index = 0;
while(fread(&density,sizeof(float),1,p),!feof(p) && !ferror(p)) {
if(density > max) max = density,index = counter;
counter += 1;
}
start = index - (pow(dim,2)+dim+1)*(n_grid/2-1);
printf("Starting pixel value: %lu, Dimension of cube: %d\n",start,
(int)dim);
printf("The maximum density is: %e with index: %lu \n", max,index);
rewind(p);
fseek(p,start*sizeof(float),SEEK_SET);
fseek(q,start*sizeof(float),SEEK_SET);
max_vel = 0.0;
min_vel = 0.0;
index1 = 0;
index2 = 0;
while(fread(&velocity,sizeof(float),1,q),!feof(q) && !ferror(q)) {
if(velocity > max_vel) max_vel = velocity,index1=counter;
if(velocity < min_vel) min_vel = velocity,index2=counter;
counter += 1;
}
printf("The maximum velocity is: %e with index: %lu\n", max_vel,index1);
printf("The minimum velocity is: %e with index: %lu\n", min_vel,index2);
density = 0.;
for (k=0;k<n_grid;k++){
fseek(p,(start+k*dim*dim)*sizeof(float),SEEK_SET);
for (j=0;j<n_grid;j++){
fseek(p,(start+j*dim+k*dim*dim)*sizeof(float),SEEK_SET);
for (i=0;i<n_grid;i++){
fread(&density,sizeof(float),1,p);
mass_g += density*vol_pix;
}
}
}
printf("The mass of the observed cube is: %e \n", mass_g);
}