Segmentation fault in function implementing Ford-Fulkerson - c

Question

I'm working on a class assignment and I've run into an issue I haven't been able to figure out. I'm implementing the Ford-Fulkerson algorithm using BFS to find max flow. But while trying to set my Residual Capacity matrix to the given capacity, I hit a segmentation fault. In the test code we received, I can see that the original capacity matrix was passed by value by its address, but I have a feeling that in my code I'm not interacting with it the way I think I am? Which leads me to believe that I may have the same issue recurring elsewhere. I worked with gdb and saw that I hit a segmentation fault on this line here in my nested for loop :
resCap[i][j] = *(capacity + i*n + j);
However, nothing I have tried has worked for me though so I am pretty stumped.
void maximum_flow(int n, int s, int t, int *capacity, int *flow)
{
int i, j, resCap[n][n], path[n]; // residual capacity and BFS augmenting path
int min_path = INT_MAX; // min of the augmenting path
// Assign residual capacity equal to the given capacity
for (i = 0; i < n; i++)
for (j = 0; j < n; j++)
{
resCap[i][j] = *(capacity + i*n + j);
*(flow + i*n + j) = 0; // no initial flow
}
// Augment path with BFS from source to sink
while (bfs(n, s, t, &(resCap[0][0]), path))
{
// find min of the augmenting path
for (j = t; j != s; j = path[j])
{
i = path[j];
min_path = min(min_path, resCap[i][j]);
}
// update residual capacities and flows on both directions
for (j = t; j != s; j = path[j])
{
i = path[j];
if(*(capacity + i*n + j) > 0)
*(flow + i*n + j) += min_flow_path;
else
*(flow + j*n + i) -= min_flow_path;
resCap[i][j] -= min_flow_path;
resCap[j][i] += min_flow_path;
}
}
}
And here is the test code provided to us in case it is needed:
int main(void)
{ int cap[1000][1000], flow[1000][1000];
int i,j, flowsum;
for(i=0; i< 1000; i++)
for( j =0; j< 1000; j++ )
cap[i][j] = 0;
for(i=0; i<499; i++)
for( j=i+1; j<500; j++)
cap[i][j] = 2;
for(i=1; i<500; i++)
cap[i][500 + (i/2)] =4;
for(i=500; i < 750; i++ )
{ cap[i][i-250]=3;
cap[i][750] = 1;
cap[i][751] = 1;
cap[i][752] = 5;
}
cap[751][753] = 5;
cap[752][753] = 5;
cap[753][750] = 20;
for( i=754; i< 999; i++)
{ cap[753][i]=1;
cap[i][500]=3;
cap[i][498]=5;
cap[i][1] = 100;
}
cap[900][999] = 1;
cap[910][999] = 1;
cap[920][999] = 1;
cap[930][999] = 1;
cap[940][999] = 1;
cap[950][999] = 1;
cap[960][999] = 1;
cap[970][999] = 1;
cap[980][999] = 1;
cap[990][999] = 1;
printf("prepared capacity matrix, now executing maxflow code\n");
maximum_flow(1000,0,999,&(cap[0][0]),&(flow[0][0]));
for(i=0; i<=999; i++)
for(j=0; j<=999; j++)
{ if( flow[i][j] > cap[i][j] )
{ printf("Capacity violated\n"); exit(0);}
}
flowsum = 0;
for(i=0; i<=999; i++)
flowsum += flow[0][i];
printf("Outflow of 0 is %d, should be 10\n", flowsum);
flowsum = 0;
for(i=0; i<=999; i++)
flowsum += flow[i][999];
printf("Inflow of 999 is %d, should be 10\n", flowsum);
printf("End Test\n");
}

Answer 1

This line is likely going to segfault, it does using Clang.
int i, j, resCap[n][n], path[n];
You're declaring a very large array on the stack. Just how big can be seen when you try and allocated it using calloc. Try this instead and don't forget to free it using the same sort of loop.
int **resCap2 = calloc(1, n * sizeof(int *));
assert(resCap2);
for (i = 0; i < n; i++) {
resCap2[i] = calloc(1, n * sizeof(int));
assert(resCap2[i]);
}
This is a lot of space ie
(1000 * sizeof(int*) * (1000 * n * sizeof(int)))