# C++ praciticing note
###### tags: `C++`
## Segmentation fault on large array sizes
a problem aries:

// C_max is define by macro 
void f() {
  T arr1[C_max][H_max][W_max];
  T arr2[C_max][C_max][K_max][K_max];
  T_out arr3[C_max][H_max][W_max];
  // process these three arrays
}

This fails when C_max, H_max, W_max goes from 64 by 32 by 32 to 512 by 228 by 228
however we changes to

T*** arr1;
arr1= new T**[C_max]; //....
for(int i = 0; i < C_max; i++) { 
    arr1[i] = new T*[H_max];  // same for arr2,3
    for(int j = 0; j < H_max; j++) {
        arr1[i][j] = new T[W_max];
    }  
  }
  // process these three arrays

it works.
So what's different between two initiation?
1. `T local_fmap[C_max][H_max][W_max];` resides in **stack address space**
2. `T*** local_fmap` resizds in **heap**, and work if your machine has enough memory
3. from reference:

* reference:
* [Segmentation fault on large array sizes](https://stackoverflow.com/questions/1847789/segmentation-fault-on-large-array-sizes)
* [What and where are the stack and heap?](https://stackoverflow.com/questions/79923/what-and-where-are-the-stack-and-heap)