Question

This is my code, which allocates space for a matrix of size specified by the parameters, and returns a pointer to said matrix:

int **allocateSpace(int row, int col){  
   int i;
   int **a;
   a = malloc(sizeof(int *) * row);
   for(i = 0; i < row; i ++)                                               
      a[i] = malloc(sizeof(int) * col);  
   return a;
}      

The above code works as intended.

However, when I change the code to the following form I get a segmentation fault:

void allocateSpace(int **a, int row, int col){
    int i;
    a = malloc(sizeof(int *) * row);
    for(i = 0; i < row; i ++)
      a[i] = malloc(sizeof(int) * col);
}

It seems like when returning from the allocateSpace() function, the memory allocated was released (since I got a segmentation fault). But why? All I am doing is allocating memory for a given pointer, and it’s all done in the subfunction.

To summarize my question: Why am I getting a segmentation fault in the second piece of code?
Thank you very much!

Answer 1

Because you need to pass in a pointer to a pointer to a pointer in order to assign the allocated memory:

void allocateSpace(int ***a, int row, int col){
    int i;
    *a = malloc(sizeof(int *) * row);
    for(i = 0; i < row; i ++)
      ( *a )[i] = malloc(sizeof(int) * col);
}

And to call it you would need to pass the address of an int**:

int** ppData;

allocateSpace( &ppData, 10, 10 );