指向位数组的指针数组

Question

I want to make an array of pointers to bit arrays. I make this func2 to test the pointers, but I get a seg fault when I try to acess an elemeny of the bit array outside the function. What am I doing wrong?

int func2(int i,  int* bit_array){

  int j;

  for(j = 0; j< i; j++)
    bit_array[j] = malloc(sizeof(int) * i);  

  for(j = 0; j< i; j++)    
   bit_array[j] = 0;

  return 1;
}

int main(){

 int** bit_root;
 bit_root = malloc(sizeof(int *) * 5);

 func2(5, bit_root);

 int n;
 for(n = 0; n < 5; n++)
   printf("%d ", bit_root[0][n]); //error

 printf("\n");

 return 0;
}

Answer 1

You are sending the array incorrect to the function func2 . func2 need to be:

int func2(int i,  int** bit_array){

  int j,k;

  for(j = 0; j< i; j++)
    bit_array[j] = malloc(sizeof(int) * i);  

  for(j = 0; j< i; j++)
    for(k = 0; k< i; k++)  
       bit_array[j][k] = 0;

  return 1;
}

int main(){

 int** bit_root;
 bit_root = malloc(sizeof(int *) * 5);

 func2(5, bit_root);

 int n;
 for(n = 0; n < 5; n++)
   printf("%d ", bit_root[0][n]); //error

 printf("\n");

 return 0;
}

Answer 2

In the lines below you allocate memory for array of int for each element of bit_array and assign pointers to int arrays to bit_array elements:

for(j = 0; j< i; j++)
  bit_array[j] = malloc(sizeof(int) * i);  

But here you assign zeroes to bit_array elements. Thus you rewrite pointers to zero as if you didn't allocate memore at all:

for(j = 0; j< i; j++)    
  bit_array[j] = 0;

To fix it replace the this last block this a following code:

int k;
for(j = 0; j< i; j++)    
  for(k = 0; k < i; k++)
    bit_array[j][k] = 0;

Here in the first loop you iterate through the array of pointers to int arrays (bit_array[j]) and in the inner loop you iterate through the array of ints (bit_array[j][k]). These changes requires changing of func2 definition - second parameter must be pointer to pointer to int instead of just a pointer. It helps you to get rid from warnings of compiler. To see what is going on clearly you can use following code:

int k, *int_array = NULL;
for(j = 0; j< i; j++)
{ 
  int_array = bit_array[j]; // get the pointer to int array   
  for(k = 0; k < i; k++)
    int_array[k] = 0; // assign values to int array
}

And don't forget to free all these memory for both inner arrays and bit_array.