** in declaration represents pointer to pointer. Pointer is itself a data type and like other data types it can have a pointer.
int i = 5, j = 6; k = 7;
int *ip1 = &i, *ip2 = &j;
int **ipp = &ip1;
Pointer to pointer are useful in case of allocating dynamic 2D array. To allocate a 10x10 2D array (may not be contiguous)
int **m = malloc(sizeof(int *)*10;
for(int i = 0; i < 10; i++)
m[i] = malloc(sizeof(int)*10
It is also used when you want to change the value of a pointer through a function.
void func (int **p, int n)
{
*p = malloc(sizeof(int)*n); // Allocate an array of 10 elements
}
int main(void)
{
int *ptr = NULL;
int n = 10;
func(&ptr, n);
if(ptr)
{
for(int i = 0; i < n; i++)
{
ptr[i] = ++i;
}
}
free(ptr);
}
Further reading: Pointer to Pointer.
