arrays - Pointer to a pointer in C not working as expected -

i learning pointers in c. have 2d array of integers , i'm trying add elements in single row. 2d array name a. know a pointer array rows, pointers integer elements. want loop on entire array a. so, declared pointer **pp equal a, since a pointer pointer. program crashing @ run time.

int main(void){ int sum=0; int a[3][3] = {{1,2,3},{4,5,6},{7,8,9}}; int **pp = null; int *p = null;  (pp=a;pp<a+3;pp++){     (p=*pp;p<*pp+3;p++)         sum += *p;      printf("sum = %d\n", sum);     sum = 0;     } } 

i know address of first object of 2d array a. address of second object a+1 , on. matrix element i,j example *(*(a+i)+j). in order words, if set pointer variable pp (a+i), know i'm in i-th row. address of first integer element of i-th row *(a+i), why made p=*pp. element (i,0) should *p same *(*(a+i)+0). so, why isn't code running should ?

following code works though needs cast :

int main(void){     int sum = 0;     int a[3][3] = { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 } };     int *pp = (int*)a;      (int = 0; < 3; ++i){         (int j = 0; j < 3; ++j){             sum += *pp;             pp++;         }         printf("sum = %d\n", sum);         sum = 0;    } } 

it little academic example because don't need pointers @ all. point is address of memory array begins. can cast first element pointer int. , following integers can reached doing pp++; moves pointer right sizeof(int). traverse multi dimentional array in linear way.