ATOI implementation in C

// Program to implement own atoi function

#include <stdio.h>

int my_atoi(char *p) {
    int k = 0;
    while (*p) {
        k = (k<<3)+(k<<1)+(*p)-'0';
        p++; // Don't increment k
    }
    return k;
}

int main(){
    char pStr[] = "987";
    int myNum = my_atoi(pStr);
    printf(" myNum = %d\n", myNum);
    return 0;
}


Explanation:
1. k = (k<<3) +  (k<<1) +  (*p) - '0';  is equivalent to 
   k = k*10 + (*p) - '0';
2. During first loop value of k is 0 and (*p) has value of char '9' 
   ( ascii  - 57) and ascii of char '0' is 48.
3. Subtraction of them gives the integer value 9 ,stored in k.
4. Increment the pointer p, makes it points to char '8'.
5. Integer k is multiplied with 10  ( k<<3 + k<<1) and 8 is added to it 
   making its value 98.
6. Again p is incremented and k finally stores integer 987.