CONVERT STRING TO NUMBER

#include <stdio.h>
#include <math.h>

void main(void)
{
    char str[6] = "12345";
    unsigned int i = 0, a = 0;
    while (str[i] != NULL && (a+=(str[i] - 48) * pow(10, strlen(str) - i - 1))) i++;
    printf("\n %d \n", a);
}