How to convert string to int without using library functions in c

How to convert string to int without using library functions in c
How to convert string to int without using library functions in c
#include<stdio.h>
int stringToInt(char[] );
int main(){
    char str[10];
    int intValue;
    printf("Enter any integer as a string: ");
    scanf("%s",str);
   intValue = stringToInt(str);
    printf("Equivalent integer value: %d",intValue);
    return 0;
}
int stringToInt(char str[]){
    int i=0,sum=0;
    while(str[i]!='\0'){
         if(str[i]< 48 || str[i] > 57){
             printf("Unable to convert it into integer.\n");
             return 0;
         }
         else{
             sum = sum*10 + (str[i] - 48);
             i++;
         }
    }
    return sum;
}

Sample output:
Enter any integer as a string: 123
Equivalent integer value: 123