here we are converting string values to the integer without using library functions.

  #include

    int stringToInt(char[] );
    int main(){

        char str[10];
        int intValue;

        printf("Enter any integer as a string: ");
        scanf("%s",str);

       intValue = stringToInt(str); //converting string value to integer type by making its function.

        printf("Equivalent integer value: %d",intValue);

        return 0;
    }

    int stringToInt(char str[])       //it is a function made to convert the string value to integer value.
       {
        int i=0,sum=0;

        while(str[i]!='\0')                       //string not equals to null
         {
             if(str[i]< 48 || str[i] > 57)             // ascii value of numbers are between 48 and 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: 456
Equivalent integer value: 456