How to write a function that takes in a string parameter and checks to see whether or not it is an integer, and if it is then return the integer value

#include <stdio.h>

int strtoint(char * s) {
  int index = 0, flag = 0;

  while ( * (s + index) != '
#include <stdio.h>
int strtoint(char * s) {
int index = 0, flag = 0;
while ( * (s + index) != '\0') {
if (( * (s + index) >= '0') &&
*
(s + index) <= '9') {
flag = 1;
index++;
} else {
flag = 0;
break;
}
}
if (flag == 1)
return atoi(s);
else
return 0;
}
main() {
printf("%d", strtoint("0123"));
printf("\n%d", strtoint("0123ii"));
}
') {
    if (( * (s + index) >= '0') &&
      *
      (s + index) <= '9') {
      flag = 1;
      index++;
    } else {
      flag = 0;
      break;
    }
  }

  if (flag == 1)
    return atoi(s);
  else
    return 0;
}

main() {
  printf("%d", strtoint("0123"));
  printf("\n%d", strtoint("0123ii"));
}