Reverse a String In-Place

#include <stdio.h>

void reverse_string(char *str) {
    // Your logic here
    int index ;
    int start = 100;
    while(start !=0 )
    {
       if(str[start] != '\0')
       {
          index = start;
          break;
       }
        start --;
    }
    
    int temp;
    for(int i =0 ; i<= index/2  ; i++)
    {
        temp = str[i];
        str[i] =  str[ index -i];
        str[index-i] = temp;
    }
}

int main() {
    char str[101];
    fgets(str, sizeof(str), stdin);

    // Remove newline
    int i = 0;
    while (str[i] != '\0') {
        if (str[i] == '\n') {
            str[i] = '\0';
            break;
        }
        i++;
    }

    reverse_string(str);
    printf("%s", str);
    return 0;
}