Error in reversing string - c

Question

I know this question is extremely common and the solution is well-known. But for a long time, I am getting an error I cannot figure out. I am trying to reverse a string in C. My code is given below:
#include <stdio.h>
char *reverse(char *);
int main(void) {
char str[] = "Hello";
char *rev;
rev = reverse(str);
printf("The reversed string is %s", rev);
return 0;
}
char *reverse(char *str){
char *end = str;
char tmp;
if(str){
while(*end){
++end;
}
--end;
while(str < end){
tmp = *str;
*str++ = *end;
*end-- = tmp;
}
}
return str;
}
As result, I am getting "leH", not "olleH". Can anyone point out why?

Answer 1

The pointer str you return in reverse() does not point to the beginning of the string, but somewhere in the middle at the end of the loop.
Another problem with your function is in case you pass an empty string: end is decremented from the end of the string and points outside the string. This invokes undefined behaviour.
You should use 2 temporary pointers to perform the task:
char *reverse(char *str) {
if (str && *str) {
char *p = str;
char *end = p + strlen(p) - 1;
while (p < end) {
char tmp = *p;
*p++ = *end;
*end-- = tmp;
}
}
return str;
}
Or if you prefer to use index variables:
char *reverse(char *str) {
if (str && *str) {
for (size_t i = 0, j = strlen(str); i < --j; i++) {
char tmp = str[i];
str[i] = str[j];
str[j] = tmp;
}
}
return str;
}

Answer 2

Because you return the incremented pointer, use another pointer or index notation and it will work, like this
char *reverse(char *str)
{
char *end;
char tmp;
int i;
end = str;
if ((str == NULL) || (*str == '\0'))
return str;
while (*end != 0)
++end;
--end;
i = 0;
while (str + i < end)
{
tmp = str[i];
str[i++] = *end;
*end-- = tmp;
}
return str;
}
When you return str, it's no longer pointing at the beginning of the string.

Answer 3

Another way of achieving reversing string is this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *reverse(const char *);
int main(void){
char *s = "hello world";
char *s_rev = reverse(s);
printf("%s => %s", s, s_rev);
free(s_rev);
return 0;
}
char *reverse(const char *s){
char *s_new = strdup(s);
char *s_begptr = &s[0];
char *s_endptr = &s[strlen(s) - 1];
char *ptr = NULL;
for (ptr = s_endptr; ptr >= s_begptr; ptr--, s_new++){
*s_new = *ptr;
}
*s_new = '\0';
s_new -= strlen(s);
return s_new;
}
Notice we are not using any temporary variables to store the value, rather, using both start and end pointers of the same string and using the loop to work backwards from the end of string in reverse.