what does compare to string here mean? - c

Question

I see a very interesting code to reverse a string,
but I don't understand here:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
void Reverse(char *s);
int main()
{
char *s=NULL;
s=(char *)malloc(sizeof(char *));
gets(s);
Reverse(s);
puts(s);
return 0;
}
void Reverse(char *s)
{
char *end=s;
char tmp;
if (s)
{
while (*end)
{
++end;
}
--end;
while (s<end) ??
{
tmp=*s;
*s++=*end;
*end--=tmp;
}
}
}
I see the this program tries to work on the same string by using end=s to change both string at the same time, but what does '*' line : while(s<end) here mean?
I use gdb and find that when we input asdfgh, when *s is fdsa and *end is fds, this is no longer true, how this line controls the program?
I just want to know what '??' line mean..
Thanks a lot !

Answer 1

Strings in C are terminated by the \0 character, which has the integer value 0. As such it is a false value.
By using while(*end) you check whether end is pointing on the termination character of the given string. If it isn't pointing on the end, you move it further (++end). To ensure that the pointer is valid you move the "cursor" backward after this.
while(s < end) will now move check whether s is further forwarded than end. If not, then you'll swap the value of both "cursor". end will move toward s and vice versa. This way you're going to reverse the string.
You're debugging output is a result of gdbs interpretation. It interpreds end as a string, not a single character. Have a look at *end while debugging.
Note that your malloc is completely wrong. You have to allocate enough memory for the string, for example s = malloc(500*sizeof(char));. Use fgets instead, where you can specify the maximum number of characters to be read. Don't forget to free all memory you allocate:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#define BUFFER_SIZE 500
void Reverse(char *s);
int main()
{
char* s = malloc(BUFFER_SIZE * sizeof(char));
fgets(s,BUFFER_SIZE,stdin);
Reverse(s);
puts(s);
free(s);
return 0;
}
void Reverse(char *s)
{
char* end=s;
char tmp;
if(s)
{
while (*end)
{
++end;
}
--end;
while (s<end)
{
tmp=*s;
*s++=*end;
*end--=tmp;
}
}
}

Answer 2

s and end are char pointer so the test (s < end) is true until the s pointer doesn't become greater than then end pointer.
Example
Assume that you want to reverse the string "hello" stored at the address 0x0000, ad the start of the function you have:
s = 0x0000
end = 0x003
now the while loop:
s=0x0000 e=0x0003 *s=o *e=h
s=0x0001 e=0x0002 *s=l *e=e
s=0x0002 e=0x0003 << while exit >>