Reversing a string in C using pointers - purpose of if statement? - c

Question

In the code below that reverses a string, I don't understand what is the purpose of the if statement. What case is this if statement trying to catch for?
Isn't this superfluous since you've already set char*end= str, and so the statement if(str) has to be true or your code would have already failed by this point?
void reverse(char* str)
{
char *end = str;
char temp;
if (str)
{
while (*end)
{
end++;
}
}
end--; //pulls back one for \0 character
while (str < end)
{
temp = *str;
*str++ = *end;
*end-- = temp;
}
}

Answer 1

It prevents dereferencing a NULL pointer, very good practice.
In case str == NULL, then *end will be undefined behavior, so checking that before dereferencing, is really a good thing to do, because ignoring such possibility will cause a bug that will be very hard to detect, except of course using a debugger, but still, why leave the need to track the problem, when you can avoid it at the expense of almost nothing, generally.
It would actually be better to do something like this
if (str == NULL)
return;

Answer 2

Isn't this superfluous since you've already set char*end= str, and so
the statement if(str) has to be true or your code would have already
failed by this point?
Actually, the line
char *end = str;
will not cause the "code to fail", even if a null pointer was passed as an argument. Then it would just assign NULL to end.
What case is this if statement trying to catch for?
Since we do not know if we were passed a NULL pointer yet, we should check before dereferencing it. That is what the if statement is for.

Answer 3

This code would work correctly:
void reverse(char* str)
{
if (str != NULL)
{
char *end = str;
while (*end != '\0')
end++;
end--; //pulls back one for \0 character
while (str < end)
{
char temp = *str;
*str++ = *end;
*end-- = temp;
}
}
}
This code would also work:
void reverse(char* str)
{
if (str == NULL)
return;
char *end = str; // Assumes C99 or later
while (*end != '\0')
end++;
end--; //pulls back one for \0 character
while (str < end)
{
char temp = *str;
*str++ = *end;
*end-- = temp;
}
}
Both ensure that you never mess with a null pointer. Decrementing a null pointer as in the original code would lead to catastrophe (or, at least, undefined behaviour). Don't risk it.
You can also use strlen() — carefully ensuring you don't run into problems with an empty string:
void reverse(char* str)
{
if (str == NULL || *str == '\0')
return;
char *end = str + strlen(str) - 1;
while (str < end)
{
char temp = *str;
*str++ = *end;
*end-- = temp;
}
}