Thread: realloc() and NULL mystery

Code:

int points_count = 1;
int *new_x = NULL;
new_x = realloc(new_x, points_count * sizeof (int));

Should it be a valid code or not?

You can realloc() a NULL pointer, if that's what you're asking.

I know that. But I saw someone complaining about exactly this code that it would cause crash, although I haven't observed such a behavior myself.

What problem did that complainer identify as the cause for the crash?

He just said that it would crash on this part of code. Then he was advised to rewrite this like

new_x = realloc(NULL, points_count * sizeof (int));
which is equivalent to
new_x = malloc(points_count * sizeof (int));

And now I just don't get it, since there would be an implicit cast to void * anyway.

Oh, in that case he is mistaken, and you can quote him the standard:

Originally Posted by C99 Section 7.20.3.4 Paragraph 3

If ptr is a null pointer, the realloc function behaves like the malloc function for the specified size.

Of course it is simpler to just use malloc() in this specific example.

laserlight
Okay, thanks then. For a couple of minutes I almost started to believe that there is a difference between passing bare NULL or a pointer initialized to it.