malloc,free pointer question.

[freeindy]

Hi, I have a thought that i'd like to clear out.

I allocate memory say;

char* p = malloc(20);

p = p + 5;

//do stuff

free(p);

Now I know this is wrong, but the question is, does the call free ignores the request? Or does it just free 20 bytes from where ever it is pointing? And one last thing, is it valid to move the allocated pointer?

Thanks
Indy

[Salem]

Yes it is wrong to do that.

> does the call free ignores the request?
Most likely, it will crash.

> is it valid to move the allocated pointer?
Sure you can, but the thing you end up freeing must be the thing you got from malloc.
Save the original pointer, and modify a copy of it.

[matsp]

 
char* p = malloc(20);

char *q = p;

p = p + 5;

//do stuff

free(q);

The above changes would be sufficient - you could do it many different ways, but if you modify the return value from malloc, you need to save it away so that you can free the same thing. As Salem said, it will most likely crash - but worse possibility is that it doesn't crash when you free, but perhaps it crashes next time you malloc, or some 20 malloc's later when the corrupted data that got stored back into the heap is needed again - those bugs can be VERY hard to find. So being careful about freeing exactly the same thing that you malloc'd is a good idea.

--
Mats

[dwks]

And one last thing, is it valid to move the allocated pointer?

As long as you remember the location of the beginning of the block that you allocated, and free this, you're okay. So you could even do this!

char *p = malloc(5);
size_t len = 0;

strcpy(p, "Name");

while(*p) putchar(*p ++), len ++;

free(p - len);

In other words, there's nothing special about the pointer that you initially assign the return value of malloc() to.

But, of course, if you write code like that you're going to hopelessly confuse yourself and everyone around you in no time at all.

[freeindy]

Thanks lads. That clears the mind :-)

Indy

[brewbuck]

Now I know this is wrong, but the question is, does the call free ignores the request? Or does it just free 20 bytes from where ever it is pointing?

It does neither -- the behavior is undefined. Most likely it will crash your program (either immediately or at some future point). But in theory, anything could happen. You might create a black hole.