Re: kfree(0) - ok?

From: Kyle Moffett
Date: Wed Aug 15 2007 - 10:34:49 EST


On Aug 15, 2007, at 10:06:49, Jan Engelhardt wrote:
On Aug 15 2007 09:58, Kyle Moffett wrote:
Irrespective of whatever the standard says, EVERY platform and compiler anybody makes nowadays has a NULL pointer value with all bits clear. Theoretically the standard allows otherwise, but such a decision would break so much code. Linux especially, we rely on the uninitialized data to have all bits clear and we depend on that producing NULL pointers; if a NULL pointer was not bitwise exactly 0 then the test "if (some_ptr != NULL)" would fail and we would start dereferencing garbage.

But if kmalloc returns NULL on failure, then testing for NULL (irrespective of being 0 or 0xDEADBEEF) is ok. What would actually concern me then is what "if (!some_ptr)" would do. Probably not the right thing.

Well, what I was referring to is:

static struct foo *some_ptr;

/* Assumes that $SOME_LOCK is held */
int initialize_foo_module()
{
if (!some_ptr) {
some_ptr = kmalloc(sizeof(*some_ptr));
if (!some_ptr)
return -ENOMEM;

/* ... */
}

/* ... */
}


We initialize all of the static data to all-bits-clear zeros during kernel init. Any platform on which the binary representations of "(unsigned long)0" and "(void *)0" are different (even in length, due to other issues) will not run the Linux kernel as it stands today.

And as to the sizeof(unsigned long) == sizeof(void *) issue, please remember that every Linux compiler is either ILP32 (int, long, and pointer are 32-bit) or LP64 (int is 32-bit and long/pointer are 64- bit). We sort of fundamentally rely on these properties in code all over the place.

So yes the Linux kernel "breaks the standard" in a bunch of places, but on the other hand we're not your average software and we don't have to worry about building on an LLP64 compiler (Windows 64-bit and some UNIXes) or other strangeness.

Cheers,
Kyle Moffett

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