Dawson Engler <engler@csl.Stanford.EDU> writes:
> enclosed are three bugs found in the 2.4.1 kernel by an extension
> that checks that kmalloc calls allocate enough memory. It examines all
> callsites of the form:
> p = [kv]malloc(nbytes);
> and issues an error if
> sizeof *p < nbytes
[...]
> struct midi_hdr *midihdr;
>
> Error --->
> if ((midihdr = (struct midi_hdr *) kmalloc(sizeof(struct midi_hdr *), GF
> P_KERNEL)) == NULL) {
This sort of thing is why the comp.lang.c approved way to call
malloc() is
foo *x = malloc (sizeof *x);
No cast is required and the sizeof usage resembles the
declaration. The following is what I say on comp.lang.c when
someone does it another way. AFAICS the recommendations apply
equally to [kv]malloc().
----------------------------------------------------------------------
When calling malloc(), I recommend using the sizeof operator on the
object you are allocating, not on the type. For instance, *don't*
write this:
int *x = malloc (sizeof (int) * 128); /* Don't do this! */
Instead, write it this way:
int *x = malloc (sizeof *x * 128);
There's a few reasons to do it this way:
* If you ever change the type that `x' points to, it's not
necessary to change the malloc() call as well.
This is more of a problem in a large program, but it's still
convenient in a small one.
* Taking the size of an object makes your sizeof call more
similar to your declaration, which makes writing the
statement less error-prone.
For instance, above, the declaration syntax is `*x' and the
sizeof operation is also written `*x'. This provides a
visual clue that the malloc() call is correct.
I don't recommend casting the return value of malloc():
* The cast is not required in ANSI C.
* Casting its return value can mask a failure to #include
<stdlib.h>, which leads to undefined behavior.
* If you cast to the wrong type by accident, odd failures can
result.
----------------------------------------------------------------------
-- Ben Pfaff <pfaffben@msu.edu> <pfaffben@debian.org> <blp@gnu.org> MSU Student - Debian GNU/Linux Maintainer - GNU Developer Personal webpage: http://www.msu.edu/user/pfaffben - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Sat Apr 07 2001 - 21:00:17 EST