From: Martin Sebor <msebor@gmail.com>
To: Bernd Edlinger <bernd.edlinger@hotmail.de>,
"gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>,
Joseph Myers <joseph@codesourcery.com>,
Jason Merrill <jason@redhat.com>, Jeff Law <law@redhat.com>,
Richard Biener <rguenther@suse.de>,
Jakub Jelinek <jakub@redhat.com>
Subject: Re: [PATCH] Implement a warning for bogus sizeof(pointer) / sizeof(pointer[0])
Date: Sat, 22 Apr 2017 06:51:00 -0000 [thread overview]
Message-ID: <776bb206-7b8e-1878-5411-3f1cdaabac05@gmail.com> (raw)
In-Reply-To: <AM4PR0701MB2162D22EC7A12EB0797E231DE41B0@AM4PR0701MB2162.eurprd07.prod.outlook.com>
On 04/20/2017 02:35 PM, Bernd Edlinger wrote:
> Hi!
>
>
> This implements a new -Wall enabled warning for a rather common, but
> completely wrong way to compute an array size by dividing the
> sizeof(pointer) / sizeof(pointer[0]) or sizeof(*pointer).
>
> It is often hard to find this kind of error by simple code inspection
> in real code, because using sizeof in this way is a quite common idiom
> to get the array size of an array variable. And furthermore this
> expression may be used in macros, which makes it even more important to
> have this warning.
>
> There is a similar warning -Wsizeof-pointer-memaccess which helped in
> implementing the infrastructure for the new warning in the C FE.
>
> However I noticed that the -Wsizeof-pointer-memaccess warning was
> missing in C, when the sizeof is used inside parentheses, which is
> different from C++, so I fixed that too.
>
> Of course, I added some test cases for that as well.
>
> To illustrate the usefulness of this warning, it revealed quite a few
> places where bogus sizeof divisions were used in our testsuite.
>
>
> Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
> Is it OK for trunk?
That seems like a useful warning. Just a few comments.
First, -Wsizeof-array-argument already diagnoses a subset of
the same problems. For example, with the patch applied, GCC
issues the two warnings below for following test case. One
should be sufficient.
$ cat y.c && gcc -S -Wall y.c
int f (int a[])
{
return sizeof a / sizeof *a;
}
y.c: In function âfâ:
y.c:3:17: warning: âsizeofâ on array function parameter âaâ will
return size of âint *â [-Wsizeof-array-argument]
return sizeof a / sizeof *a;
^
y.c:1:12: note: declared here
int f (int a[])
^
y.c:3:19: warning: dividing the pointer size by the element size
[-Wsizeof-pointer-div]
return sizeof a / sizeof *a;
^
Second, I would suggest mentioning the actual types of the operands
rather than referring to "pointer size" and "element size." Maybe
something like:
division 'sizeof (int*) / sizeof (int)' does not compute the number
of array elements
I suggest avoiding "element size" because the pointed-to argument
need not be an array. Mentioning the types should help users better
understand the problem (especially in C++ where types are often
obscured by layers of templates). It might also be a nice touch
to add a note pointing to the declaration of the first sizeof
operand (if it's an object).
Martin
next prev parent reply other threads:[~2017-04-21 23:50 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-20 21:33 Bernd Edlinger
2017-04-22 6:51 ` Martin Sebor [this message]
2017-04-22 17:50 ` Bernd Edlinger
2017-04-28 3:23 ` Martin Sebor
2017-04-28 14:42 ` Bernd Edlinger
2017-04-28 16:27 ` Martin Sebor
2017-04-28 17:20 ` Bernd Edlinger
2017-05-01 15:54 ` Jason Merrill
2017-05-02 13:38 ` Bernd Edlinger
2017-05-03 13:15 ` Jason Merrill
2017-05-12 16:56 ` [PING] " Bernd Edlinger
2017-06-01 16:03 ` [PING**2] " Bernd Edlinger
2017-06-01 22:09 ` Joseph Myers
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