linux-kernel - Re: [PATCH] hugetlb_cgroup: convert comma to semicolon

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Message-ID: <a6fb2b6dcabb7cfd59b16a6fa1b4c49bbc73bb8f.camel@perches.com>
Date:   Sun, 23 Aug 2020 09:04:51 -0700
From:   Joe Perches <joe@...ches.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Giuseppe Scrivano <gscrivan@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Xu Wang <vulab@...as.ac.cn>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] hugetlb_cgroup: convert comma to semicolon

On Sun, 2020-08-23 at 16:21 +0100, Matthew Wilcox wrote:
> On Wed, Aug 19, 2020 at 10:14:11AM +0200, Giuseppe Scrivano wrote:
> > > > -	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]),
> > > > +	cft->file_offset = offsetof(struct hugetlb_cgroup, events_file[idx]);
> > > >  	cft->flags = CFTYPE_NOT_ON_ROOT;
> > 
> > I think in this case having two expressions as part of the same
> > statement is equivalent to having two separate statements.  Both
> > cft->file_offset and cft->flags get the expected value.
> 
> That's not how the comma operator works.
> 
> It will evaluate offsetof(struct hugetlb_cgroup, events_file[idx]) and
> then discard the result.  Since it has no side-effects, this is effectively
> doing:
> 
> 	cft->file_offset = cft->flags = CFTYPE_NOT_ON_ROOT;

$ gcc -x c -
#include <stdio.h>
#include <stdlib.h>

struct foo {
	int a;
	char b[50];
};

int main(int argc, char **argv)
{
	int a;
	int b;

	a = sizeof(struct foo), b = 1;

	printf("a: %d, b: %d\n", a, b);

	return 0;
}
$ ./a.out
a: 56, b: 1