linux-kernel - Re: Style Question

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Date:	Mon, 12 Mar 2007 13:37:43 +0800
From:	"Cong WANG" <xiyou.wangcong@...il.com>
To:	"Jan Engelhardt" <jengelh@...ux01.gwdg.de>,
	linux-kernel@...r.kernel.org
Subject: Re: Style Question

2007/3/12, Jan Engelhardt <jengelh@...ux01.gwdg.de>:
>
> On Mar 11 2007 22:15, Cong WANG wrote:
> >
> > I have a question about coding style in linux kernel. In
> > Documention/CodingStyle, it is said that "Linux style for comments is
> > the C89 "/* ... */" style. Don't use C99-style "// ..." comments."
> > _But_ I see a lot of '//' style comments in current kernel code.
> >
> > Which is wrong? The documentions or the code, or neither? And why?
>
> The code. And because it's not always reviewed but silently pushed.
>
> > Another question is about NULL. AFAIK, in user space, using NULL is
> > better than directly using 0 in C. In kernel, I know it used its own
> > NULL, which may be defined as ((void*)0), but it's _still_ different
> > from raw zero.
>
> In what way?

The following code is picked from drivers/kvm/kvm_main.c:

static struct kvm_vcpu *vcpu_load(struct kvm *kvm, int vcpu_slot)
{
       struct kvm_vcpu *vcpu = &kvm->vcpus[vcpu_slot];

       mutex_lock(&vcpu->mutex);
       if (unlikely(!vcpu->vmcs)) {
               mutex_unlock(&vcpu->mutex);
               return 0;
       }
       return kvm_arch_ops->vcpu_load(vcpu);
}

Obviously, it used 0 rather than NULL when returning a pointer to
indicate an error. Should we fix such issue?

>
> >So can I say using NULL is better than 0 in kernel?
>
> On what basis? Do you even know what NULL is defined as in
> (C, not C++) userspace? Think about it.
>

I think it's more clear to indicate we are using a pointer rather than
an integer when we use NULL in kernel. But in userspace, using NULL is
for portbility of the program, although most (*just* most, NOT all) of
NULL's defination is ((void*)0). ;-)
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