Re: limits / PIPE_BUF?

[Vadim Lobanov <vlobanov@speakeasy.net>]

On Thu, 4 May 2006, Arjan van de Ven wrote:

> On Thu, 2006-05-04 at 10:50 -0700, Vadim Lobanov wrote:
> > On Thu, 4 May 2006, Arjan van de Ven wrote:
> >
> > > On Thu, 2006-05-04 at 09:39 -0700, Vadim Lobanov wrote:
> > > > How does the kernel
> > > > code ensure that this value is honored, considering that PIPE_BUF is
> > > > not
> > > > referenced in any of the pipe code?
> > >
> > >
> > > the kernel implementation guarantees one page basically, and on all
> > > architectures that I know of that's at least 4096 bytes
> > >
> >
> > Alright, so sounds like this constant should remain inside the
> > include/linux/limits.h file. What about #defining it to be equal to
> > PAGE_SIZE, like ARM (include/linux-arm/limits.h, for example) does?
>
> there is a certain elegance in providing the same value on all
> architectures; it means apps don't suddenly break if you port it to
> a "lesser" one. Also there is a problem with PAGE_SIZE itself, that's a
> config option on several architectures, so it'd have to be a define for
> get_page_size() or something, at which point you change semantics since
> apps can't do
>
> char foo[PIPE_BUF];
>
> anymore
>

Good point.

I suppose this means that the limits.h header file is used, either
directly or indirectly, by both us (the kernel) and by user-space? If
so, blech! What's the policy on keeping around config values that don't
do anything inside the kernel anymore? The short list is:
 NR_OPEN
 ARG_MAX
 CHILD_MAX
 OPEN_MAX
 LINK_MAX
 MAX_CANON
 MAX_INPUT
 RTSIG_MAX

- Vadim Lobanov