linux-kernel - Re: [RFC PATCH 4/6] seq_file: add function to write binary data

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Date:	Tue, 06 May 2008 20:37:04 +0200
From:	Peter Oberparleiter <peter.oberparleiter@...ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	ltp-list@...ts.sourceforge.net, ltp-coverage@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC PATCH 4/6] seq_file: add function to write binary data

Andrew Morton wrote:
> On Mon, 05 May 2008 17:24:37 +0200 Peter Oberparleiter <peter.oberparleiter@...ibm.com> wrote:
> 
>> From: Peter Oberparleiter <peter.oberparleiter@...ibm.com>
>> Index: linux-2.6.26-rc1/fs/seq_file.c
>> ===================================================================
>> --- linux-2.6.26-rc1.orig/fs/seq_file.c
>> +++ linux-2.6.26-rc1/fs/seq_file.c
>> @@ -554,6 +554,18 @@ int seq_puts(struct seq_file *m, const c
>>  }
>>  EXPORT_SYMBOL(seq_puts);
>>  
>> +int seq_write(struct seq_file *m, const void *s, size_t len)
> 
> Most of the other seq_file interface functions are nicely documented.

Documentation will be added with the next resend.

>> +{
>> +	if (m->count + len < m->size) {
> 
> Are you sure that shouldn't be >=?

If I understood seq_read() correctly, then < is correct:
m->count == m->size seems to be used as special marker for seq_read()
and >= doesn't make sense to me in this place. Also seq_printf() and
seq_puts() follow the same scheme.

>> +		memcpy(m->buf + m->count, s, len);
>> +		m->count += len;
>> +		return 0;
>> +	}
>> +	m->count = m->size;
>> +	return -1;
>> +}
>> +EXPORT_SYMBOL(seq_write);
> 
> Usually when a write-style function is passed too much data it will write
> as much as it can and will then return a smaller-than-requested value.

A write function in the context of the seq_file interface seems to be
defined more as an all-or-nothing business: user wants to read, buffer
is half-full, write is called, item doesn't fit, buffer is emptied,
write is called again, buffer doesn't fit, buffer size is doubled,
write is called again, etc.

> That's inappropriate for your application of seq_write(), but perhaps is
> appropriate for other future callers?

seq_write() is almost identical to the already existing seq_puts() which
led me to believe that it would fit the overall logic.

> This function has an upper limit of PAGE_SIZE bytes, I think?  The covering
> documentation should explain such things.

seq_read() will double its internal buffer size if an item doesn't fit.


Regards,
  Peter
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