linux-kernel - Re: [PATCH 1/2] tools/nolibc: Add fread() to stdio.h

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Message-ID: <20260105110142.127eaba3@pumpkin>
Date: Mon, 5 Jan 2026 11:01:42 +0000
From: David Laight <david.laight.linux@...il.com>
To: Daniel Palmer <daniel@...ngy.jp>
Cc: w@....eu, linux@...ssschuh.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] tools/nolibc: Add fread() to stdio.h

On Mon, 5 Jan 2026 18:43:03 +0900
Daniel Palmer <daniel@...ngy.jp> wrote:

> Hi David,
> 
> On Mon, 5 Jan 2026 at 18:27, David Laight <david.laight.linux@...il.com> wrote:
> > But you've deleted the partial bytes from the input.
> > I'm sure that isn't right.
> > Normally a FILE is buffered and the bytes are saved for the next read.
> > Remember you can be reading from a pipe that is being written using
> > 'block buffering' - so it is valid for only a partial 'item' be read.
> > (I'm sure non-blocking IO is also valid...)  
> 
> I see now. If a partial read happens, the next call to fread() will
> read from after the end of the partial read that happened and it'll be
> broken.
> Since in nolibc the FILE pointer that gets used isn't really a pointer
> but the file descriptor I'm not sure where we'd stash the partial part
> so we need to avoid doing the partial read entirely.

Except you can't really avoid the partial read.
Doing multiple read() system calls doesn't help.
The situation where it can happen probably doesn't happen for nolibc.
Is there support for ferror() and/or feof() ?
(a global 'u8 fstate[64]' indexed by fd number would probably suffice.)
If so you could set the 'error' bit and then error any further fread()s.

	David

> 
> Thanks,
> 
> Daniel