AW: [PATCH] fread.3: return value

[Walter Harms <wharms@bfs.de>]

________________________________________
Von: linux-man-owner@vger.kernel.org [linux-man-owner@vger.kernel.org] im Auftrag von Arkadiusz Drabczyk [arkadiusz@drabczyk.org]
Gesendet: Donnerstag, 18. Juni 2020 13:43
An: Walter Harms
Cc: linux-man@vger.kernel.org
Betreff: Re: [PATCH] fread.3: Explain that file position is moved after calling fread()/fwrite()

On Thu, Jun 18, 2020 at 08:37:02AM +0000, Walter Harms wrote:
>> Isn't it already covered by this fragment:
>>
>> "If an error occurs, or the end of the file is reached, the return
>> value is a short item count (or zero)."
>>
>> IMHO no,
>> what does a "short item count" mean ?

>"items of data" refers to nmemb:

>"The function fread() reads nmemb items of data"

>so "item count" means number of size-long sequences of bytes and
>"short" means fewer than requested. But I think you know that well and
>you just think that "short item count" is not clear enough, right?

>(And of course I hope that I get it right.)

When reading that i try to "ignore" what i know and get the position of an first time reader,
so i noticed that this was fishy (i guess short item count is derived from short read) 

>> E.g. my size is 4 bytes and there are 3 bytes left to read. what is
>> the return value?

>If the requested size is 4 but there are only 3 bytes left then
>fread() will return 0, no matter what's the value of nmemb because
>it's not able to read even a single item and feof() will return
>non-zero. I think that this is explained clearly enough in the
>manpage.

IMHO i would be more clear to drop that "short item count". 
"If an error occurs, or the end of the file is reached, the  return value is zero"

>> and will the last few bytes ever read ?

>That is interesting. POSIX
>https://pubs.opengroup.org/onlinepubs/9699919799.2008edition/ says
>that "If a partial element is read, its value is unspecified.". But as
>fread() returns number of items it has successfully read the returned
>value should always be checked and it's different than requested nmemb
>it means an error and I guess there is no way to tell how many bytes
>have been partially read - for example if size == 4 and nmemb == 1 and
>returned value is 0 you don't know if fread() read 1, 2 or 3 bytes.

the GNU version fills the buffer with the remaining bytes but returns 0.


>Anyway, the patch that I sent explains that file pointer is moved
>after fread()/fwrite(). The discussion about return value should be
>started in a separate thread.

i have changed the subject.

--
Arkadiusz Drabczyk <arkadiusz@drabczyk.org>