Re: [Qemu-devel] [PATCH 1/2] qga: flush implicitely when needed

[Eric Blake]

On 11/24/2015 10:52 AM, Marc-André Lureau wrote:
> Hi
> 
> ----- Original Message -----
>> On 11/24/2015 09:34 AM, address@hidden wrote:
>>> From: Marc-André Lureau <address@hidden>
>>
>> In the subject: s/implicitely/implicitly/ if you are fixing the typo, or
>> s/implicitely/explicitly/ if you are trying to make it match what the
>> patch actually does.
>>
> 
> ok, I'll switch to explicitely (it depends on the point of view, I was 
> commenting from the qga API user pov, but I get your point)

I was trying to point out not only the 2 points of view, but also the
typo (it's explicitly, not explicitely) :)

>>>      fh = gfh->fh;
>>> +
>>> +    if (!gfh->writing) {
>>> +        int ret = fseek(fh, 0, SEEK_CUR);
>>
>> Seems a bit odd to use fflush() in one place and fseek() in the other,
>> but the net result is the same either way.
> 
> "and input is not directly followed by output without an intervening call to 
> a file positioning function, unless the input operation encounters 
> end-of-file."
> 
> so I tried to follow what the spec said.

POSIX currently specifies the behavior of fflush() on seekable input
files, but did not always do so; and it has been a source of bugs on
several libc implementations (it is still undefined to use fflush() on a
non-seekable file, but I don't know if anyone is using qga guest-file-*
on non-seekable files, at least in a situation where they are both
reading and writing to the same file handle).  So on further thought, I
actually prefer avoiding fflush() after input when possible, to avoid
confusing older libc, and as a result, your asymmetry is probably the
best choice after all.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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