Re: [PATCH] manual: fopen with the x flag does not follow ISO C11

[Florian Weimer <fweimer at redhat dot com>]

* Paul Eggert:

> On 12/9/19 11:16 AM, Florian Weimer wrote:
>
>> -The @samp{x} modifier is part of @w{ISO C11}.
>> +@strong{Portability Note:} Some standards describe the @samp{x}
>> +specifier with different semantics, requiring that exclusive,
>> +mandatory file locking is enabled for the new file.  The
>> +implementation in @theglibc{} follows existing practice and only
>> +passes the @code{O_EXCL} flag to the kernel.
>
> As far as I can see, the intent of C11 is that 'fopen' with "x" simply
> maps to POSIX 'open' with O_EXCL. There's no requirement in C11 that
> the program must have exclusive use of the file during the entire
> period that the file is open. Instead, the exclusion can be during the
> original access, during the opening of the file so that we know we
> created the file.
>
> Also, the word "mandatory" doesn't seem right here. C11 says that
> 'fopen' with "x" gets you exclusive access "to the extent that the
> underlying system supports exclusive access", which doesn't sound like
> "mandatory". POSIX declined to standardize mandatory locking, and C11
> doesn't standardize it either.

To be honest, I'm not really interested in what the C standard says.

However, it seems that people think that glibc implements the x
specifier, and that the standard requires various additional things that
glibc does not implement, ranging from locking to special file modes
(similar to mkstemp).

I think it is worth clarifying that glibc's x specifier only results in
O_EXCL and nothing else.

Thanks,
Florian