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Re: [PATCH v2 1/2] manual: Add preadv and pwritev documentation
On 04/05/2017 09:51, Florian Weimer wrote:
> * Adhemerval Zanella:
>
>> +@comment sys/uio.h
>> +@comment BSD
>> +@deftypefun ssize_t pwritev (int @var{fd}, const struct iovec *@var{iov}, int @var{iovcnt}, off_t @var{offset})
>> +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
>> +@c This is a syscall for Linux 3.2 for all architectures but microblaze
>> +@c (which was added on 3.15). The sysdeps/posix fallback emulation
>> +@c is also MT-Safe since it calls pwrite, and it is now a syscall on all
>> +@c targets.
>> +
>> +This function is similar to the @code{writev} function, with the difference
>> +it adds an extra @var{offset} parameter of type @code{off_t} similar to
>> +@code{pwrite}. The data is written to the file starting at position
>> +@var{offset}. The position of the file descriptor itself is not affected
>> +by the operation. The value is the same as before the call.
>
> This description is incorrect for O_APPEND descriptors. pwrite
> ignores the offset for them, and so does pwritev. Maybe you can fix
> pwrite as well in this commit?
>
> (The GNU implementation is not POSIX-conforming, probably because the
> POSIX specification introduces a security hole.)
Right, I will update the documentation.
>
>> +When the source file is compiled with @code{_FILE_OFFSET_BITS == 64} the
>> +@code{pwritev} function is in fact @code{pwritev64} and the type
>> +@code{off_t} has 64 bits, which makes it possible to handle files up to
>> +@twoexp{63} bytes in length.
>> +
>> +The return value is a count of bytes (@emph{not} buffers) written, @math{0}
>> +indicating end-of-file, or @math{-1} indicating an error. The possible
>> +errors are the same as in @code{writev} and @code{pwrite}.
>
> The end-of-file part does not make sense here. I don't know what the
> out-of-space behavior of our implementation is (could be 0, could be
> ENOSPC).
>
AFAIk Linux pwritev does return ENOSPC, however I did not check our fallback
implementation regarding it. It is currently used only on microblaze, so
I am not sure if we just update the documentation to reflect it or if we
aim to check/fix the default sysdeps/posix as well.