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Re: [PATCH v2 1/2] manual: Add preadv and pwritev documentation
- From: Florian Weimer <fw at deneb dot enyo dot de>
- To: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- Cc: libc-alpha at sourceware dot org
- Date: Thu, 04 May 2017 14:51:05 +0200
- Subject: Re: [PATCH v2 1/2] manual: Add preadv and pwritev documentation
- Authentication-results: sourceware.org; auth=none
- References: <1493901791-13438-1-git-send-email-adhemerval.zanella@linaro.org>
* 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.)
> +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).