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Date: Mon, 19 Aug 2019 20:56:35 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] Add copy_file_range system call

On Mon, Aug 19, 2019 at 11:41:14PM +0000, Árni Dagur wrote:
> This patch was based on commit 53147f9, which added splice and vmsplice.
> ---
>  The function signature in the glibc manpage specifies `loff_t` instead
>  of `off_t`, for both `copy_file_range` and `splice`. In musl, however, 
>  the function signature for `splice` specifies `off_t`, so I did the
>  same here. I'm not an experienced C programmer, so that may have been
>  wrong.

I think this looks ok. Regarding loff_t vs off_t, loff_t is the
kernel's API type for functions that take a 64-bit offset
unconditionally rather than glibc providing 32-bit off_t and 64-bit
off_t versions of them. This is gratuitous for musl where off_t is
always 64-bit. We provide loff_t as a macro that expands to off_t, but
even if it were a typedef the types woule be the same type, so it's
fine to use off_t here, and I think it's the cleanest and most
consistent with what we're doing elsewhere even if it's not textually
the same as the man page.

>  include/unistd.h            | 1 +
>  src/linux/copy_file_range.c | 8 ++++++++
>  2 files changed, 9 insertions(+)
>  create mode 100644 src/linux/copy_file_range.c
> 
> diff --git a/include/unistd.h b/include/unistd.h
> index 9485da7a..00cc7042 100644
> --- a/include/unistd.h
> +++ b/include/unistd.h
> @@ -188,6 +188,7 @@ char *get_current_dir_name(void);
>  int syncfs(int);
>  int euidaccess(const char *, int);
>  int eaccess(const char *, int);
> +ssize_t copy_file_range(int fd_in, off_t *off_in, int fd_out, off_t *off_out, size_t len, unsigned flags);
>  #endif

Is there a reason for the choice to put it in unistd.h? Similar
functions seem to have gone in fcntl.h. unistd.h does not make the
loff_t type available which could be problematic to callers using it,
since they really should (for API compatibility) be declaring the
objects whose addresses they pass as loff_t.

If glibc does it here and exposes loff_t in unistd.h we might need to
consider doing that too with _GNU_SOURCE.

Rich

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