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Use Wrapping for intended underflow of unsigned integer value. #369

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May 6, 2016
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Use Wrapping for intended underflow of unsigned integer value. #369

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8 changes: 6 additions & 2 deletions src/unistd.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -114,8 +114,12 @@ pub fn chdir<P: ?Sized + NixPath>(path: &P) -> Result<()> {
#[inline]
pub fn chown<P: ?Sized + NixPath>(path: &P, owner: Option<uid_t>, group: Option<gid_t>) -> Result<()> {
let res = try!(path.with_nix_path(|cstr| {
// We use `0 - 1` to get `-1 : {u,g}id_t` which is specified as the no-op value for chown(3).
unsafe { libc::chown(cstr.as_ptr(), owner.unwrap_or(0 - 1), group.unwrap_or(0 - 1)) }
// According to the POSIX specification, -1 is used to indicate that
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I think this comment is still relevant. What about you @fiveop?

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I thought about it; but then, we are not documenting all the other functions properly. So, for consistency I removed it. :)

If you do mind, I'll put it in again.

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I think this is kind of a weird value for a no-op and that line makes it a little clearer what we're trying to do with (Wrapping(0 as uid_t) - Wrapping(1 as uid_t)).0. Any opinions @kamalmarhubi?

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I'll add a comment.

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@kamalmarhubi kamalmarhubi May 6, 2016 •
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Any opinions @kamalmarhubi?

I remember asking for the comment back in #274 (comment) so... :-)

(I think it warrants the comment because it's a weird thing to do.)

// owner and group, respectively, are not to be changed. Since uid_t and
// gid_t are unsigned types, we use wrapping_sub to get '-1'.
unsafe { libc::chown(cstr.as_ptr(),
owner.unwrap_or((0 as uid_t).wrapping_sub(1)),
group.unwrap_or((0 as gid_t).wrapping_sub(1))) }
}));

Errno::result(res).map(drop)
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