Introduce `mmap_nofd` to work around `<F: AsFd>`?

[newpavlov]

Today when I use nix's mmap for creating anonymous mappings I have to use mmap::<BorrowedFd>(...), which feels... quite awkward. It would be nice to have a variant of mmap with the following signature:

pub unsafe fn mmap_nofd(
    addr: Option<NonZeroUsize>,
    length: NonZeroUsize,
    prot: ProtFlags,
    flags: MapFlags,
) -> Result<*mut c_void>

The offset argument has meaning only for file-backed mappings, so we can remove it as well.

[newpavlov]

Alternatively, it can be named as mmap_anonymous.

[asomers]

For that matter, is F: AsFd the best argument type for non-anonymous mappings? Would anybody ever want to supply an OwnedFd? If not, then maybe the argument type should be Option<BorrowedFd>.

[SteveLauC]

For anonymous mmap

Today when I use nix's mmap for creating anonymous mappings I have to use mmap::<BorrowedFd>(...), which feels quite awkward

It is indeed awkward to have to specify the generic argument even though we don't use it at all, so perhaps a dedicated anonymous function would be great here?

BTW, if we are gonna add this function, mmap_anonymous seems to be a better name than mmap_nofd.

---

For file-backed (non-anonymous) mmap

Is F: AsFd the best argument type for non-anonymous mappings? Would anybody ever want to supply an OwnedFd? If not, then maybe the argument type should be Option<BorrowedFd>.

From the Linux man page:

After the mmap() call has returned, the file descriptor, fd, can be closed immediately without invalidating the mapping.

It seems to be ok to close that fd after calling mmap(..., fd, ...)

[JonathanWoollett-Light]

mmap_anonymous makes the most sense to me.