Added fd-aliasing for Un/ManagedFD

Fortune for mapped-file's current commit: Great blessing − 大吉

Cargo.toml

@@ -2,7 +2,7 @@
 name = "mapped-file"
 description = "Construct a memory mapping over any file object"
 keywords = ["unix", "mmap", "generic", "file", "fd"]
-version = "0.0.2"
+version = "0.0.3"
 edition = "2021"
 repository="https://github.com/notflan/mapped-file"
 license="MIT"

src/file/managed.rs

@@ -24,6 +24,8 @@ impl Clone for ManagedFD {
     }
 }
 
+//TODO: io::Read/io::Write impls for ManagedFD
+
 impl ManagedFD
 {
     #[inline] 
@@ -32,6 +34,18 @@ impl ManagedFD
 	Self(UnmanagedFD::new_unchecked(fd))
     }
 
+    /// Duplicate a file-descriptor, aliasing the open resource for the lifetime of the returned `ManagedFD`..
+    #[inline]
+    pub fn alias(file: &(impl AsRawFd + ?Sized)) -> io::Result<Self>
+    {
+	let r = unsafe { libc::dup(file.as_raw_fd()) };
+	if let Some(r) = UnmanagedFD::new_raw(r) {
+	    Ok(Self(r))
+	} else {
+	    Err(io::Error::last_os_error())
+	}
+    }
+
     #[inline] 
     pub const fn take_raw(fd: RawFd) -> Self
     {
@@ -101,6 +115,3 @@ impl From<ManagedFD> for std::fs::File
 	}
     }
 }
-
-
-//TODO: implement the rest of ManagedFD from `memfd` module in `utf8encode`

src/file/memory.rs

@@ -234,4 +234,4 @@ impl From<MemoryFile> for std::fs::File
     }
 }
 
-//TODO: implement `memfd` from `utf8encode`.
+//TODO: io::Read/io::Write impls for MemoryFile

src/file/unmanaged.rs

@@ -79,4 +79,12 @@ impl AsRawFd for UnmanagedFD
     }
 }
 
+impl From<UnmanagedFD> for ManagedFD {
+    #[inline]
+    fn from(from: UnmanagedFD) -> Self {
+	Self::take(unsafe { UnmanagedFD::new_unchecked( c_try!(libc::dup(from.get()) => if |x| x < 0; "dup(): failed to duplicate file descriptor {}", from.get()) ) })
+
+    }
+}
+
 //TODO: implement a full version of the temporary struct `UnmanagedFD` from `utf8encode`