feat: Add UPPER_FDTBL_TAG

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "redox_syscall"
-version = "0.5.16"
+version = "0.5.17"
 description = "A Rust library to access raw Redox system calls"
 license = "MIT"
 authors = ["Jeremy Soller <jackpot51@gmail.com>"]

src/flag.rs

@@ -401,3 +401,6 @@ bitflags! {
         const FD_UPPER = 1 << 14;
     }
 }
+
+/// The tag for the fd number in the upper file descriptor table.
+pub const UPPER_FDTBL_TAG: usize = 1 << (usize::BITS - 2);