Add a runtime::sigrt function for constructing no-libc "realtime" s…

…ignals.

src/runtime.rs

@@ -613,3 +613,24 @@ pub const SIGRTMAX: u32 = {
         linux_raw_sys::general::_NSIG - 1
     }
 };
+
+/// Return a signal corresponding to `SIGRTMIN + n`.
+///
+/// This is similar to [`Signal::rt`], but uses the raw OS `SIGRTMIN` value
+/// instead of the libc `SIGRTMIN` value. Don't use this unless you know your
+/// code won't share a process with a libc (perhaps because you yourself are
+/// implementing a libc).
+#[cfg(linux_raw)]
+#[doc(alias = "SIGRTMIN")]
+pub fn sigrt(n: u32) -> Option<Signal> {
+    let sig = SIGRTMIN.wrapping_add(n);
+    if (SIGRTMIN..=SIGRTMAX).contains(&sig) {
+        // SAFETY: We've checked that `sig` is in the expected range. It could
+        // still conflict with libc's reserved values, however users of the
+        // `runtime` module here must already know that there's no other libc
+        // to conflict with.
+        Some(unsafe { Signal::from_raw_unchecked(sig as i32) })
+    } else {
+        None
+    }
+}