Prepare for calling select(2)

Cargo.lock

@@ -109,4 +109,5 @@ name = "xremap"
 version = "0.1.0"
 dependencies = [
  "evdev",
+ "libc",
 ]

Cargo.toml

@@ -7,3 +7,4 @@ edition = "2021"
 
 [dependencies]
 evdev = "0.11.3"
+libc = "0.2"

src/input.rs

@@ -1,5 +1,7 @@
 extern crate evdev;
 
+use std::os::unix::io::{AsRawFd, RawFd};
+use std::{io, mem, ptr};
 use evdev::Device;
 
 pub fn select_device() -> Device {

src/main.rs

@@ -1,10 +1,14 @@
 use std::error::Error;
 
 mod input;
+mod select;
 
 fn main() -> Result<(), Box<dyn Error>> {
     let mut device = input::select_device();
     device.grab()?;
+    if select::is_readable(&mut device) {
+        println!("event!")
+    }
     device.ungrab()?;
     Ok(())
 }

src/select.rs

@@ -0,0 +1,61 @@
+extern crate evdev;
+
+use std::os::unix::io::{AsRawFd, RawFd};
+use std::{io, mem, ptr};
+use evdev::Device;
+
+// A call of this function blocks until the argument device becomes readable.
+// TODO: support selecting multiple devices
+pub fn is_readable(device: &mut Device) -> bool {
+    let mut fd_set = FdSet::new(); // TODO: maybe this should be prepared in the caller
+    fd_set.set(device.as_raw_fd());
+
+    let result = match unsafe {
+        libc::select(
+            device.as_raw_fd() + 1,
+            to_fdset_ptr(Some(&mut fd_set)),
+            to_fdset_ptr(None),
+            to_fdset_ptr(None),
+            to_ptr::<libc::timeval>(None) as *mut libc::timeval,
+        )
+    } {
+        -1 => Err(io::Error::last_os_error()),
+        res => Ok(res as usize),
+    }.unwrap();
+    return result == 1 && fd_set.is_set(device.as_raw_fd())
+}
+
+fn to_fdset_ptr(opt: Option<&mut FdSet>) -> *mut libc::fd_set {
+    match opt {
+        None => ptr::null_mut(),
+        Some(&mut FdSet(ref mut raw_fd_set)) => raw_fd_set,
+    }
+}
+
+fn to_ptr<T>(opt: Option<&T>) -> *const T {
+    match opt {
+        None => ptr::null::<T>(),
+        Some(p) => p,
+    }
+}
+
+struct FdSet(libc::fd_set);
+
+impl FdSet {
+    fn new() -> FdSet {
+        unsafe {
+            // Is the memory released properly?
+            let mut raw_fd_set = mem::MaybeUninit::<libc::fd_set>::uninit();
+            libc::FD_ZERO(raw_fd_set.as_mut_ptr());
+            FdSet(raw_fd_set.assume_init())
+        }
+    }
+
+    fn set(&mut self, fd: RawFd) {
+        unsafe { libc::FD_SET(fd, &mut self.0) }
+    }
+
+    fn is_set(&mut self, fd: RawFd) -> bool {
+        unsafe { libc::FD_ISSET(fd, &mut self.0) }
+    }
+}