diff --git a/src/ptp.rs b/src/ptp.rs
index 7807ea9..732abfb 100644
--- a/src/ptp.rs
+++ b/src/ptp.rs
@@ -280,3 +280,74 @@ pub use statime_linux_mod::{spawn_statime, StatimeDaemon, StatimeOptions};
 
 #[cfg(not(target_os = "linux"))]
 pub use statime_nonlinux_mod::{spawn_statime, StatimeDaemon, StatimeOptions};
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn display_formats() {
+        let io_err = std::io::Error::new(std::io::ErrorKind::Other, "oops");
+        let ptp_err: PtpError = io_err.into();
+        let s = ptp_err.to_string();
+        assert!(s.starts_with("I/O error: "));
+        assert!(s.contains("oops"));
+
+        assert_eq!(PtpError::ChronoOutOfRange.to_string(), "Chrono out-of-range error");
+        assert_eq!(PtpError::Unsupported.to_string(), "PTP is unsupported on this platform");
+    }
+
+    #[test]
+    #[cfg(target_os = "linux")]
+    fn supported_on_linux() {
+        assert!(is_supported());
+    }
+
+    #[test]
+    #[cfg(not(target_os = "linux"))]
+    fn unsupported_on_non_linux() {
+        assert!(!is_supported());
+    }
+
+    #[test]
+    #[cfg(not(target_os = "linux"))]
+    fn non_linux_ptp_clock_ops_unsupported() {
+        let r = PtpClock::open("/dev/ptp0");
+        match r {
+            Err(PtpError::Unsupported) => {}
+            other => panic!("expected Unsupported on non-Linux, got {:?}", other),
+        }
+
+        let opts = StatimeOptions::new("lo").with_phc_index(0).with_args(["--debug"]);
+        let r = spawn_statime(opts);
+        match r {
+            Err(PtpError::Unsupported) => {}
+            other => panic!("expected Unsupported on non-Linux, got {:?}", other),
+        }
+    }
+
+    #[test]
+    #[cfg(target_os = "linux")]
+    fn linux_open_invalid_device_returns_io() {
+        let r = PtpClock::open("/dev/this_device_should_not_exist_ptp99999");
+        match r {
+            Err(PtpError::Io(_)) => {}
+            Ok(_) => panic!("expected Io error for invalid device, got Ok"),
+            Err(e) => panic!("expected Io error, got {:?}", e),
+        }
+    }
+
+    #[test]
+    #[cfg(target_os = "linux")]
+    fn linux_spawn_statime_ok_or_not_found() {
+        let opts = StatimeOptions::new("lo");
+        let res = spawn_statime(opts);
+        match res {
+            Ok(mut d) => {
+                let _ = d.stop();
+            }
+            Err(PtpError::Io(_)) => { /* not found or failed to spawn: acceptable */ }
+            Err(e) => panic!("unexpected error: {:?}", e),
+        }
+    }
+}