Prevent multiple initialization

README.md

@@ -8,7 +8,7 @@ Target side implementation of the RTT (Real-Time Transfer) I/O protocol. RTT imp
 
 ## Platform support
 
-To use the global `rprintln!` macro or the defmt implementation, a platform-specific [`critical-section`](https://github.com/rust-embedded/critical-section) implementation is needed for locking.
+A platform-specific [`critical-section`](https://github.com/rust-embedded/critical-section) implementation is needed to use this library.
 
 Output directly to a channel object with `write!` or the binary `write` method does not require locking and therefore does not need any platform-specific critical section.
 

rtt-target/src/init.rs

@@ -121,6 +121,7 @@ macro_rules! rtt_init {
     } => {{
         use core::mem::MaybeUninit;
         use core::ptr;
+        use core::cell::Cell;
         use $crate::UpChannel;
         use $crate::DownChannel;
         use $crate::rtt::*;
@@ -137,6 +138,20 @@ macro_rules! rtt_init {
         #[export_name = "_SEGGER_RTT"]
         pub static mut CONTROL_BLOCK: MaybeUninit<RttControlBlock> = MaybeUninit::uninit();
 
+        #[allow(unused)]
+        #[export_name = "rtt_init_must_not_be_called_multiple_times"]
+        fn rtt_init_must_not_be_called_multiple_times() { }
+
+        use ::rtt_target::export::critical_section;
+
+        static INITIALIZED: critical_section::Mutex<Cell<bool>> = critical_section::Mutex::new(Cell::new(false));
+        critical_section::with(|cs| {
+            if INITIALIZED.borrow(cs).get() {
+                panic!("rtt_init! must not be called multiple times");
+            }
+            INITIALIZED.borrow(cs).set(true);
+        });
+
         unsafe {
             ptr::write_bytes(CONTROL_BLOCK.as_mut_ptr(), 0, 1);
 

rtt-target/src/lib.rs

@@ -438,3 +438,10 @@ impl Drop for TerminalWriter<'_> {
         }
     }
 }
+
+/// Used to reexport items for use in macros. Do not use directly.
+/// Not covered by semver guarantees.
+#[doc(hidden)]
+pub mod export {
+    pub use critical_section;
+}