Fix Issue 15939

src/core/memory.d

@@ -38,7 +38,7 @@
  *
  * Notes_to_implementors:
  * $(UL
- * $(LI On POSIX systems, the signals SIGUSR1 and SIGUSR2 are reserved
+ * $(LI On POSIX systems, the signals `SIGRTMIN` and `SIGRTMIN + 1` are reserved
  *   by this module for use in the garbage collector implementation.
  *   Typically, they will be used to stop and resume other threads
  *   when performing a collection, but an implementation may choose

src/core/thread.d

@@ -1908,7 +1908,7 @@ version( CoreDdoc )
 {
     /**
      * Instruct the thread module, when initialized, to use a different set of
-     * signals besides SIGUSR1 and SIGUSR2 for suspension and resumption of threads.
+     * signals besides `SIGRTMIN` and `SIGRTMIN + 1` for suspension and resumption of threads.
      * This function should be called at most once, prior to thread_init().
      * This function is Posix-only.
      */
@@ -1966,12 +1966,13 @@ extern (C) void thread_init()
     {
         if( suspendSignalNumber == 0 )
         {
-            suspendSignalNumber = SIGUSR1;
+            suspendSignalNumber = SIGRTMIN;
         }
 
         if( resumeSignalNumber == 0 )
         {
-            resumeSignalNumber = SIGUSR2;
+            resumeSignalNumber = SIGRTMIN + 1;
+            assert(resumeSignalNumber <= SIGRTMAX);
         }
 
         int         status;