Make tweaks suggested by @wmeissner.

* Use HashMap + synchronized block for signal handlers.
* Bind native signal function with intptr_t return type.

src/main/java/jnr/posix/BaseNativePOSIX.java

@@ -19,7 +19,8 @@
 import java.nio.ByteBuffer;
 import java.nio.charset.Charset;
 import java.util.Collection;
-import java.util.Hashtable;
+import java.util.HashMap;
+import java.util.Map;
 import jnr.constants.platform.Signal;
 
 abstract class BaseNativePOSIX extends NativePOSIX implements POSIX {
@@ -29,7 +30,7 @@ abstract class BaseNativePOSIX extends NativePOSIX implements POSIX {
     protected final POSIXHandler handler;
     protected final JavaLibCHelper helper;
 
-    private final Hashtable<Signal, SignalHandler> signalHandlers = new Hashtable();
+    protected final Map<Signal, SignalHandler> signalHandlers = new HashMap();
 
     BaseNativePOSIX(String libraryName, LibCProvider libcProvider, POSIXHandler handler) {
         this.handler = handler;
@@ -223,19 +224,21 @@ public int kill(int pid, int signal) {
     }
 
     public SignalHandler signal(Signal sig, final SignalHandler handler) {
-        SignalHandler old = signalHandlers.get(sig);
-
-        int result = libc().signal(sig.intValue(), new LibC.LibCSignalHandler() {
-            public void signal(int sig) {
-                handler.handle(sig);
+        synchronized (signalHandlers) {
+            SignalHandler old = signalHandlers.get(sig);
+
+            long result = libc().signal(sig.intValue(), new LibC.LibCSignalHandler() {
+                public void signal(int sig) {
+                    handler.handle(sig);
+                }
+            });
+
+            if (result != -1) {
+                signalHandlers.put(sig, handler);
             }
-        });
-
-        if (result != -1) {
-            signalHandlers.put(sig, handler);
+
+            return old;
         }
-
-        return old;
     }
 
     public int lchmod(String filename, int mode) {

src/main/java/jnr/posix/LibC.java

@@ -41,6 +41,7 @@
 
 import java.nio.ByteBuffer;
 import jnr.ffi.annotations.Delegate;
+import jnr.ffi.types.intptr_t;
 
 public interface LibC {
     int chmod(CharSequence filename, int mode);
@@ -79,7 +80,7 @@ public interface LibC {
     public interface LibCSignalHandler {
         @Delegate void signal(int sig);
     }
-    int signal(int sig, LibCSignalHandler handler);
+    @intptr_t long signal(int sig, LibCSignalHandler handler);
     int lchmod(CharSequence filename, int mode);
     int lchown(CharSequence filename, int user, int group);
     int link(CharSequence oldpath, CharSequence newpath);