diff --git a/bin/qasim-gui.rb b/bin/qasim-gui.rb
index c201c59..0cf8ac7 100755
--- a/bin/qasim-gui.rb
+++ b/bin/qasim-gui.rb
@@ -224,14 +224,18 @@ module Qasim
 		rescue Errno::EEXIST => e
 			# test if the other process still exist
 			masterpid = File.read(lockfname).strip
-			Process.kill(0, masterpid.to_i)
-			warn "error: Another instance of %s is already running." % APP_NAME
-			exit 1
-		rescue Errno::ESRCH => e
-			f = IO.open(fd)
-			f.syswrite( "#{Process.pid}\n" )
-			f.close
-			@app.exec
+			begin
+				Process.kill(0, masterpid.to_i)
+				warn "error: Another instance of %s is already running." % APP_NAME
+				exit 1
+			rescue Errno::ESRCH => e
+				fd = IO::sysopen( lockfname,
+								 Fcntl::O_WRONLY | Fcntl::O_EXCL )
+				f = IO.open(fd)
+				f.syswrite( "#{Process.pid}\n" )
+				f.close
+				@app.exec
+			end
 		ensure
 			masterpid = File.read(lockfname).strip
 			if masterpid.to_i == Process.pid then