[foreign] RFR 8211060: Library.getDefault() does not work on Windows

Jorn Vernee jbvernee at xs4all.nl
Mon Sep 24 12:34:35 UTC 2018

I agree with the usability point. In C++ it's as simple to call puts as 

     #include <stdio.h>

     int main() {
         puts("Hello World!");

And I think the optimal Java equivalent would be something like:

     import static org.openjdk.stdio.*;

     public class Main {

         public static void main(String[] args) {
             puts("Hello World!");


This can be facilitated by creating a 'singleton facade' for the library 
interface like so:

     public class stdio {

         private static final stdioImpl lib = Libraries.bind(lookup(), 

         public static int puts (String message) {
             try(Scope scope = Scope.newNativeScope()) {
                 Pointer<Byte> msg = scope.toCString(message);
                 return lib.puts(msg);


Such a facade class could be shipped with the JDK or perhaps as an 
artifact on maven central, or maybe it could be an additional output of 

But there is only so much you can do automagically from the Java side. 
When working from C/C++ you have the compiler filling in the blanks. For 
instance, it automatically allocates storage for string literals. Java 
does that as well for Java string literals `String s = "Hello";`, but it 
can not do that for native strings, and you have to use the Scope API to 
do that manually. In some cases, like the above, you can write glue-code 
to make that automatic, but I think at some point things become too 
complex for that, and there will always be some usability barrier to 


Samuel Audet schreef op 2018-09-24 13:38:
> FWIW, I think the factory method pattern should be reconsidered
> entirely. In C/C++, when we want to call say getpid(), we don't start
> loading stuff up before calling getpid(), we call getpid()! Why not do
> the same from Java? From a usability point of view, not loading stuff
> manually works fine for JavaCPP...
> Now, I know you're going to start taking about interfaces and what
> not. You said that you had plans to introduce an entirely new array
> type just to make it more friendly with vector instructions and native
> libraries. Why not start thinking about an "interface" that would be
> friendly to native libraries as well? Why stop at arrays?
> Samuel
> On 09/24/2018 08:10 PM, Maurizio Cimadamore wrote:
>> Hi Jorn,
>> thanks for the patch. As mentioned last time we have two options here: 
>> one is to follow the approach forward in your patch; another would be 
>> to ditch Library.getDefault() entirely and adopt a more explicit 
>> approach - that is to always require 'implicit' libraries to be 
>> mentioned - either by jextract artifacts or by API points.
>> A note on the latter - when you do:
>> Libraries.bindRaw(lookup, Foo.class)
>> the code lookup the @NativeHeader annotation on Foo.class and tries to 
>> extract required library names from there. Currently, we do not add 
>> library names for standard libraries such as "c" or "m" (math), which 
>> is what led us down the (slippery?) slope of having a 
>> Library.getDefault somewhere.
>> Another option would be to have jextract to always generate the names 
>> of the libraries an artifact depends on, and then the API should throw 
>> an exception if a @NativeHeader is found with no libraries. More 
>> specifically, jextract should always add "c" to the set of used 
>> libraries in the @NativeHeader annotation (other libraries can be 
>> explicitly supplied using the -l flag).
>> Note that I'm not saying "the binder should always add in 'clib'" for 
>> you, because that's kind of a lame assumption: it works obviously well 
>> for C but it doesn't make a lot of sense if you want to use Panama for 
>> other purposes/languages. Which seems to suggest that, as far as the 
>> binder is concerned, library dependencies should always be explicit.
>> Thoughts?
>> Maurizio
>> On 24/09/18 11:52, Jorn Vernee wrote:
>>> Hello,
>>> I'd like to contribute a patch that adds support for the default 
>>> library on windows.
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8211060
>>> Diff: 
>>> https://gist.github.com/JornVernee/7d45780df082cbfb27417b437c7b13a8
>>> As mentioned before [1], this fixes 2 bugs:
>>> 1.) When no library was loaded ClassLoader#NativeLibrary#getFromClass
>>> threw an NPE (at least on windows). That is fixed by returning
>>> defaultLibrary.fromClass when the nativeLibraryContext is empty.
>>> 2.) The default library search was not working on windows. It was 
>>> using
>>> a default handle, which works on unix (dlsym(RTLD_DEFAULT)), but not 
>>> on
>>> windows (see https://stackoverflow.com/q/23437007). I have changed 
>>> the
>>> implementation from using a default handle to using a new native
>>> function findEntryInProcess, which does the right thing for Windows
>>> (iterate over all loaded modules), and does the same thing it used to
>>> for unix. findEntry is now a Java method, and the original findEntry 
>>> is
>>> renamed to findEntry0. The NativeLibrary implementation of findEntry
>>> forwards to findEntry0, and the anonymous class of the default 
>>> library
>>> overrides to forward to findEntryInProcess.
>>> Please test this patch on unix as well, since I don't have the 
>>> ability to do so.
>>> Jorn
>>> [1] : 
>>> http://mail.openjdk.java.net/pipermail/panama-dev/2018-September/002644.html

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