System.loadLibrary and CLinker.lookup exception

[Ty Young]

Getting a symbol from a specific library has been possible for over a 
month in the preview branch:


https://github.com/openjdk/panama-foreign/blob/foreign-preview/src/java.base/share/classes/java/lang/foreign/SymbolLookup.java


Specifically this static method or the Path overload version:


https://github.com/openjdk/panama-foreign/blob/foreign-preview/src/java.base/share/classes/java/lang/foreign/SymbolLookup.java#L126 



There are no builds yet of the preview branch AFAIK so you'll need to 
build from source.


Downcall/upcall don't really have anything to do with finding a 
function. Maybe "CLinker" is going to get renamed or its content moved 
at some point in order to reflect the new API...?


On 4/26/22 10:02 AM, Radosław Smogura wrote:
> Hi all,
>
> I wanted to touch this topic before, because I got catch in this trap, too.
>
> I wonder if there’s a way to slightly refactor API to make it more “browsable” – I mean that potential user of API would see that there are two different lookups – system one, and loader one.
>
> One thing which came from top of my head is to separate SymbolLookup from Clinker, in particularly:
>
> Make SymbolLookup having following static methods: loaderLookup and systemLookup and instance - lookup.
>
> Narrow Clinker responsibility only to create up & down call stubs based on lookups from particular “version” of SymbolLookup.
>
> Kind regards,
> Radoslaw Smogura
>
> Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows
>
> ________________________________
> From: Jack Andrews <effbiae at gmail.com>
> Sent: Tuesday, April 26, 2022 1:23:25 PM
> To: Radosław Smogura <mail at smogura.eu>
> Cc: panama-dev at openjdk.java.net <panama-dev at openjdk.java.net>
> Subject: Re: System.loadLibrary and CLinker.lookup exception
>
> On Tue, 26 Apr 2022 at 20:51, Radosław Smogura <mail at smogura.eu<mailto:mail at smogura.eu>> wrote:
>
> Hi Jack,
>
>
>
> Can you try SymbolLookup (Java SE 18 & JDK 18) (oracle.com)<https://docs.oracle.com/en/java/javase/18/docs/api/jdk.incubator.foreign/jdk/incubator/foreign/SymbolLookup.html#loaderLookup()>
>
>
> that works: SymbolLookup.loaderLookup() lets me find kinit
>
> has CLinker.systemCLinker().lookup() been deprecated?
> if so,
>    https://github.com/openjdk/panama-foreign/blob/foreign-jextract/doc/panama_ffi.md
> needs updating
>
> $ cat Example.java
> import jdk.incubator.foreign.*;
> import java.lang.invoke.*;
> import static jdk.incubator.foreign.ValueLayout.*;
>
> public class Example {
>
>      static CLinker LINKER = CLinker.systemCLinker();
>
>      public static void main(String[] args) throws Throwable {
>          System.loadLibrary("k");
>          SymbolLookup L = SymbolLookup.loaderLookup();
>          NativeSymbol S = L.lookup("kinit").get();
>          System.out.println(S);
>          MethodHandle kinit = LINKER.downcallHandle(
>                  L.lookup("kinit").get(),
>                  FunctionDescriptor.of(JAVA_INT)
>          );
>          int r = (int) kinit.invoke();
>          System.out.println(r);
> /*20*/  NativeSymbol T = LINKER.lookup("kinit").get();
>          System.out.println(T);
>      }
> }
> $ LD_LIBRARY_PATH=. java --add-modules jdk.incubator.foreign --enable-native-access=ALL-UNNAMED Example.java
> WARNING: Using incubator modules: jdk.incubator.foreign
> warning: using incubating module(s): jdk.incubator.foreign
> 1 warning
> NativeSymbolImpl[name=kinit, address=MemoryAddress{ offset=0x7fad2ac4a6f0 }, scope=jdk.internal.foreign.ResourceScopeImpl$GlobalScopeImpl at 3b2c72c2]
> 717736192
> Exception in thread "main" java.util.NoSuchElementException: No value present
> at java.base/java.util.Optional.get(Optional.java:143)
> at Example.main(Example.java:20)
>
>