Question about critical native function behavior with for-loop variables

[David]

Hi,

I have a question about the behavior of critical functions inside a
for-loop. I marked open() as critical (I know this is not an empty function
like the java docs tells me i should use critical for, but I really wanted
to try it). Wanting to see if it speeds things up. What I didn't expect was
that it doesn't work well with variables created inside the loop itself, or
at least that seems to be the case. Open() fails returning -1. To work
around this issue I created a "stableBuffer" just outside the loop, which
makes the code work all of the time.

I just have two questions. Is this expected behavior for critical
functions? Why does the stable buffer approach work consistently while
using variables inside the loop fail after a few iterations?

The loop that causes issues:

    @Test
    void test() {
        var paths = filesTooRead;

        byte[] stableBuffer = new byte[4096];

        for (var path : paths) {

            byte[] pathBytes = path.getBytes();
            Arrays.fill(stableBuffer, (byte) 0);
            System.arraycopy(pathBytes, 0, stableBuffer, 0, pathBytes.length);

            // works all the time
            int fd = openFile(stableBuffer, 0, 0);

            // works for a couple of iterations
//            int fd = openFile(path.getBytes(), 0, 0);
//            int fd =
openFile("/media/david/Data2/text_files/file_2299.bin".getBytes(), 0,
0);

            MemorySegment buffer = Arena.ofAuto().allocate(4);

            read(fd, buffer, 4);
            buffer.set(ValueLayout.JAVA_BYTE, 3, (byte) 0);

         //   System.out.println(path + "content " + buffer.getString(0));

            if (fd < 0) {
                System.err.println("Failed to open file:");
            }

            closeFile(fd);

        }
    }

OS: Pop!_OS 22.04 LTS
Java version: JDK 24 EA Build 35 (2025/2/4)


Thank you for your time and feedback.

Kind regards,
David Vlijmincx

The entire class:

package bench;

import org.junit.jupiter.api.Test;

import java.io.IOException;
import java.lang.foreign.*;
import java.lang.invoke.MethodHandle;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.Arrays;
import java.util.stream.Stream;

import static java.lang.foreign.ValueLayout.*;

public class CriticalLoopTest {

    public static final String BENCHMARK_FILE_EXTENSION = ".bin";
    public static final Path BASE_BENCHMARK_FILES_DIR =
Path.of("/media/david/Data2/text_files");
    public static final String[] filesTooRead;

    private static final MethodHandle open;
    private static final MethodHandle close;
    private static final MethodHandle read;

    static {
        Linker linker = Linker.nativeLinker();

        open = linker.downcallHandle(
                linker.defaultLookup().find("open").orElseThrow(),
                FunctionDescriptor.of(JAVA_INT, ADDRESS, JAVA_INT, JAVA_INT),
                Linker.Option.critical(true)
        );

        read = linker.downcallHandle(
                linker.defaultLookup().find("read").orElseThrow(),
                FunctionDescriptor.of(JAVA_INT, JAVA_INT, ADDRESS, JAVA_INT)
        );

        close = linker.downcallHandle(
                linker.defaultLookup().find("close").orElseThrow(),
                FunctionDescriptor.ofVoid(JAVA_INT)
        );

        try (Stream<Path> files = Files.walk(BASE_BENCHMARK_FILES_DIR)){
            filesTooRead = files
                    .filter(p ->
p.getFileName().toString().endsWith(BENCHMARK_FILE_EXTENSION))
                    .map(Path::toString)
                    .toArray(String[]::new);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }


    @Test
    void test() {
        var paths = filesTooRead;

        byte[] stableBuffer = new byte[4096];

        for (var path : paths) {

            byte[] pathBytes = path.getBytes();
            Arrays.fill(stableBuffer, (byte) 0);
            System.arraycopy(pathBytes, 0, stableBuffer, 0, pathBytes.length);

            // works all the time
            int fd = openFile(stableBuffer, 0, 0);

            // works sometimes
//            int fd = openFile(path.getBytes(), 0, 0);
//            int fd =
openFile("/media/david/Data2/text_files/file_2299.bin".getBytes(), 0,
0);

            MemorySegment buffer = Arena.ofAuto().allocate(4);

            read(fd, buffer, 4);
            buffer.set(ValueLayout.JAVA_BYTE, 3, (byte) 0);

         //   System.out.println(path + "content " + buffer.getString(0));

            if (fd < 0) {
                System.err.println("Failed to open file:");
            }

            closeFile(fd);

        }
    }

    public int read(int fd, MemorySegment mem, int len) {
        try {
            return (int) read.invokeExact(fd, mem, len);
        } catch (Throwable e) {
            throw new RuntimeException(e);
        }
    }

    public int openFile(byte[] filePath, int flags, int mode) {
        try {
            int fd = (int)
open.invokeExact(MemorySegment.ofArray(filePath), flags, mode);
            if (fd < 0) {
                throw new RuntimeException("Failed to open file fd=" + fd);
            }
            return fd;
        } catch (Throwable e) {
            throw new RuntimeException(e);
        }
    }

    public static void closeFile(int fd) {
        try {
            close.invokeExact(fd);
        } catch (Throwable e) {
            throw new RuntimeException("Could not close file with FD:" + fd, e);
        }
    }
}
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