A simple jextract API plugin to generate all struct layouts in a given header file.

JStruct.java

import jdk.incubator.foreign.GroupLayout;
import jdk.incubator.foreign.MemoryLayout;
import jdk.incubator.foreign.SequenceLayout;
import jdk.incubator.foreign.ValueLayout;
import jdk.incubator.jextract.Declaration;
import jdk.incubator.jextract.JextractTask;

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

/**
 * Simple jextract API plugin to generate all struct layouts in a given header file. For simplicity,
 * all layouts are considered to be little endian. Use like this:
 * <blockquote><pre>{@code
 * $ <PANAMA_JDK>/bin/java --add-modules jdk.incubator.jextract src/JStruct.java <header-file>
 * }</pre></blockquote>
 */
public class JStruct {
    public static void main(String[] args) {
        if (args.length == 0) {
            System.err.println("Expected header file");
            System.exit(2);
        }
        Path header = Paths.get(args[0]);
        if (!Files.isReadable(header)) {
            System.err.println("Cannot read header file: " + args[0]);
            System.exit(2);
        }

        var task = JextractTask.newTask(false, header);
        Path builtinInc = Paths.get(System.getProperty("java.home"), "conf", "jextract");
        var decl = task.parse("-I" + builtinInc);
        decl.accept(new StructPrinter(), null);
    }

    static class StructPrinter implements Declaration.Visitor<Void, Void> {

        static String ALIGN_STR = "                                                                                            ";
        
        int align = 0;
        
        void indent() {
            print(ALIGN_STR.substring(0, align));
        }
        
        void incrAlign() {
            align += 4;
        }
        
        void decrAlign() {
            align -= 4;
        }
        
        void print(String s) {
            System.out.print(s);
        }
        
        @Override
        public Void visitScoped(Declaration.Scoped d, Void aVoid) {
            if (d.layout().isPresent()) {
                printLayout(d.name(), d.layout().get());
            } else {
                d.members().stream()
                        .filter(m -> m instanceof Declaration.Scoped)
                        .forEach(m -> m.accept(this, null));
            }
            return null;
        }

        protected void printLayout(String elementName, MemoryLayout layout) {
            print("MemoryLayout " + elementName + "$LAYOUT = ");
            printLayout(layout);
            print(";\n\n");
        }

        private void printLayout(MemoryLayout l) {
            if (l instanceof ValueLayout) {
                String valueLayout = switch ((int)l.byteSize()) {
                    case 1 -> "MemoryLayouts.BITS_8_LE";
                    case 2 -> "MemoryLayouts.BITS_16_LE";
                    case 4 -> "MemoryLayouts.BITS_32_LE";
                    case 8 -> "MemoryLayouts.BITS_64_LE";
                    default -> "MemoryLayout.ofValueBits(" + l.bitSize() + ", ByteOrder.LITTLE_ENDIAN)";
                };
                print(valueLayout);
            } else if (l instanceof SequenceLayout) {
                print("MemoryLayout.ofSequence(");
                if (((SequenceLayout) l).elementCount().isPresent()) {
                    print(((SequenceLayout) l).elementCount().getAsLong() + ", ");
                }
                printLayout(((SequenceLayout) l).elementLayout());
                print(")");
            } else if (l instanceof GroupLayout) {
                if (((GroupLayout) l).isStruct()) {
                    print("MemoryLayout.ofStruct(\n");
                } else {
                    print("MemoryLayout.ofUnion(\n");
                }
                incrAlign();
                String delim = "";
                for (MemoryLayout e : ((GroupLayout) l).memberLayouts()) {
                    print(delim);
                    indent();
                    printLayout(e);
                    delim = ",\n";
                }
                print("\n");
                decrAlign();
                indent();
                print(")");
            } else {
                //padding
                print("MemoryLayout.ofPaddingBits(" + l.bitSize() + ")");
            }
            if (l.name().isPresent()) {
                print(".withName(\"" +  l.name().get() + "\")");
            }
        }
    }
}