如何使用Java 17外部函数与内存API连接由Linux C++应用通过ftok()和shmget()创建的共享内存？

Great question! Let's break this down clearly, since Linux System V shared memory (the kind your C++ app uses with ftok() and shmget()) behaves differently than the file-backed shared memory Java's MemorySegment.mapFile() was designed for.

Key Background #

First, a quick clarification: MemorySegment.mapFile() works with file-backed shared memory (like POSIX shared memory created with shm_open()/mmap()), which is tied to a visible file in /dev/shm. System V shared memory, though it has a hidden mapping in /dev/shm, is managed via kernel keys and IDs—not directly via file paths. That means you can't use mapFile() to connect to it directly.

Connecting to System V Shared Memory with Java 17 FFM #

To link your Java app to the existing System V shared memory, you do need to call the same Linux system functions (ftok(), shmget(), shmat()) via Java's Foreign Function & Memory (FFM) API. Here's a step-by-step implementation:

Step 1: Link to libc Functions #

First, we'll define method handles for the libc functions we need to call:

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

public class SysVSharedMemoryClient {
    private static final Linker NATIVE_LINKER = Linker.nativeLinker();
    private static final SymbolLookup LIBC = NATIVE_LINKER.defaultLookup();

    // Method handle for ftok() - generates a key from a file and project ID
    private static final MethodHandle FTOK = NATIVE_LINKER.downcallHandle(
        LIBC.find("ftok").orElseThrow(() -> new RuntimeException("ftok not found in libc")),
        FunctionDescriptor.of(JAVA_INT, ADDRESS, JAVA_INT)
    );

    // Method handle for shmget() - retrieves the shared memory ID for a key
    private static final MethodHandle SHMGET = NATIVE_LINKER.downcallHandle(
        LIBC.find("shmget").orElseThrow(() -> new RuntimeException("shmget not found in libc")),
        FunctionDescriptor.of(JAVA_INT, JAVA_INT, JAVA_LONG, JAVA_INT)
    );

    // Method handle for shmat() - attaches the shared memory segment to our process
    private static final MethodHandle SHMAT = NATIVE_LINKER.downcallHandle(
        LIBC.find("shmat").orElseThrow(() -> new RuntimeException("shmat not found in libc")),
        FunctionDescriptor.of(ADDRESS, JAVA_INT, ADDRESS, JAVA_INT)
    );

    // Method handle for shmdt() - detaches the shared memory segment
    private static final MethodHandle SHMDT = NATIVE_LINKER.downcallHandle(
        LIBC.find("shmdt").orElseThrow(() -> new RuntimeException("shmdt not found in libc")),
        FunctionDescriptor.of(JAVA_INT, ADDRESS)
    );

Step 2: Connect to the Shared Memory #

Next, replicate the key generation and shared memory attachment logic from your C++ app:

public static void main(String[] args) throws Throwable {
        // Match the exact file path and project ID used in your C++ app
        String sharedFilePath = "/path/to/your/cpp/app/shared/file";
        int projectId = 0x42; // Must be identical to the proj_id in your C++ ftok() call
        long shmSize = 4096; // Must match the size used in your C++ shmget() call

        try (Arena arena = Arena.ofConfined()) {
            // 1. Generate the same key as the C++ app
            MemorySegment pathSegment = arena.allocateFrom(sharedFilePath);
            int sharedKey = (int) FTOK.invokeExact(pathSegment.address(), projectId);
            if (sharedKey == -1) {
                throw new RuntimeException("ftok failed - check file path exists and permissions");
            }

            // 2. Retrieve the shared memory ID
            int shmFlags = 0666; // Match the permissions from your C++ app (no IPC_CREAT needed for existing shm)
            int shmId = (int) SHMGET.invokeExact(sharedKey, shmSize, shmFlags);
            if (shmId == -1) {
                throw new RuntimeException("shmget failed - shared memory may not exist or permissions are wrong");
            }

            // 3. Attach the shared memory segment to a Java MemorySegment
            MemorySegment sharedMemory = (MemorySegment) SHMAT.invokeExact(shmId, 0L, 0);
            if (sharedMemory.address() == -1) {
                throw new RuntimeException("shmat failed - could not attach shared memory");
            }

            try {
                // Example: Read an integer from the start of the shared memory
                int value = sharedMemory.get(JAVA_INT, 0);
                System.out.printf("Read value from shared memory: %d%n", value);

                // Example: Write an integer back to the shared memory
                sharedMemory.set(JAVA_INT, 0, 98765);
                System.out.println("Written new value to shared memory");
            } finally {
                // 4. Always detach the shared memory when done
                int detachResult = (int) SHMDT.invokeExact(sharedMemory.address());
                if (detachResult == -1) {
                    throw new RuntimeException("shmdt failed - could not detach shared memory");
                }
            }
        }
    }
}

Critical Notes #

• Consistency is key: The file path, project ID, and shared memory size must be identical to what your C++ app uses. If even one differs, ftok() will generate a different key, and you won't find the shared memory.
• Permissions: Ensure your Java process has read/write access to the file used in ftok() and the shared memory segment itself.
• Cleanup: Always call shmdt() to detach the segment—failing to do so can leave resources hanging.

Is There a Simpler Alternative? #

If you have the flexibility to modify your C++ app, switching to POSIX shared memory (using shm_open() and mmap() instead of ftok()/shmget()) lets you use Java's MemorySegment.mapFile() directly. Here's why:

• POSIX shared memory creates a visible file in /dev/shm, which Java can map like any other file.
• You'd skip the FFM function calls entirely, using code like:

  try (MemorySegment sharedMem = MemorySegment.mapFile(
      Path.of("/dev/shm/your-shared-mem-file"),
      0,
      4096,
      FileChannel.MapMode.READ_WRITE,
      Arena.ofConfined()
  )) {
      // Read/write to sharedMem directly
  }
  

This is far simpler, but it requires updating your C++ code to use POSIX shared memory instead of System V.

Final Takeaway #

• If you can't modify the C++ app: You must use Java's FFM API to call ftok(), shmget(), and shmat()—there's no built-in shortcut for System V shared memory.
• If you can modify the C++ app: Switching to POSIX shared memory lets you use MemorySegment.mapFile() for a much cleaner implementation.

内容的提问来源于stack exchange，提问作者the.unknowing