Posting to a POSIX semaphore using JNA

Question

I am trying to post to a semaphore using JNA on a Linux machine. For some reason, I always receive a 22 error (invalid argument) even for this simple example. In my understanding, should the below code not open a POSIX semaphore, post to it and close it again?

public class Sample {

  private static final int O_CREAT = 0x40;

  public static void main(String[] args) throws Exception {
    File notifier = new File("/tmp", "_test" + new Random().nextInt());
      if (!notifier.isFile() && !notifier.createNewFile()) {
        throw new IllegalStateException("Could not create notifier: " + notifier);
      }

      SempahoreLibrary library = Native.load("c", SempahoreLibrary.class);
      Pointer semaphore = library.sem_open(notifier.getAbsolutePath(), O_CREAT, 666, 0);
      try {
        library.sem_post(semaphore);
      } finally {
        library.sem_close(semaphore);
      }
  }

  interface SempahoreLibrary extends Library {
    Pointer sem_open(String name, int flags, int mode, int value) throws LastErrorException;
    int sem_post(Pointer pointer) throws LastErrorException;
    int sem_close(Pointer pointer) throws LastErrorException;
  }
}

Answer 1

I initially couldn't make it work with JNR either (strongly recommended over JNA), and got curious. Writing it in C helped.. :)

An strace on the C port made it clear you don't have to create a file upfront and then "map" the semaphore to it. Also using the full path is wrong, because semaphores are created in /dev/shm and the "/" in the path screws up everything:

futex(0x7f731b1190d0, FUTEX_WAKE_PRIVATE, 2147483647) = 0
openat(AT_FDCWD, "/dev/shm/sem.sema", O_RDWR|O_NOFOLLOW) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=32, ...}) = 0

So you should be able to remove the whole file/path creation and just use a regular non-path name for the semaphore in sem_open. Also the file mode should be octal, and you should make sure to also load the pthread library - it's required.

Here is a working example in C:

// clang -Wall sema.c -lpthread

#include <fcntl.h>
#include <sys/stat.h>
#include <semaphore.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>

int main(int argc, char** argv)
{
    sem_t* s = sem_open("notifier", O_CREAT, 0644, 0);

    if (!s) {
        perror("sem_open");
        exit(errno);
    }

    printf("s: %p\n", s);

    sem_post(s);

    int value = -1;
    sem_getvalue(s, &value);
    printf("value: %d\n", value);

    sem_wait(s);
    sem_getvalue(s, &value);
    printf("value: %d\n", value);

    sem_close(s);

    exit(EXIT_SUCCESS);
}

Here is a working Java version using JNR:

import jnr.ffi.LastError;
import jnr.ffi.LibraryLoader;
import jnr.ffi.Pointer;
import jnr.ffi.Runtime;

public class Semaphore
{
    private static final int O_CREAT = 0x40;

    public interface SempahoreLibrary
    {
        Pointer sem_open(String name, int flags, int mode, int value);
        int sem_post(Pointer pointer);
        int sem_close(Pointer pointer);
    }

    public static void main(String[] args) throws Exception
    {

        LibraryLoader<SempahoreLibrary> loader = LibraryLoader.create(SempahoreLibrary.class);
        loader.library("c");
        loader.library("pthread");

        SempahoreLibrary library = loader.load();
        jnr.ffi.Runtime runtime = Runtime.getRuntime(library);

        Pointer semaphore = library.sem_open("notifier", O_CREAT, 0644, 0);
        if (semaphore == null)
        {
            int errno = LastError.getLastError(runtime);
            System.out.println("sem_open: " + errno);
            System.exit(errno);
        }

        System.out.println("semaphore: " + Long.toHexString(semaphore.address()));

        try
        {
            int error = library.sem_post(semaphore);
            System.out.println("post: " + (error == 0 ? "OK" : LastError.getLastError(runtime)));
        }
        finally
        {
            int error = library.sem_close(semaphore);
            System.out.println("close: " + (error == 0 ? "OK" : LastError.getLastError(runtime)));
        }
    }
}