How to memory map (mmap) a linux block device (e.g. /dev/sdb) in Java?

Question

I can read/write a linux block device with Java using java.nio. The following code works:

Path fp = FileSystems.getDefault().getPath("/dev", "sdb");
FileChannel fc = null;
try {
  fc = FileChannel.open(fp, EnumSet.of(StandardOpenOption.READ, StandardOpenOption.WRITE));
} catch (Exception e) {
  System.out.println("Error opening file: " + e.getMessage());
}
ByteBuffer buf = ByteBuffer.allocate(50);
try {
  if(fc != null)
    fc.write(buf);
} catch (Exception e) {
  System.out.println("Error writing to file: " + e.getMessage());
}

However, memory mapping does not work. The following code fails:

MappedByteBuffer mbb = null;
try {
  mbb = fc.map(FileChannel.MapMode.READ_WRITE, 0, 100);
} catch (IOException e) {
  System.out.println("Error mapping file: " + e.getMessage());
}

This fails with Error:

java.io.IOException: Invalid argument
    at sun.nio.ch.FileDispatcherImpl.truncate0(Native Method)
    at sun.nio.ch.FileDispatcherImpl.truncate(FileDispatcherImpl.java:79)
    at sun.nio.ch.FileChannelImpl.map(FileChannelImpl.java:817)

Is there a work around to this? Perhaps by using a different library? I read somewhere that maybe by using JNI I could do this, but I couldn't find any sources.

Answer 1

According to the documentation the mechanics of actually mapping a file are left to the implementation. It appears that the implementation is attempting to truncate the file (maybe because the block device size is not the same as the size you specify?).

I am curious why you are reading from the block device directly (unless you are trying to write some sort of filesystem utility or something that needs to do raw I/O). If you need to read from the block device as a memory mapped file directly, you may need to write some C/C++ code to map the file and handle reading/writing to it and use a Java/JNI bridge class to bridge calls to your C/C++ code. That way you handle calling mmap() yourself and can specify any options you need. Looking at the mmap() documentation you may not be able to specify block devices on your platform (I'm guessing Linux but I could be wrong).

If you absolutely need to do this within Java you may need to do read() calls and write() calls of the appropriate length and offset.