Read/Write to linux Pipe using Java

Question

My query is on what is the best way to read / write to a linux Pipe in Java? I already am using the java.io.RandomAccessFile like

   RandomAccessFile file = new RandomAccessFile("/dev/zap/16", "rw");

and then passing it to worker thread which reads it after every 2ms as

  byte[] buffer = new byte[16];
  file.read(buffer);

It does read it from Pipe, but I suspect that some bytes are overwritten. Do you know how linux (ubuntu) handles the buffer for pipe's?

Actually the /dev/zap/16 is created by Zaptel driver used for Asterisk when you have SS7 card installed on your machine. Asterisk has module that reads this pipe but is in native code. I am trying to get this working on Java. Out of topic but if any of you trying to install Asterisk + SS7 here is my experience http://amitbhayani.blogspot.com/2009/04/asterisk-ss7.html — amit.bhayani, Oct 28 '09 at 02:02

score 11 · Answer 1 · answered Oct 27 '09 at 16:40

11

I haven't ever tried this myself but what your doing feels just wrong. Linux pipes are First in - First out (FIFO) by definition. Hence you should only be able to read bytes in the same order as you've written them - not randomly. I'd suggest to use a normal File instead and it should work fine.

answered Oct 27 '09 at 16:40

sfussenegger

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Do you mean a normal `FileInputStream`? That's what I'd try first, too. – erickson Oct 27 '09 at 16:41
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`FileInputStream` and `FileOutputStream` respectively, yes – sfussenegger Oct 27 '09 at 16:50
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Interesting... Every tutorial I found on pipes in java is using RandomAccessFile. – Belovoj Feb 06 '18 at 09:43

score 6 · Answer 2 · answered Oct 27 '09 at 16:42

Pipes aren't handled in any way special by Java as far as I know. You simply open the file for writing and write to it.

You can't really "overwrite" anything in a pipe, since you can't seek in a pipe. For the same reason a RandomAccessFile isn't the smartest choice to use (since a pipe is explicitely not a random access file). I'd suggest using a FileOutputStream instead.

Also note that read() is not guaranteed to read until the buffer is full! It can read a single byte as well and you need to check its return value and possibly loop to read the full buffer.

score 0 · Answer 3 · answered Oct 27 '09 at 17:09

I think you may not be flushing after writing, so do OutputStream.flush() often and read may be a byte at time, at least to see if your data is getting thru. e.g. to start with open a named pipe in readonly mode(FileInputStream) in process1, open it in write mode(FileOutputStream ) in process2, so anything you write in process2 will be read in process1.

Read/Write to linux Pipe using Java

3 Answers3