wait free synchronization question

shogun1234 · 08-19-2007, 09:08 AM

I have two programmes. One (a C programme) writes data into a file (e.g., A) . Then the other programme (a Java programme) would read data from the file A written by the C programme.

In order to prevent concurrency problem (Java and C programmes read/ write the file simultaneously), I make the C programme create another file called .lck and the Java programme must read the .lck file first. If the .lck file exists, the Java programme tries to read the .lck file again (constantly read whether the .lck exists or not) until the .lck file is deleted by the C programme. Then the Java programme reads the data of the file A. Beside, the way how the Java programme probes whether the .lck file exists or not is done through a while loop.

Code:

// create lck file object ...
boolean doLoop = true;
while(doLoop){
    if(lckFile.exists()){
        doLoop = false;
    }
}

It looks fine up to the present. However, I read some document saying that by using while loop would have performance implication, which is what I need to take into account. Although I have found some resources discussing the mechanism of wait-free synchronization that can be applied to such case. Unfortunately, it looks like those mechanism only provide for a single language. For example, java.util.concurrent.atomic. Moreover, the platforms currently my application runs on only support JDK1.4, which seems not support such methods. What should I do if I want to solve this problem?

I appreciate any suggestion,

Thank you very much.

taylor_venable · 08-19-2007, 09:53 AM

You have to think up some other way of communicating between the two processes. How about a Unix socket? Your Java program, when it gets to the point that it needs the file, waits for the C program to send it a message on the socket. The C program, on the other hand, writes the file, then sends a message to the socket when it's done. To avoid the same problem of waiting busily in filesystem checks, the socket should be made available before the C program needs it, so the C program never has to check to make sure the socket exists.

I think this might work, although I must admit (caveat emptor) that I've not done anything like this before! A better solution would be to move the functionality of your C program into your Java program (or vice-versa) but in the absence of this, message passing seems like a good idea.

chrism01 · 08-19-2007, 08:16 PM

As above, alternately, forget the file and just send the data over a socket.

bigearsbilly · 08-20-2007, 09:37 AM

well, at least you could use a FIFO.
they are a file and look after themselves as reading and writing and
act like a pipe.

though you can only have one reader and one writer at a time.
probably you can get away without changing your code.

But i would recommend creating and deleting the fifo in the
application each time.

mkfifo FIFO creates one.