How to check if an application is in waiting

Question

I have two applications running on my machine. One is supposed to hand in the work and other is supposed to do the work. How can I make sure that the first application/process is in wait state. I can verify via the resources its consuming, but that does not guarantee so. What tools should I use?

Thanks

Answer 1

you can use named pipes - the first app will read from it but it will be blank and hence it will keep waiting (blocked). The second app will write into it when it wants the first one to continue.

Answer 2

Your 2 applications shoud communicate. There are a lot of ways to do that:

• Send messages through sockets. This way the 2 processes can run on different machines if you use normal network sockets instead of local ones.
• If you are using C you can use semaphores with semget/semop/semctl. There should be interfaces for that in other languages.
• Named pipes block until there is both a read and a write operation in progress. You can use that for synchronisation.
• Signals are also good for this. In C it is called sendmsg/recvmsg.
• DBUS can also be used and has bindings for variuos languages.

Update: If you can't modify the processing application then it is harder. You have to rely on some signs that indicate the progress. (I am assuming you processing application reads a file, does some processing then writes the result to an output file.) Do you know the final size the result should be? If so you need to check the size repeatedly (or whenever it changes).

If you don't know the size but you know how the processing works you may be able to use that. For example the processing is done when the output file is closed. You can use strace to see all the system calls including the close. You can replace the close() function with the LD_PRELOAD environment variable (on windows you have to replace dlls). This way you can sort of modify the processing program without actually recompiling or even having access to its source.

Answer 3

Nothing can guarantee that your application is in waiting state. You have to pass it some work and get back a response. It might be transactions or not - application can confirm that it got the message to process before it starts to process it or after it was processed (successfully or not). If it does not wait, passing a piece of work should fail. Whether when trying to write to a TCP/IP socket or other means, or if timeout occurs. This depends on implementation, what kind of transport you are using and other requirements.

There is actually a way of figuring out if the process (thread) is in blocking state and waiting for data on a socket (or other source), but that means that client should be on the same computer and have access privileges required to do that, but that makes no sense other than debugging, which you can do using any debugger anyway.

Overall, the idea of making sure that application is waiting for data before trying to pass it that data smells bad. Not to mention the racing condition - what if you checked and it was OK, and when you actually tried to send the data, you found out that application is not waiting at that time (even if that is microseconds).