what is meant by address space??? if we create a thread for process,does the thread and process share address space??? do we need IPC between process on the same address space???

"address space" generally is talking about the memory map of a process.

On Linux you can run cat /proc/<PID>/maps to see the memory map for the given process ID. (Use ps -ef to see the PIDs running on your system). On Linux threads can show up as separate PIDs. To verify they are using the same memory map you can just diff the maps - if they are the same then you know they are threads.

IPC = Interprocess Communication - As noted by this name it is used to communicate between processes. You'd want to do more reading on IPC (shared memory, semaphores and message queues) for more detail about using these in programs.

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IME, using kernel 2.4.x, threads show up as separate processes; using 2.6.x they don't show up at all, only the orig process is seen using eg ps or top cmds..

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Actually you can see the threads with various ps options for example with "ps -eLF" - the LWP stuff in the output relates the thread ID and the number of threads. If you do this you can still see /proc/<LWP>/... info just as you could in the past.

I agree however that the default display changed so it doesn't show threads by default as it did. Actually I had a couple of old 2.4.x systems a long time ago and was confused back then because on one of them I had to add a flag (-m as I recall) to see the threads in process list but the other was showing them by default.

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Yeah, I should have said that threads are not full processes (in the std defn of a proc) as of 2.6.x.

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