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Is there something equivalent to SIGSTOP and SICONT for threads? Am using pthreads. Thanks

An edit: I am implementing a crude form of file access syncronization among threads. So if a file is already opened by a thread, and another thread wants to open it again, I need to halt or pause the second thread at that point of its execution. When the first thread has completed its work it will check what other threads wanted to use a file it released and "wake" them up. The second thread then resumes execution from exactly that point. I use my own book keeping datastructures.

Vladimir Panteleev
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Lipika Deka
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3 Answers3

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I'm going to tell you how to do things instead of answering the question. (Look up the "X Y problem".)

You are trying to prevent two threads from accessing the same file at the same time. In other words, access is MUTually EXclusive. A "mutex" is designed to do this. In general, it is easier to find help if you search for what you are trying to do (prevent two threads from accessing the same resource simultaneously) rather than searching for how you want to do it (make one thread wait for the other).

Edit: It sounds like you actually want many readers but one writer. This is probably the second most common synchronization problem (after the "producer-consumer" problem). Use a pthread_rwlock: readers call pthread_rdlock and writers call pthread_wrlock.

If you're doing something this sophisticated, you really should start reading the relevant literature. If you think you can do multithreaded programming some serious reading, you are much smarter than me and you don't need my help. I recommend "The Little Book of Semaphores" which is a free download (source). It's not about pthreads, but it's good stuff. The readers-writers problem you are asking about is found under §4.2 in the chapter "Classical Synchronization Problems" (heck, this problem is even mentioned in the blurb).

Multithreaded programing is HARD with capital letters and a bold font.

Matthew Iselin
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Dietrich Epp
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  • Thanks a lot. But, its slighty more complicated then that. I allow many readers to access the file at the same time but just one writer and there may be more then one thread waiting to ccess a file. So there is a queue. – Lipika Deka Jun 16 '11 at 05:59
  • @Juggler: Then what you want is a [readers/writer lock](http://en.wikipedia.org/wiki/Readers-writer_lock), which every modern threading library provides; e.g. [pthread version](http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_destroy.html). Edit: Dietrich beat me to it :-) – Nemo Jun 16 '11 at 06:25
  • +1 by the way. There is no such thing as "crude" synchronization. There is only "correct" and "incorrect" synchronization... – Nemo Jun 16 '11 at 06:34
  • Thanks...am aware of the difficulties involved but I have to do it and also do it within a certain specified time. Appreciate you pointers – Lipika Deka Jun 16 '11 at 06:41
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Well, there is pthread_kill.

But you almost certainly do not want to do this. What if the other thread holds (e.g.) a mutex for the heap, and you try to call new while it is stopped?

Since you do not know what the runtime is doing with mutexes, there is no way to avoid this kind of problem in general unless you completely avoid the standard library.

[edit]

Actually, come to think of it, I am not sure what happens if you target a specific thread with SIGSTOP, since that signal usually affects the whole process.

So to update my answer, I do not believe there is any standard mechanism for suspending a thread asynchronously... And for the reason mentioned above, I do not think you want one.

Nemo
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Depending on your application, Pthreads supports what can be considered more refined mechanisms, such as http://www.unix.com/man-page/all/3t/pthread_suspend/ and Mutex mechnisms

Ofir
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