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    <p>This does indeed depend on the implementation - so since you have asked about Linux specifically, my comments are refer to the current NPTL implementation of pthreads, which is used in modern glibc.</p> <p>There are two related, but separate, issues here. Firstly, there is this situation:</p> <ul> <li>There are read locks currently held, and writers waiting. A new thread tries to take a read lock.</li> </ul> <p>The default action here is to allow the reader to proceed - effectively "jumping the queue" over the writer. You can, however, override this. If you use the <code>pthread_rwlockattr_setkind_np()</code> function to set the <code>PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP</code> flag on the <code>attr</code> that you pass to <code>pthread_rwlock_init()</code>, then your rwlock will block the reader in the above situation.</p> <p>The second situation is:</p> <ul> <li>The last holder releases the lock, and there are both readers and writers waiting.</li> </ul> <p>In this situation, NPTL will always wake up a writer in preference to a reader.</p> <p>Taken together, the above means that if you use the <code>PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP</code> flag, your writers shouldn't be starved (of course, now a continuous stream of writers can starve the readers. <em>C'est la vie</em>). You can confirm all this by checking the sources (it's all very readable<sup>1</sup>) in <a href="https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/pthread_rwlock_rdlock.c;h=31eb508a0d34bbeaa8c3112c6cde58c0d06a7734;hb=e28c88707ef0529593fccedf1a94c3fce3df0ef3" rel="nofollow noreferrer">pthread_rwlock_rdlock.c</a> and <a href="https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/pthread_rwlock_unlock.c;h=a7ef71a11382c507d4883bad4102bb3c0c85db05;hb=e28c88707ef0529593fccedf1a94c3fce3df0ef3" rel="nofollow noreferrer">pthread_rwlock_unlock.c</a>.</p> <p>Note that there is also a <code>PTHREAD_RWLOCK_PREFER_WRITER_NP</code>, but it appears <em>not</em> to have the right effect - quite possibly a bug (<em>or possibly not - see <a href="https://stackoverflow.com/questions/2190090/how-to-prevent-writer-starvation-in-a-read-write-lock-in-pthreads/2190271#comment-6945209">comment by jilles below</a></em>).</p> <p><hr> <sup>1. ...or at least it was, back when I wrote this answer in 2010. The latest versions of NPTL are considerably more complex and I haven't re-done the analysis.</sup></p>
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    1. POHow to prevent writer starvation in a read write lock in pthreads
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    1. COthanks a lot. This is what i was looking for. One more thing, is the function pthread_rwlockattr_setkind_np() a POSIX api or only linux specific ? I could not see it in the /usr/include/pthread.h header file on my linux system(may be its old). And what does "np" stands for ? Thanks a lot :)
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    2. COIt's listed as a UNIX98 extension here: http://nptl.bullopensource.org/Tests/Optimization-level-in-nptl.html but I believe that's an error, and it's actually a GNU extension (as are all the other functions ending in `_np`). Certainly in practice it's not available elsewhere. I presume `_np` is short for `NPTL`, which means "Native POSIX Threads Library". I'm not sure exactly which version of glibc introduced it, but glibc 2.7 on my box has it. Maybe ask on the glibc mailing list?
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    3. COThanks a lot caf :)
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