question about blocking i/o in a thread

I'm using pthreads on Linux, and one of my threads periodically calls the write function on a device file descriptor. If the write call takes a while to finish, will my th开发者_JAVA技巧read be suspended so other threads can run? I didn't set any of the scheduling features of pthreads, so my question is about default thread behavior.

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So long as nothing else is trying to write to the same resource, the other threads should run while the writing thread waits for its write to complete.

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If a write() call blocks, only the calling thread is suspended. This is documented in the POSIX spec for write():

If there is enough space for all the data requested to be written immediately, the implementation should do so. Otherwise, the calling thread may block; that is, pause until enough space is available for writing.

Note that it says calling thread, not calling process.

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See if blocking behavior is explicitly defined here http://www.akkadia.org/drepper/nptl-design.pdf

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In principle, YES, other threads can run.

But be aware that some filesystems have locking mechnisms which permit only one concurrent IO operation on a single file. So if another thread does another IO on the same file (even if it's via a different file descriptor) it MAY block it for some of the duration of the write() system call.

There are also other in-kernel locks for other facililties. Most of them will not block other threads running unless they're doing closely related activities, however.

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If your device file descriptor is a shared resource, you have to take care of locking. But once it's thread-safe, calls to such shared resource are serialized, thus if one thread writes, the rest are blocked. If locking is not implemented, the data may be garbled.