Dr. Dobb's Journal - December 2008 - (Page 43)

d12babk_p4db 10/10/08 10:53 AM Page 43 using it (for example, Posix has it only as a compatibility call and advises against using it in the new programs). So it would have portability issues. Even if your library supports it, the related synchronization may be tricky. All this means that this isn’t a very good option. The next option is to use a select() call. But a simple select() on a single file descriptor is not going to fix anything. It would get blocked just as easily as the simple read(). So the further option is to make a select() with a timeout. Maybe every second the timeout would expire, the thread would wake up and check the state of the control block, and if asked to stop, it would stop. Not exactly an optimal solution, either. On one hand, there is a noticeable delay. One second may not seem like much but it quickly accumulates. If you’re closing 100 files, it would take quite a while. On the other hand, every second there are a bunch of threads that wake up, check their states, and go back to sleep— this accumulates, too. The accumulation of a delay can be dealt with by splitting the stopping of the thread into two library calls: One call to set the exit indication, another call to wait for the read thread to exit. Then the first call can be applied to all the sockets first, the read threads would “time out in parallel,” and the second calls will quickly collect them. This splitting is useful even with the other approaches. A better solution is to call select() on two file descriptors: the socket and the indicator. The indicator can be the read side of a pipe. When the reader thread needs to be stopped, write some data into the pipe, and it will wake up select(). The downside is that now you need three times the number of file descriptors: one for the original socket, one for the read side of the pipe, and one for the write side of the pipe. If you’re trying to be portable to Windows, you may have issues with the pipes. On the other hand, Windows has an analog of select() that can work on both files and synchronization primitives (such as semaphores), so you can avoid the overhead of extra file descriptors. If you need to stop a thread that is listening for the new connections, you may not even need the extra file descriptor. Just have Listing One // "state" contains the library-level socket state while(true) { int len = read(state.fd, inbuf, sizeof(inbuf)); if (len <= 0) if (len < 0) { // report the error } state.open = false; close(state.fd); break; } processInput(inbuf, len); } Listing Two shutdown(fd, SHUT_RDWR); joinReaderThread(); // wait for it to exit destroyState(state); Listing Three // reader thread: // "state" contains the library-level socket state while(true) { int fd; lock(state.mutex); fd = state.open? state.fd : -1; unlock(state.mutex); if (fd < 0) break; int len = read(fd, inbuf, sizeof(inbuf)); if (len <= 0) if (state.open && len < 0) { // report the error } lock(state.mutex); if (state.open) { state.open = false; close(state.fd); } unlock(state.mutex); break; } processInput(inbuf, len); } // stopping code: lock(state.mutex); if (state.open) { state.open = false; close(state.fd); } unlock(state.mutex); Listing Four // stopping code: lock(state.mutex); if (state.open) { state.open = false; usleep(20000); // 20 msec close(state.fd); } unlock(state.mutex); December 2008 l www.ddj.com l Dr. Dobb’s Journal 43 http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - December 2008

Dr. Dobb's Journal - December 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Conversations The Man Who Sold the Sky Performance on Rails LINQ-to-SQL and T-SQL A Remote Java RMI Registry Beyond B-Trees File Descriptors and Multithreaded Programs Effective Concurrency The Agile Edge Swaine's Flames

Dr. Dobb's Journal - December 2008

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