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POReading from FIFO in C: select() doesn't return?
 

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  1. POReading from FIFO in C: select() doesn't return?
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    <p>I've got a C program I'm writing. Here's what it does:</p> <ul> <li>Create n fifos using mkfifo</li> <li>Open them for read (with the O_NONBLOCK flag set)</li> <li>Open them for write</li> <li>Spawn a thread</li> </ul> <p>In the thread, run in a loop:</p> <ul> <li>Create an fd_set of the file descriptors for all n fifos</li> <li>Call select(n, &amp;my_set, NULL, NULL, NULL)</li> <li>For each fd ready for I/O (FD_ISSET(fd, &amp;my_set)): <ul> <li>Read a string from the fd (read(fd, buf, buf_len))</li> <li>Print the string</li> <li>If string == "kill", mark the fd as dead and remove it from the list (n--)</li> <li>If n == 0, terminate the thread</li> </ul></li> </ul> <p>In the main program:</p> <ul> <li>For i = 0 to n <ul> <li>Write to fds[i] with a string (write(fds[i], buf, buf_len))</li> </ul></li> <li>For i = 0 to n <ul> <li>Write to fds[i] with the string "kill"</li> </ul></li> <li>Join on the thread I created</li> <li>Exit</li> </ul> <p>The behavior I'm seeing is that select() will return once with a length of 1, being the first fd in the list. The second time through the loop, select will just sit there forever.</p> <p>Here's my output:</p> <pre><code>thread created Waiting on 4 file descriptors &gt; Wrote 'Hello to target 0 from writer 0' to 0 &gt; Wrote 'Hello to target 0 from writer 1' to 1 &gt; Wrote 'Hello to target 1 from writer 0' to 2 &gt; Wrote 'Hello to target 1 from writer 1' to 3 &gt; Sending kill to 0:0 (#0) &gt; Sending kill to 0:1 (#1) &gt; Sending kill to 1:0 (#2) &gt; Sending kill to 1:1 (#3) &lt; Got string: 'Hello to target 0 from writer 0' Waiting on 4 file descriptors ^C </code></pre> <p>The OS is Linux, in case it matters.</p> <p>Link to the code: <a href="https://dl.getdropbox.com/u/188590/fifotest.c" rel="nofollow noreferrer">https://dl.getdropbox.com/u/188590/fifotest.c</a> (Sorry it's a bit heinous)</p> <p>Thanks, Nathan</p>
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    1. COI think we're going to need to see code, sanitized or not. The log information isn't completely clear - I'm sure what writer 0 and 1 is about, and it seems surprising if the 'to 0' means 'to file descriptor 0' as that's standard input. Are you scrupulously checking for errors from every system call?
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      1. USJonathan Leffler
    2. COOK, I put a link to the code in the question. It's at https://dl.getdropbox.com/u/188590/fifotest.c I believe I am scrupulously checking for errors. The numbers (such as 0) aren't the fd numbers, they're the index into the array.
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      1. USNathan
    3. COGot the code: got the output ending: < Got string: 'Hello to target 0 from writer 0' Waiting on 4 file descriptors < Got string: 'kill 0:0' Waiting on 3 file descriptors ...which is a bit further than you got...looking now.
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      1. USJonathan Leffler
 

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