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POblocking select() from multiple sockets
 

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    <p>Unix/C question here. </p> <p>I have multiple sockets that I am trying to poll for periodic data. I don't want select to wait indefinitely so I have a timeout in place and I'm running in a loop. I have found that once a socket is ready to read, <strong>it is always ready to read</strong>. As in, I cannot have select go to sleep when there is no data to be read from any of the sockets. </p> <pre><code>for (i = 0; i &lt; n_connections; i++) { FD_SET( sockfd[i], &amp;master ); if (sockfd[i] &gt; fdmax) fdmax = sockfd[i]; } for(;;) { int nready = 0; timeout.tv_sec = 1; timeout.tv_usec = 0; read_fds = master; if ( (nready = select(fdmax+1, &amp;read_fds, NULL, NULL, NULL)) == -1 ) { fprintf( stderr, "Select Error\n" ); return FAILURE; } printf( "Number of ready descriptors: %d\n", nready ); for (i = 0; i &lt;= fdmax; i++) { if (FD_ISSET(i, &amp;read_fds)) { if (( nbytes = recv(i, buf, sizeof(buf), 0)) &lt;= 0 ) { if (nbytes == 0) { //connection closed printf("Socket %d hung up\n", i ); } else { fprintf( stderr, "Recv Error %d\n", nbytes); } } else { printf( "Data Received on %d: %s\n", i, buf ); } } } // end file descriptor loop </code></pre> <p>It seems that after my first read, the 1 second timeout no longer applies and the socket is always "ready to read", even if there are 0 bytes available. How can I get select to sleep until data comes in (for the one second, or by switching the final argument to NULL, indefinitely waiting for data to come in on the socket?)</p> <p>Output: </p> <pre><code>Number of Ready Descriptors: 2 Data Received on 4: GreetingsChap Data Received on 5: HiMatengsChap Loop... Number of Ready Descriptors: 2 Socket 4 hung up Socket 5 hung up Loop... Number of Ready Descriptors: 2 Socket 4 hung up Socket 5 hung up Loop... </code></pre> <p>Thank you,</p> <p>Note: Code updated for clarity Updated based on @yvesBraumes suggestions - still doesn't work.</p>
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    1. COAre you calling `FD_SET()` before each `select()`? I notice you're simply assigning `read_fds` to `master`. `FD_COPY()` might be more appropriate.
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    2. COWhat do you mean by "ready to read"? What is select() returning and what is read() returning in this case?
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    3. CO@chrisaycock, I am using FD_SET before. I'm FD_SETing the master list of file descriptors, then assigning (which is what you see). Posix allows for this and most examples I have found use it.
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