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Entry  01 Oct 2013, Konstantin Olchanski, Info, MacOS select() problem 
    Reply  25 Oct 2013, Konstantin Olchanski, Info, MacOS select() problem 
Message ID: 917     Entry time: 01 Oct 2013     Reply to this: 922
Author: Konstantin Olchanski 
Topic: Info 
Subject: MacOS select() problem 
The following code found in mhttpd does not work on MacOS (BSD UNIX).

On Linux, the do-loop will finish after 2 seconds as expected. On MacOS (and other BSD systems), it will 
loop forever.

The cause is the MIDAS watchdog alarm() signal that fires every 1 second and always interrupts the 2 
second sleep of select(). The Linux select() updates it's timeout argument to reflect time already slept, so 
eventually we finish. The MacOS (BSD) select() does not update the timeout argument and select goes back 
to sleep for another 2 seconds (to be again interrupted half-way through).

The POSIX standard (specification for select() & co) permits either behaviour. Compare "man select" on 
MacOS and on Linux.

If the select() timeout were not 2 seconds, but 0.9 seconds; or if the MIDAS watchdog alarm fired every 
2.1 seconds, this problem would also not exist.

I think there are several places in MIDAS with code like this. An audit is required.

                  FD_SET(_sock, &readfds);

                  timeout.tv_sec = 2;
                  timeout.tv_usec = 0;

                  do {
                     status = select(FD_SETSIZE, &readfds, NULL, NULL, &timeout);
                     /* if an alarm signal was cought, restart with reduced timeout */
                  } while (status == -1 && errno == EINTR);

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