[gpsd-dev] Implementing pselect() in gpsctl (bug #35765) - what is wrong with my understanding of code?

[Andrew Evdokimov]

Hi all,

I'm currently working on implementing pselect() for gpsctl (see bug #35765) but 
have faced some difficulties understanding how this program currently works. 
gpsctl has the get_packet() function that is supposed to return a packet mask 
when gpsd_poll() will return one with ONLINE_SET set. Current code is as 
follows (only significant lines are left):

{
    static fd_set rfds;
    gps_mask_t fieldmask;
    struct timeval tv;

    FD_ZERO(&rfds);
    for (;;) {
        FD_CLR(session->gpsdata.gps_fd, &rfds);
...
        tv.tv_sec = 2;
        tv.tv_usec = 0;
        errno = 0;
        if (select(session->gpsdata.gps_fd + 1, &rfds, NULL, NULL, &tv) == -1) {
            if (errno == EINTR || !FD_ISSET(session->gpsdata.gps_fd, &rfds))
                continue;
            gpsd_report(LOG_ERROR, "select %s\n", strerror(errno));
            exit(2);
        }

        fieldmask = gpsd_poll(session);
...
        if ((fieldmask &~ ONLINE_SET)!=0)
            return fieldmask;
    }
}

So before entering a for-loop we're zeroing fd_set rfds (OK with this), then in 
a loop we're removing 'session->gpsdata.gps_fd' file descriptor from 
already-empty fd_set and then calling select() with 2-seconds timeout that will 
wait until some of our file descriptors will become ready for IO. As soon as 
rfds does not contain any file descriptors the select() call will timeout in 2 
seconds passing control to gpsd_poll() etc. In other words, select() here is 
just a 2-second sleep. gpsd_poll() does not use our rfds thus at the end of a 
loop we have an empty fd_set which will be cleared of session->gpsdata.gps_fd 
at next iteration etc...

Where am I wrong? I'm new to this project and may not fully understand some 
internal logic, so please help. Thanks in advance!

WBR,
Andrew