The Linux SCSI Generic (sg) HOWTO
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# 7.7. fcntl(sg_fd, F_SETFL, oflags | FASYNC)

fcntl(int sg_fd, int cmd, long arg). There are several uses for this system call in association with a sg file descriptor. The following pseudo code shows code that is useful for scanning the sg devices, taking care not to be caught in a wait for an O_EXCL lock by another process, and when the appropriate device is found, switching to normal blocked io. A working example of this logic is in the sg_scan utility program.

    open("/dev/sg0", O_RDONLY | O_NONBLOCK)
    /* check device, EBUSY means some other process has O_EXCL lock on it */
    /* when the device you want is found then ... */
    flags = fcntl(sg_fd, F_GETFL)
    fcntl(sg_fd, F_SETFL, flags & (~ O_NONBLOCK))
    /* since, with simple apps, it is easier to use normal blocked io */

The sg driver supports asynchronous notification. This is a non-blocking mode of operation in which, when the driver receives data back from a device so that a read() can be done, it sends a SIGPOLL (aka SIGIO) signal to the owning process. Here is a code snippet from the sg_poll test program.

    sigemptyset(&sig_set)
    sigaddset(&sig_set, SIGPOLL)
    sigaction(SIGPOLL, &s_action, 0)
    fcntl(sg_fd, F_SETOWN, getpid())
    flags = fcntl(sg_fd, F_GETFL);
    fcntl(sg_fd, F_SETFL, flags | O_ASYNC)

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