192 lines
4.0 KiB
HTML
192 lines
4.0 KiB
HTML
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>Asynchronous usage of sg</TITLE
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NAME="ASYNC">Chapter 12. Asynchronous usage of sg</H1
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><P
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>It is recommended that synchronous sg-based applications use the new
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SG_IO ioctl() command. Existing applications (which are mainly synchronous)
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can continue to use the older sg_header based interface which is still
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supported.</P
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><P
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>Asynchronous usage allows multiple SCSI commands to be queued up to the
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device. If the device supports command queuing then there can be a major
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performance gain. Even if the device doesn't support command queuing (or
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is temporarily busy) then queuing up commands in the mid level or the host
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driver can be a minor performance win (since there will be a lower latency
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to transmit the next command when the device becomes free).</P
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><P
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>Asynchronous usage usually starts with setting the O_NONBLOCK flag on open()
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[or thereafter by using the fcntl(fd, SETFD, old_flags | O_NONBLOCK)
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system call]. A similar effect can be obtained without using O_NONBLOCK
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when POSIX threads are used. There are several strategies that can then
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be followed:
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<P
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></P
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><OL
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TYPE="1"
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><LI
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><P
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>set O_NONBLOCK and use a poll() loop</P
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></LI
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><LI
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><P
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>set O_NONBLOCK and use SIGPOLL signal to alert app when readable</P
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></LI
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><LI
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><P
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>use POSIX threads and a single sg file descriptor</P
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></LI
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><LI
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><P
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>use POSIX threads and multiple sg file descriptors to same device</P
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></OL
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></P
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><P
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>The O_NONBLOCK flag also permits open(), write() and read() [but not
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the ioctl(SG_IO)] to access a SCSI device even though it has been
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marked offline. SCSI devices are marked offline when they are detected
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and don't respond to the initial SCSI commands as expected, or,
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some SCSI error condition is detected on that device and the
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mid level error recovery logic is unable to "resurrect" the device.
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A SCSI device that is being reset (and still settling) could be
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accessed during this period by using the O_NONBLOCK flag; this
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could lead to unexpected behaviour so the sg user should take care.</P
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><P
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>In Linux SIGIO and SIGPOLL are the same signal. If POSIX real time
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signals are used (e.g. when SA_SIGINFO is used with sigaction() and
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fcntl(fd, F_SETSIG, SIGRTMIN + <n>) ) then the file descriptor with which
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the signal is associated is available to the signal handler. The
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associated file descriptor is in the si_fd member of the siginfo_t
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structure. The poll() system call that is often used after a signal
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is received can thus be bypassed.</P
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