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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>usb_unlink_urb</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1" /><link rel="home" href="index.html" title="The Linux-USB Host Side API" /><link rel="up" href="usbcore.html" title="Chapter 5. USB Core APIs" /><link rel="prev" href="API-usb-submit-urb.html" title="usb_submit_urb" /><link rel="next" href="API-usb-kill-urb.html" title="usb_kill_urb" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center"><span class="phrase">usb_unlink_urb</span></th></tr><tr><td width="20%" align="left"><a accesskey="p" href="API-usb-submit-urb.html">Prev</a> </td><th width="60%" align="center">Chapter 5. USB Core APIs</th><td width="20%" align="right"> <a accesskey="n" href="API-usb-kill-urb.html">Next</a></td></tr></table><hr /></div><div class="refentry"><a id="API-usb-unlink-urb"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>usb_unlink_urb —
abort/cancel a transfer request for an endpoint
</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><div class="funcsynopsis"><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">int <strong class="fsfunc">usb_unlink_urb </strong>(</code></td><td>struct urb * <var class="pdparam">urb</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1"><a id="idm140259812442064"></a><h2>Arguments</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="parameter"><code>urb</code></em></span></dt><dd><p>
pointer to urb describing a previously submitted request,
may be NULL
</p></dd></dl></div></div><div class="refsect1"><a id="idm140259812438912"></a><h2>Description</h2><p>
This routine cancels an in-progress request. URBs complete only once
per submission, and may be canceled only once per submission.
Successful cancellation means termination of <em class="parameter"><code>urb</code></em> will be expedited
and the completion handler will be called with a status code
indicating that the request has been canceled (rather than any other
code).
</p><p>
Drivers should not call this routine or related routines, such as
<code class="function">usb_kill_urb</code> or <code class="function">usb_unlink_anchored_urbs</code>, after their disconnect
method has returned. The disconnect function should synchronize with
a driver's I/O routines to insure that all URB-related activity has
completed before it returns.
</p><p>
This request is asynchronous, however the HCD might call the -><code class="function">complete</code>
callback during unlink. Therefore when drivers call <code class="function">usb_unlink_urb</code>, they
must not hold any locks that may be taken by the completion function.
Success is indicated by returning -EINPROGRESS, at which time the URB will
probably not yet have been given back to the device driver. When it is
eventually called, the completion function will see <em class="parameter"><code>urb</code></em>->status ==
-ECONNRESET.
Failure is indicated by <code class="function">usb_unlink_urb</code> returning any other value.
Unlinking will fail when <em class="parameter"><code>urb</code></em> is not currently <span class="quote">“<span class="quote">linked</span>”</span> (i.e., it was
never submitted, or it was unlinked before, or the hardware is already
finished with it), even if the completion handler has not yet run.
</p><p>
The URB must not be deallocated while this routine is running. In
particular, when a driver calls this routine, it must insure that the
completion handler cannot deallocate the URB.
</p></div><div class="refsect1"><a id="idm140259812429760"></a><h2>Unlinking and Endpoint Queues</h2><p>
</p><p>
[The behaviors and guarantees described below do not apply to virtual
root hubs but only to endpoint queues for physical USB devices.]
</p><p>
Host Controller Drivers (HCDs) place all the URBs for a particular
endpoint in a queue. Normally the queue advances as the controller
hardware processes each request. But when an URB terminates with an
error its queue generally stops (see below), at least until that URB's
completion routine returns. It is guaranteed that a stopped queue
will not restart until all its unlinked URBs have been fully retired,
with their completion routines run, even if that's not until some time
after the original completion handler returns. The same behavior and
guarantee apply when an URB terminates because it was unlinked.
</p><p>
Bulk and interrupt endpoint queues are guaranteed to stop whenever an
URB terminates with any sort of error, including -ECONNRESET, -ENOENT,
and -EREMOTEIO. Control endpoint queues behave the same way except
that they are not guaranteed to stop for -EREMOTEIO errors. Queues
for isochronous endpoints are treated differently, because they must
advance at fixed rates. Such queues do not stop when an URB
encounters an error or is unlinked. An unlinked isochronous URB may
leave a gap in the stream of packets; it is undefined whether such
gaps can be filled in.
</p><p>
Note that early termination of an URB because a short packet was
received will generate a -EREMOTEIO error if and only if the
URB_SHORT_NOT_OK flag is set. By setting this flag, USB device
drivers can build deep queues for large or complex bulk transfers
and clean them up reliably after any sort of aborted transfer by
unlinking all pending URBs at the first fault.
</p><p>
When a control URB terminates with an error other than -EREMOTEIO, it
is quite likely that the status stage of the transfer will not take
place.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="API-usb-submit-urb.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="usbcore.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="API-usb-kill-urb.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top"><span class="phrase">usb_submit_urb</span> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> <span class="phrase">usb_kill_urb</span></td></tr></table></div></body></html>
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