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authorGravatar Rob Landley <rob@landley.net>2006-02-12 00:45:39 +0000
committerGravatar Rob Landley <rob@landley.net>2006-02-12 00:45:39 +0000
commitc29a0f371a8b5409f79e88f26d00c7d9fc2caa4f (patch)
tree2fff7e8094b6878dd88a5579991269f8ef8b863b /docs
parent4926d643ea9ad56fc6b2173c9a3ce3719d6bb39f (diff)
downloadbusybox-c29a0f371a8b5409f79e88f26d00c7d9fc2caa4f.tar.gz
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More random documentation.
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@@ -16,6 +16,7 @@
<ul>
<li><a href="#tips_encrypted_passwords">Encrypted Passwords</a></li>
<li><a href="#tips_vfork">Fork and vfork</a></li>
+ <li><a href="#tips_short_read">Short reads and writes</a></li>
</ul>
</ul>
@@ -298,6 +299,39 @@ each other while traversing the free memory lists). The thing about vfork is
that it's a big red flag warning "there be dragons here" rather than
something subtle and thus even more dangerous.)</p>
+<h2><a name="tips_sort_read">Short reads and writes</a></h2>
+
+<p>Busybox has special functions, bb_full_read() and bb_full_write(), to
+check that all the data we asked for got read or written. Is this a real
+world consideration? Try the following:</p>
+
+<pre>while true; do echo hello; sleep 1; done | tee out.txt</pre>
+
+<p>If tee is implemented with bb_full_read(), tee doesn't display output
+in real time but blocks until its entire input buffer (generally a couple
+kilobytes) is read, then displays it all at once. In that case, we _want_
+the short read, for user interface reasons. (Note that read() should never
+return 0 unless it has hit the end of input, and an attempt to write 0
+bytes should be ignored by the OS.)</p>
+
+<p>As for short writes, play around with two processes piping data to each
+other on the command line (cat bigfile | gzip > out.gz) and suspend and
+resume a few times (ctrl-z to suspend, "fg" to resume). The writer can
+experience short writes, which are especially dangerous because if you don't
+notice them you'll discard data. They can also happen when a system is under
+load and a fast process is piping to a slower one. (Such as an xterm waiting
+on x11 when the scheduler decides X is being a CPU hog with all that
+text console scrolling...)</p>
+
+<p>So will data always be read from the far end of a pipe at the
+same chunk sizes it was written in? Nope. Don't rely on that. For one
+counterexample, see <a href="http://www.faqs.org/rfcs/rfc896.html">rfc 896</p>
+for Nagle's algorithm</a>, which waits a fraction of a second or so before
+sending out small amounts of data through a TCP/IP connection in case more
+data comes in that can be merged into the same packet. (In case you were
+wondering why action games that use TCP/IP set TCP_NODELAY to lower the latency
+on their their sockets, now you know.)</p>
+
<br>
<br>
<br>