svn commit: trunk/busybox/docs/busybox.net

[landley at busybox.net]

Author: landley
Date: 2006-02-11 16:45:39 -0800 (Sat, 11 Feb 2006)
New Revision: 13918

Log:
More random documentation.


Modified:
   trunk/busybox/docs/busybox.net/programming.html


Changeset:
Modified: trunk/busybox/docs/busybox.net/programming.html
===================================================================
--- trunk/busybox/docs/busybox.net/programming.html	2006-02-11 22:56:39 UTC (rev 13917)
+++ trunk/busybox/docs/busybox.net/programming.html	2006-02-12 00:45:39 UTC (rev 13918)
@@ -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 @@
 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>