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diff --git a/docs/busybox.net/programming.html b/docs/busybox.net/programming.html index 6dbe693..99fdaac 100644 --- a/docs/busybox.net/programming.html +++ b/docs/busybox.net/programming.html @@ -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> |