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The version of XMMS shipped with Red Hat 8.0 and 9.0 and Fedora no longer knows how to play MP3s because the folks at Red Hat were worried about the licensing issues related to the MP3 codec. To get an XMMS with MP3 support on these versions of Linux, you can grab the source from http://www.xmms.org and build it yourself. Or, see http://www.fedorafaq.org/#xmms-mp3 for information about other possibilities.
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To further confuse matters, there's a different streaming protocol called
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Technically, the implementation in this chapter will also be manipulated from two threads—the AllegroServe thread running the Shoutcast server and the REPL thread. But you can live with the race condition for now. I'll discuss how to use locking to make code thread safe in the next chapter.
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Another thing you may want to do while working on this code is to evaluate the form (net.aserve::debug-on :notrap). This tells AllegroServe to not trap errors signaled by your code, which will allow you to debug them in the normal Lisp debugger. In SLIME this will pop up a SLIME debugger buffer just like any other error.
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Shoutcast headers are usually sent in lowercase, so you need to escape the names of the keyword symbols used to identify them to AllegroServe to keep the Lisp reader from converting them to all uppercase. Thus, you'd write :|icy-metaint| rather than :icy-metaint. You could also write :ic y-me ai
, but that'd be silly.
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The function turn-off-chunked-transfer-encoding is a bit of a kludge. There's no way to turn off chunked transfer encoding via AllegroServe's official APIs without specifying a content length because any client that advertises itself as an HTTP/1.1 client, which iTunes does, is supposed to understand it. But this does the trick.
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Most MP3-playing software will display the metadata somewhere in the user interface. However, the XMMS program on Linux by default doesn't. To get XMMS to display Shoutcast metadata, press Ctrl+P to see the Preferences pane. Then in the Audio I/O Plugins tab (the leftmost tab in version 1.2.10), select the MPEG Layer 1/2/3 Player (libmpg123.so) and hit the Configure button. Then select the Streaming tab on the configuration window, and at the bottom of the tab in the SHOUTCAST/Icecast section, check the 'Enable SHOUTCAST/Icecast title streaming' box.
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Folks coming to Common Lisp from Scheme might wonder why play-current can't just call itself recursively. In Scheme that would work fine since Scheme implementations are required by the Scheme specification to support 'an unbounded number of active tail calls.' Common Lisp implementations are allowed to have this property, but it isn't required by the language standard. Thus, in Common Lisp the idiomatic way to write loops is with a looping construct, not with recursion.
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This function assumes, as has other code you've written, that your Lisp implementation's internal character encoding is ASCII or a superset of ASCII, so you can use CHAR-CODE to translate Lisp CHARACTER objects to bytes of ASCII data.
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The intricacies of concurrent programming are beyond the scope of this book. The basic idea is that if you have multiple threads of control—as you will in this application with some threads running the shoutcast function and other threads responding to requests from the browser—then you need to make sure only one thread at a time manipulates an object in order to prevent one thread from seeing the object in an inconsistent state while another thread is working on it. In this function, for instance, if two new MP3 clients are connecting at the same time, they'd both try to add an entry to *playlists* and might interfere with each other. The with-process-lock ensures that each thread gets exclusive access to the hash table for long enough to do the work it needs to do.
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This approach also assumes that every client machine has a unique IP address. This assumption should hold as long as all the users are on the same LAN but may not hold if clients are connecting from behind a firewall that
