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<H2><A NAME="s6">6.</A> <A HREF="distcc.html#toc6">distcc Internals</A></H2>

<H2><A NAME="ss6.1">6.1</A> <A HREF="distcc.html#toc6.1">          Protocol</A>

<P>distcc uses a simple, application-specific protocol running
directly over a TCP socket.  A new request socket is opened
for each job.</P>
<P>The request and response begin with a magic number and
version number, allowing incompatible versions or
misconfigurations to be identified.  At the moment there is
only one deployed protocol version, and no attempt to
support backward or forward compatibility, though this could
be added in the future.</P>
<P>The request and response consist of tagged, length-preceded
elements.  Each element of the request contains a
four-character ASCII token, an eight-digit ASCII hexadecimal
length or value, and, depending on the tag, a byte stream
whose length is determined by the hexadecimal field.</P>
<P>The complete request is sent to the server before the reply
begins.  Opening the TCP socket is performed concurrently
with execution of the preprocessor on the client.</P>
<P>The request from the client contains</P>
<LI>Magic number and version</LI>
<LI>Compiler command line</LI>
<LI>Preprocessed source code</LI>
<P>The response from the server contains</P>
<LI>Magic number and version</LI>
<LI>Compiler exit code &amp; status</LI>
<LI>Compiler error messages</LI>
<LI>Compiler stdout</LI>
<LI>Object file (if any)</LI>
<P>Consult the source for more information.</P>
<H2><A NAME="ss6.2">6.2</A> <A HREF="distcc.html#toc6.2">          Working files</A>

<P>distcc stores working files in a subdirectory of
<CODE>/tmp</CODE>.  These include synchronization files, and
compiler input/output temporary files.</P>
<P>Temporary files should normally be cleaned up when the
program exits.  If distcc misbehaves, these files may be
useful in tracking down the cause.  Any that remain can be
removed by the system's temporary file reaper, or by hand.</P>
<H2><A NAME="ss6.3">6.3</A> <A HREF="distcc.html#toc6.3">          Lock files</A>

<P>distcc uses lock files to allow each client to balance its
jobs across available volunteer machines.  For each
volunteer host, a zero-length file is created.  Clients
using that volunteer hold a <CODE>flock</CODE> lock on the file
while running.</P>
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