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<?xml version="1.0" encoding="iso-8859-1"?>
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<!DOCTYPE chapter PUBLIC "-//Samba-Team//DTD DocBook V4.2-Based Variant V1.0//EN" "http://www.samba.org/samba/DTD/samba-doc">
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<firstname>Chris</firstname><surname>Hertel</surname>
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<pubdate>July 1998</pubdate>
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<title>The samba DEBUG system</title>
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<title>New Output Syntax</title>
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The syntax of a debugging log file is represented as:
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<para><programlisting>
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>debugfile< :== { >debugmsg< }
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>debugmsg< :== >debughdr< '\n' >debugtext<
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>debughdr< :== '[' TIME ',' LEVEL ']' FILE ':' [FUNCTION] '(' LINE ')'
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>debugtext< :== { >debugline< }
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>debugline< :== TEXT '\n'
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</programlisting></para>
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TEXT is a string of characters excluding the newline character.
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LEVEL is the DEBUG level of the message (an integer in the range
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FILE is the name of the file from which the debug message was
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FUNCTION is the function from which the debug message was generated.
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LINE is the line number of the debug statement that generated the
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<para>Basically, what that all means is:</para>
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A debugging log file is made up of debug messages.
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Each debug message is made up of a header and text. The header is
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separated from the text by a newline.
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The header begins with the timestamp and debug level of the
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message enclosed in brackets. The filename, function, and line
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number at which the message was generated follow. The filename is
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terminated by a colon, and the function name is terminated by the
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parenthesis which contain the line number. Depending upon the
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compiler, the function name may be missing (it is generated by the
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__FUNCTION__ macro, which is not universally implemented, dangit).
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The message text is made up of zero or more lines, each terminated
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<para>Here's some example output:</para>
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<para><programlisting>
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[1998/08/03 12:55:25, 1] nmbd.c:(659)
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Netbios nameserver version 1.9.19-prealpha started.
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Copyright Andrew Tridgell 1994-1997
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[1998/08/03 12:55:25, 3] loadparm.c:(763)
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Initializing global parameters
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</programlisting></para>
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Note that in the above example the function names are not listed on
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the header line. That's because the example above was generated on an
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SGI Indy, and the SGI compiler doesn't support the __FUNCTION__ macro.
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<title>The DEBUG() Macro</title>
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Use of the DEBUG() macro is unchanged. DEBUG() takes two parameters.
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The first is the message level, the second is the body of a function
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call to the Debug1() function.
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<para>That's confusing.</para>
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<para>Here's an example which may help a bit. If you would write</para>
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<para><programlisting>
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printf( "This is a %s message.\n", "debug" );
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</programlisting></para>
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to send the output to stdout, then you would write
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<para><programlisting>
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DEBUG( 0, ( "This is a %s message.\n", "debug" ) );
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</programlisting></para>
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to send the output to the debug file. All of the normal printf()
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formatting escapes work.
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Note that in the above example the DEBUG message level is set to 0.
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Messages at level 0 always print. Basically, if the message level is
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less than or equal to the global value DEBUGLEVEL, then the DEBUG
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statement is processed.
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The output of the above example would be something like:
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<para><programlisting>
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[1998/07/30 16:00:51, 0] file.c:function(128)
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This is a debug message.
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</programlisting></para>
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Each call to DEBUG() creates a new header *unless* the output produced
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by the previous call to DEBUG() did not end with a '\n'. Output to the
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debug file is passed through a formatting buffer which is flushed
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every time a newline is encountered. If the buffer is not empty when
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DEBUG() is called, the new input is simply appended.
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...but that's really just a Kludge. It was put in place because
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DEBUG() has been used to write partial lines. Here's a simple (dumb)
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example of the kind of thing I'm talking about:
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<para><programlisting>
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DEBUG( 0, ("The test returned " ) );
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DEBUG(0, ("False") );
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</programlisting></para>
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Without the format buffer, the output (assuming test() returned true)
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would look like this:
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<para><programlisting>
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[1998/07/30 16:00:51, 0] file.c:function(256)
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[1998/07/30 16:00:51, 0] file.c:function(258)
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[1998/07/30 16:00:51, 0] file.c:function(261)
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</programlisting></para>
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<para>Which isn't much use. The format buffer kludge fixes this problem.
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<title>The DEBUGADD() Macro</title>
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In addition to the kludgey solution to the broken line problem
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described above, there is a clean solution. The DEBUGADD() macro never
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generates a header. It will append new text to the current debug
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message even if the format buffer is empty. The syntax of the
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DEBUGADD() macro is the same as that of the DEBUG() macro.
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<para><programlisting>
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DEBUG( 0, ("This is the first line.\n" ) );
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DEBUGADD( 0, ("This is the second line.\nThis is the third line.\n" ) );
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</programlisting></para>
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<para>Produces</para>
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<para><programlisting>
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[1998/07/30 16:00:51, 0] file.c:function(512)
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This is the first line.
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This is the second line.
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This is the third line.
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</programlisting></para>
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<title>The DEBUGLVL() Macro</title>
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One of the problems with the DEBUG() macro was that DEBUG() lines
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tended to get a bit long. Consider this example from
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<para><programlisting>
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DEBUG(3,("send_local_master_announcement: type %x for name %s on subnet %s for workgroup %s\n",
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type, global_myname, subrec->subnet_name, work->work_group));
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</programlisting></para>
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One solution to this is to break it down using DEBUG() and DEBUGADD(),
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<para><programlisting>
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DEBUG( 3, ( "send_local_master_announcement: " ) );
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DEBUGADD( 3, ( "type %x for name %s ", type, global_myname ) );
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DEBUGADD( 3, ( "on subnet %s ", subrec->subnet_name ) );
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DEBUGADD( 3, ( "for workgroup %s\n", work->work_group ) );
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</programlisting></para>
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A similar, but arguably nicer approach is to use the DEBUGLVL() macro.
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This macro returns True if the message level is less than or equal to
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the global DEBUGLEVEL value, so:
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<para><programlisting>
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dbgtext( "send_local_master_announcement: " );
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dbgtext( "type %x for name %s ", type, global_myname );
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dbgtext( "on subnet %s ", subrec->subnet_name );
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dbgtext( "for workgroup %s\n", work->work_group );
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</programlisting></para>
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<para>(The dbgtext() function is explained below.)</para>
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<para>There are a few advantages to this scheme:</para>
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The test is performed only once.
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You can allocate variables off of the stack that will only be used
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within the DEBUGLVL() block.
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Processing that is only relevant to debug output can be contained
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within the DEBUGLVL() block.
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<title>New Functions</title>
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<title>dbgtext()</title>
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This function prints debug message text to the debug file (and
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possibly to syslog) via the format buffer. The function uses a
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variable argument list just like printf() or Debug1(). The
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input is printed into a buffer using the vslprintf() function,
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and then passed to format_debug_text().
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If you use DEBUGLVL() you will probably print the body of the
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message using dbgtext().
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<title>dbghdr()</title>
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This is the function that writes a debug message header.
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Headers are not processed via the format buffer. Also note that
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if the format buffer is not empty, a call to dbghdr() will not
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produce any output. See the comments in dbghdr() for more info.
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It is not likely that this function will be called directly. It
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is used by DEBUG() and DEBUGADD().
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<title>format_debug_text()</title>
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This is a static function in debug.c. It stores the output text
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for the body of the message in a buffer until it encounters a
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newline. When the newline character is found, the buffer is
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written to the debug file via the Debug1() function, and the
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buffer is reset. This allows us to add the indentation at the
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beginning of each line of the message body, and also ensures
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that the output is written a line at a time (which cleans up