Chapter 4. The samba DEBUG system

Chapter 4. The samba DEBUG system
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Chris Hertel

July 1998

Table of Contents

New Output Syntax

The DEBUG() Macro

The DEBUGADD() Macro

The DEBUGLVL() Macro

New Functions

dbgtext()
dbghdr()
format_debug_text()

New Output Syntax

+ The syntax of a debugging log file is represented as: +

+  >debugfile< :== { >debugmsg< }
+
+  >debugmsg<  :== >debughdr< '\n' >debugtext<
+
+  >debughdr<  :== '[' TIME ',' LEVEL ']' FILE ':' [FUNCTION] '(' LINE ')'
+
+  >debugtext< :== { >debugline< }
+
+  >debugline< :== TEXT '\n'
+

+TEXT is a string of characters excluding the newline character. +

+LEVEL is the DEBUG level of the message (an integer in the range + 0..10). +

+TIME is a timestamp. +

+FILE is the name of the file from which the debug message was +generated. +

+FUNCTION is the function from which the debug message was generated. +

+LINE is the line number of the debug statement that generated the +message. +

Basically, what that all means is:

+A debugging log file is made up of debug messages. +
+Each debug message is made up of a header and text. The header is +separated from the text by a newline. +
+The header begins with the timestamp and debug level of the +message enclosed in brackets. The filename, function, and line +number at which the message was generated follow. The filename is +terminated by a colon, and the function name is terminated by the +parenthesis which contain the line number. Depending upon the +compiler, the function name may be missing (it is generated by the +__FUNCTION__ macro, which is not universally implemented, dangit). +
+The message text is made up of zero or more lines, each terminated +by a newline. +

Here's some example output:

+    [1998/08/03 12:55:25, 1] nmbd.c:(659)
+      Netbios nameserver version 1.9.19-prealpha started.
+      Copyright Andrew Tridgell 1994-1997
+    [1998/08/03 12:55:25, 3] loadparm.c:(763)
+      Initializing global parameters
+

+Note that in the above example the function names are not listed on +the header line. That's because the example above was generated on an +SGI Indy, and the SGI compiler doesn't support the __FUNCTION__ macro. +

The DEBUG() Macro

+Use of the DEBUG() macro is unchanged. DEBUG() takes two parameters. +The first is the message level, the second is the body of a function +call to the Debug1() function. +

That's confusing.

Here's an example which may help a bit. If you would write

+printf( "This is a %s message.\n", "debug" );
+

+to send the output to stdout, then you would write +

+DEBUG( 0, ( "This is a %s message.\n", "debug" ) );
+

+to send the output to the debug file. All of the normal printf() +formatting escapes work. +

+Note that in the above example the DEBUG message level is set to 0. +Messages at level 0 always print. Basically, if the message level is +less than or equal to the global value DEBUGLEVEL, then the DEBUG +statement is processed. +

+The output of the above example would be something like: +

+    [1998/07/30 16:00:51, 0] file.c:function(128)
+      This is a debug message.
+

+Each call to DEBUG() creates a new header *unless* the output produced +by the previous call to DEBUG() did not end with a '\n'. Output to the +debug file is passed through a formatting buffer which is flushed +every time a newline is encountered. If the buffer is not empty when +DEBUG() is called, the new input is simply appended. +

+...but that's really just a Kludge. It was put in place because +DEBUG() has been used to write partial lines. Here's a simple (dumb) +example of the kind of thing I'm talking about: +

+    DEBUG( 0, ("The test returned " ) );
+    if( test() )
+      DEBUG(0, ("True") );
+    else
+      DEBUG(0, ("False") );
+    DEBUG(0, (".\n") );
+

+Without the format buffer, the output (assuming test() returned true) +would look like this: +

+    [1998/07/30 16:00:51, 0] file.c:function(256)
+      The test returned
+    [1998/07/30 16:00:51, 0] file.c:function(258)
+      True
+    [1998/07/30 16:00:51, 0] file.c:function(261)
+      .
+

Which isn't much use. The format buffer kludge fixes this problem. +

The DEBUGADD() Macro

+In addition to the kludgey solution to the broken line problem +described above, there is a clean solution. The DEBUGADD() macro never +generates a header. It will append new text to the current debug +message even if the format buffer is empty. The syntax of the +DEBUGADD() macro is the same as that of the DEBUG() macro. +

+    DEBUG( 0, ("This is the first line.\n" ) );
+    DEBUGADD( 0, ("This is the second line.\nThis is the third line.\n" ) );
+

Produces

+    [1998/07/30 16:00:51, 0] file.c:function(512)
+      This is the first line.
+      This is the second line.
+      This is the third line.
+

The DEBUGLVL() Macro

+One of the problems with the DEBUG() macro was that DEBUG() lines +tended to get a bit long. Consider this example from +nmbd_sendannounce.c: +

+  DEBUG(3,("send_local_master_announcement: type %x for name %s on subnet %s for workgroup %s\n",
+            type, global_myname, subrec->subnet_name, work->work_group));
+

+One solution to this is to break it down using DEBUG() and DEBUGADD(), +as follows: +

+  DEBUG( 3, ( "send_local_master_announcement: " ) );
+  DEBUGADD( 3, ( "type %x for name %s ", type, global_myname ) );
+  DEBUGADD( 3, ( "on subnet %s ", subrec->subnet_name ) );
+  DEBUGADD( 3, ( "for workgroup %s\n", work->work_group ) );
+

+A similar, but arguably nicer approach is to use the DEBUGLVL() macro. +This macro returns True if the message level is less than or equal to +the global DEBUGLEVEL value, so: +

+  if( DEBUGLVL( 3 ) )
+    {
+    dbgtext( "send_local_master_announcement: " );
+    dbgtext( "type %x for name %s ", type, global_myname );
+    dbgtext( "on subnet %s ", subrec->subnet_name );
+    dbgtext( "for workgroup %s\n", work->work_group );
+    }
+

(The dbgtext() function is explained below.)

There are a few advantages to this scheme:

+The test is performed only once. +
+You can allocate variables off of the stack that will only be used +within the DEBUGLVL() block. +
+Processing that is only relevant to debug output can be contained +within the DEBUGLVL() block. +

New Functions

dbgtext()

+This function prints debug message text to the debug file (and +possibly to syslog) via the format buffer. The function uses a +variable argument list just like printf() or Debug1(). The +input is printed into a buffer using the vslprintf() function, +and then passed to format_debug_text(). + +If you use DEBUGLVL() you will probably print the body of the +message using dbgtext(). +

dbghdr()

+This is the function that writes a debug message header. +Headers are not processed via the format buffer. Also note that +if the format buffer is not empty, a call to dbghdr() will not +produce any output. See the comments in dbghdr() for more info. +

+It is not likely that this function will be called directly. It +is used by DEBUG() and DEBUGADD(). +

format_debug_text()

+This is a static function in debug.c. It stores the output text +for the body of the message in a buffer until it encounters a +newline. When the newline character is found, the buffer is +written to the debug file via the Debug1() function, and the +buffer is reset. This allows us to add the indentation at the +beginning of each line of the message body, and also ensures +that the output is written a line at a time (which cleans up +syslog output). +

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