m4: Debug Output

1 
1 7.4 Saving debugging output
1 ===========================
1 
1 Debug and tracing output can be redirected to files using either the
1 '--debugfile' option to 'm4' (⇒Invoking m4 Debugging options.), or
1 with the builtin macro 'debugfile':
1 
1  -- Builtin: debugfile ([FILE])
1      Sends all further debug and trace output to FILE, opened in append
1      mode.  If FILE is the empty string, debug and trace output are
1      discarded.  If 'debugfile' is called without any arguments, debug
1      and trace output are sent to standard error.  This does not affect
1      warnings, error messages, or 'errprint' output, which are always
1      sent to standard error.  If FILE cannot be opened, the current
1      debug file is unchanged, and an error is issued.
1 
1      The expansion of 'debugfile' is void.
1 
1      $ m4 -d
1      traceon(`divnum')
1      =>
1      divnum(`extra')
1      error->m4:stdin:2: Warning: excess arguments to builtin `divnum' ignored
1      error->m4trace: -1- divnum(`extra') -> `0'
1      =>0
1      debugfile()
1      =>
1      divnum(`extra')
1      error->m4:stdin:4: Warning: excess arguments to builtin `divnum' ignored
1      =>0
1      debugfile
1      =>
1      divnum
1      error->m4trace: -1- divnum -> `0'
1      =>0
1