(m4.info)Debug Output

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7.4 Saving debugging output
===========================

Debug and tracing output can be redirected to files using either the
'--debugfile' option to 'm4' (Note: Invoking m4.), or
with the builtin macro 'debugfile':

 -- Builtin: debugfile ([FILE])
     Sends all further debug and trace output to FILE, opened in append
     mode.  If FILE is the empty string, debug and trace output are
     discarded.  If 'debugfile' is called without any arguments, debug
     and trace output are sent to standard error.  This does not affect
     warnings, error messages, or 'errprint' output, which are always
     sent to standard error.  If FILE cannot be opened, the current
     debug file is unchanged, and an error is issued.

     The expansion of 'debugfile' is void.

     $ m4 -d
     traceon(`divnum')
     =>
     divnum(`extra')
     error->m4:stdin:2: Warning: excess arguments to builtin `divnum' ignored
     error->m4trace: -1- divnum(`extra') -> `0'
     =>0
     debugfile()
     =>
     divnum(`extra')
     error->m4:stdin:4: Warning: excess arguments to builtin `divnum' ignored
     =>0
     debugfile
     =>
     divnum
     error->m4trace: -1- divnum -> `0'
     =>0