m4: Debug Output
7.4 Saving debugging output
===========================
Debug and tracing output can be redirected to files using either the
'--debugfile' option to 'm4' (⇒Invoking m4 Debugging options.), 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