Debug and tracing output can be redirected to files using either the
--debugfile option to
m4 (see Invoking m4), or with the builtin macro
Sends all further debug and trace output to file, opened in append
mode. If file is the empty string, debug and trace output are
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