File: autoconf.info,  Node: Debugging via autom4te,  Prev: Programming in M4sugar,  Up: Programming in M4

8.4 Debugging via autom4te
==========================

At times, it is desirable to see what was happening inside m4, to see
why output was not matching expectations.  However, post-processing done
by ‘autom4te’ means that directly using the m4 builtin ‘m4_traceon’ is
likely to interfere with operation.  Also, frequent diversion changes
and the concept of forbidden tokens make it difficult to use ‘m4_defn’
to generate inline comments in the final output.

   There are a couple of tools to help with this.  One is the use of the
‘--trace’ option provided by ‘autom4te’ (as well as each of the programs
that wrap ‘autom4te’, such as ‘autoconf’), in order to inspect when a
macro is called and with which arguments.  For example, when this
paragraph was written, the autoconf version could be found by:

     $ autoconf --trace=AC_INIT
     configure.ac:23:AC_INIT:GNU Autoconf:2.63b.95-3963:bug-autoconf@gnu.org
     $ autoconf --trace='AC_INIT:version is $2'
     version is 2.63b.95-3963

   Another trick is to print out the expansion of various m4 expressions
to standard error or to an independent file, with no further m4
expansion, and without interfering with diversion changes or the
post-processing done to standard output.  ‘m4_errprintn’ shows a given
expression on standard error.  For example, if you want to see the
expansion of an autoconf primitive or of one of your autoconf macros,
you can do it like this:

     $ cat <<\EOF > configure.ac
     AC_INIT
     m4_errprintn([The definition of AC_DEFINE_UNQUOTED:])
     m4_errprintn(m4_defn([AC_DEFINE_UNQUOTED]))
     AC_OUTPUT
     EOF
     $ autoconf
     error→The definition of AC_DEFINE_UNQUOTED:
     error→_AC_DEFINE_Q([], $@)