21.5.2 Debugging with M4
After writing a new macro or a `configure.in' template, the generated `configure' script may not contain what you expect. Frequently this is due to a problem in quoting (see section 21.3.3 Quoting), but the interactions between macros can be complex. When you consider that the arguments to GNU Autotools macros are often shell scripts, things can get rather hairy. A number of techniques exist for helping you to debug these kinds of problems.
Expansion problems due to over-quoting and under-quoting can be
difficult to pinpoint. Autoconf half-heartedly tries to detect this
condition by scanning the generated `configure' script for any
remaining invocations of the
M4 provides a comprehensive facility for tracing expansions. This makes it possible to see how macro arguments are expanded and how a macro is finally expanded. Often, this can be half the battle in discovering if the macro definition or the invocation is at fault. Autoconf 2.15 will include this tracing mechanism. To trace the generation of `configure', Autoconf can be invoked like so:
Autoconf provides fine control over which macros are traced and the format of the trace output. You should refer to the Autoconf manual for further details.
Another situation that can arise is the presence of shell syntax errors
in the generated `configure' script. These errors are usually
obvious, as the shell will abort `configure' when the syntax error
is encountered. The task of then locating the troublesome shell code in
the input files can be potentially quite difficult. If the erroneous
shell code appears in `configure.in', it should be easy to
spot--presumably because you wrote it recently! If the code is imported
from a third party macro, though, it may only be present because you
invoked that macro. A trick to help locate these kinds of errors is to
place some magic text (
Finally, due to an error on your part,
Semantic errors of this kind are usually easy to solve once you can spot
them. A fast and simple way of tracing the shell execution is to use
the shell's `-x' and `-v' options to turn on its own
tracing. This can be done by explicitly placing the required
This kind of tracing is invaluable in debugging shell code containing semantic errors.