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With a good grasp of M4 concepts, we may turn our attention to applying these principles to writing ‘configure.in’ files and new ‘.m4’ macro files. There are some differences between writing generic M4 input files and macros within the GNU Autotools framework and these will be covered in this section, along with some useful hints on working within the framework. This section ties in closely with Writing New Macros for Autoconf.
Now that you are familiar with the capabilities of M4, you can
forget about the names of the built-in M4 macros–they should
be avoided in the GNU Autotools framework. Where appropriate, the
framework provides a collection of macros that are laid on top of the
M4 built-ins. For instance, the macros in the AC_
family are just regular M4 macros that take a number of
arguments and rely on an extensive library of AC_
support macros.
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Some conventions have grown over the life of the GNU Autotools, mostly as a disciplined way of avoiding M4 pitfalls. These conventions are designed to make your macros more robust, your code easier to read and, most importantly, improve your chances for getting things to work the first time! A brief list of recommended conventions appears below:
changequote
. Any good macro
will already perform sufficient quoting.
dnl
to prevent unwanted whitespace from accumulating in
‘configure’.
AC_
macros, and others which emulate their good
behavior, permit default values for unspecified arguments. It is
considered good style to explicitly show your intention to use an empty
argument by using a pair of quotes, such as []
.
autoconf
and inspect
the results.
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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 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 AC_
and AM_
families of
macros. However, this only works for the AC_
and AM_
macros and not for third party macros.
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 --trace=AC_PROG_CC |
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.
GNU m4
also provides a debugging mode that can be helpful in
discovering problems such as infinite recursion. This mode is activated
with the ‘-d’ option. In order to pass options to m4
,
invoke Autoconf like so:
$ M4='m4 -dV' autoconf |
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 (__MAGIC__
) throughout ‘configure.in’:
AC_INIT AC_PROG_CC __MAGIC__ MY_SUSPECT_MACRO __MAGIC__ AC_OUTPUT(Makefile) |
After autoconf
has generated ‘configure’, you can search
through it for the magic text to determine the extremities of the
suspect macro. If your erroneous code appears within the magic text
markers, you’ve found the culprit! Don’t be afraid to hack up
‘configure’. It can easily be regenerated.
Finally, due to an error on your part, m4
may generate a
‘configure’ script that contains semantic errors. Something as
simple as inverted logic may lead to a nonsense test result:
checking for /etc/passwd... no |
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 set
commands into ‘configure.in’:
AC_INIT AC_PROG_CC set -x -v MY_BROKEN_MACRO set +x +v AC_OUTPUT(Makefile) |
This kind of tracing is invaluable in debugging shell code containing semantic errors.
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