8.3.10 Forbidden Patterns

M4sugar provides a means to define suspicious patterns, patterns describing tokens which should not be found in the output. For instance, if an Autoconf configure script includes tokens such as ‘AC_DEFINE’, or ‘dnl’, then most probably something went wrong (typically a macro was not evaluated because of overquotation).

M4sugar forbids all the tokens matching ‘^_?m4_’ and ‘^dnl$’. Additional layers, such as M4sh and Autoconf, add additional forbidden patterns to the list.

Macro: m4_pattern_forbid (pattern)

Declare that no token matching pattern must be found in the output. The output file is (temporarily) split into one word per line as part of the autom4te post-processing, with each line (and therefore word) then being checked against the Perl regular expression pattern. If the regular expression matches, and m4_pattern_allow does not also match, then an error is raised.

Comments are not checked; this can be a problem if, for instance, you have some macro left unexpanded after an ‘#include’. No consensus is currently found in the Autoconf community, as some people consider it should be valid to name macros in comments (which doesn’t make sense to the authors of this documentation: input, such as macros, should be documented by ‘dnl’ comments; reserving ‘#’-comments to document the output).

As an example, if you define your own macros that begin with ‘M_’ and are composed from capital letters and underscores, the specification of m4_pattern_forbid([^M_[A-Z_]+]) will ensure all your macros are expanded when not used in comments.

As an example of a common use of this macro, consider what happens in packages that want to use the pkg-config script via the third-party PKG_CHECK_MODULES macro. By default, if a developer checks out the development tree but has not yet installed the pkg-config macros locally, they can manage to successfully run autoconf on the package, but the resulting configure file will likely result in a confusing shell message about a syntax error on the line mentioning the unexpanded PKG_CHECK_MODULES macro. On the other hand, if configure.ac includes m4_pattern_forbid([^PKG_]), the missing pkg-config macros will be detected immediately without allowing autoconf to succeed.

Of course, you might encounter exceptions to these generic rules, for instance you might have to refer to ‘$m4_flags’.

Macro: m4_pattern_allow (pattern)

Any token matching pattern is allowed, including if it matches an m4_pattern_forbid pattern.

For example, Gnulib uses m4_pattern_forbid([^gl_]) to reserve the gl_ namespace for itself, but also uses m4_pattern_allow([^gl_ES$]) to avoid a false negative on the valid locale name.