Quotation Rule Of Thumb
-----------------------
To conclude, the quotation rule of thumb is:
_One pair of quotes per pair of parentheses._
Never over-quote, never
under-quote, in particular in the definition of macros. In the few
places where the macros need to use brackets (usually in C program text
or regular expressions), properly quote _the arguments_!
It is common to read Autoconf programs with snippets like:
AC_TRY_LINK(
changequote(<<, >>)dnl
<<#include <time.h>
#ifndef tzname /* For SGI. */
extern char *tzname[]; /* RS6000 and others reject char **tzname. */
#endif>>,
changequote([, ])dnl
[atoi (*tzname);], ac_cv_var_tzname=yes, ac_cv_var_tzname=no)
which is incredibly useless since `AC_TRY_LINK' is _already_ double
quoting, so you just need:
AC_TRY_LINK(
[#include <time.h>
#ifndef tzname /* For SGI. */
extern char *tzname[]; /* RS6000 and others reject char **tzname. */
#endif],
[atoi (*tzname);],
[ac_cv_var_tzname=yes],
[ac_cv_var_tzname=no])
The M4-fluent reader will note that these two examples are rigorously
equivalent, since `m4' swallows both the `changequote(<<, >>)' and `<<'
`>>' when it "collects" the arguments: these quotes are not part of the
arguments!
Simplified, the example above is just doing this:
changequote(<<, >>)dnl
<<[]>>
changequote([, ])dnl
instead of simply:
[[]]
With macros that do not double quote their arguments (which is the
rule), double-quote the (risky) literals:
AC_LINK_IFELSE([AC_LANG_PROGRAM(
[[#include <time.h>
#ifndef tzname /* For SGI. */
extern char *tzname[]; /* RS6000 and others reject char **tzname. */
#endif]],
[atoi (*tzname);])],
[ac_cv_var_tzname=yes],
[ac_cv_var_tzname=no])
See Note:Quadrigraphs, for what to do if you run into a hopeless
case where quoting does not suffice.
When you create a `configure' script using newly written macros,
examine it carefully to check whether you need to add more quotes in
your macros. If one or more words have disappeared in the `m4' output,
you need more quotes. When in doubt, quote.
However, it's also possible to put on too many layers of quotes. If
this happens, the resulting `configure' script will contain unexpanded
macros. The `autoconf' program checks for this problem by doing `grep
AC_ configure'.