Using Configuration Names

While feature tests are definitely the best approach, a configure script may occasionally have to make a decision based on a configuration name. This may be necessary if certain code must be compiled differently based on something which can not be tested using a standard Autoconf feature test. For instance, the expect package needs to find information about the system's tty implementation; this can't reliably be done when cross compiling without examining the particular configuration name.

It is normally better to test for particular features, rather than to test for a particular system type. This is because as Unix and other operating systems evolve, different systems copy features from one another.

When there is no alternative to testing the configuration name in a configure script, it is best to define a macro which describes the feature, rather than defining a macro which describes the particular system. This permits the same macro to be used on other systems which adopt the same feature (see the chapter called Writing New Macros for Autoconf).

Testing for a particular system is normally done using a case statement in the autoconf configure.in file. The case statement might look something like the following, assuming that host is a shell variable holding a canonical configuration system--which will be the case if configure.in uses the AC_CANONICAL_HOST or AC_CANONICAL_SYSTEM macros.

case "${host}" in i[[3456]]86-*-linux-gnu*) do something ;; sparc*-sun-solaris2.[[56789]]*) do something ;; sparc*-sun-solaris*) do something ;; mips*-*-elf*) do something ;; esac

Note the doubled square brackets in this piece of code. These are used to work around an ugly implementation detail of autoconf--it uses M4 under the hood. Without these extra brackets, the square brackets in the case statement would be swallowed by M4, and would not appear in the resulting configure. This nasty detail is discussed at more length in the chapter called M4.

It is particularly important to use * after the operating system field, in order to match the version number which will be generated by config.guess. In most cases you must be careful to match a range of processor types. For most processor families, a trailing * suffices, as in mips* above. For the i386 family, something along the lines of i[34567]86 suffices at present. For the m68k family, you will need something like m68*. Of course, if you do not need to match on the processor, it is simpler to just replace the entire field by a *, as in *-*-irix*.