13.3 Using the System Type

In configure.ac the system type is generally used by one or more case statements to select system-specifics. Shell wildcards can be used to match a group of system types.

For example, an extra assembler code object file could be chosen, giving access to a CPU cycle counter register. $(CYCLE_OBJ) in the following would be used in a makefile to add the object to a program or library.

     case $host in
       alpha*-*-*) CYCLE_OBJ=rpcc.o ;;
       i?86-*-*)   CYCLE_OBJ=rdtsc.o ;;
       *)          CYCLE_OBJ= ;;
     esac
     AC_SUBST([CYCLE_OBJ])

AC_CONFIG_LINKS (see Configuration Links) is another good way to select variant source files, for example optimized code for some CPUs. The configured CPU type doesn't always indicate exact CPU types, so some runtime capability checks may be necessary too.

     case $host in
       alpha*-*-*)   AC_CONFIG_LINKS([dither.c:alpha/dither.c]) ;;
       powerpc*-*-*) AC_CONFIG_LINKS([dither.c:powerpc/dither.c]) ;;
       *-*-*)        AC_CONFIG_LINKS([dither.c:generic/dither.c]) ;;
     esac

The host system type can also be used to find cross-compilation tools with AC_CHECK_TOOL (see Generic Programs).

The above examples all show ‘$host’, since this is where the code is going to run. Only rarely is it necessary to test ‘$build’ (which is where the build is being done).

Whenever you're tempted to use ‘$host’ it's worth considering whether some sort of probe would be better. New system types come along periodically or previously missing features are added. Well-written probes can adapt themselves to such things, but hard-coded lists of names can't. Here are some guidelines,

• Availability of libraries and library functions should always be checked by probing.
• Variant behavior of system calls is best identified with runtime tests if possible, but bug workarounds or obscure difficulties might have to be driven from ‘$host’.
• Assembler code is inevitably highly CPU-specific and is best selected according to ‘$host_cpu’.
• Assembler variations like underscore prefix on globals or ELF versus COFF type directives are however best determined by probing, perhaps even examining the compiler output.

‘$target’ is for use by a package creating a compiler or similar. For ordinary packages it's meaningless and should not be used. It indicates what the created compiler should generate code for, if it can cross-compile. ‘$target’ generally selects various hard-coded CPU and system conventions, since usually the compiler or tools under construction themselves determine how the target works.