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A configure script for a cross compilation tool will use the --target option to control how it is built, so that the resulting program will produce programs which run on the appropriate system. In this section we explain how you can write your own configure scripts to support the --target option.
You must start by putting AC_CANONICAL_SYSTEM in configure.in.
AC_CANONICAL_SYSTEM will look for a --target option and canonicalize it using the config.sub shell script (for more information about configuration names, canonicalizing them, and config.sub, see the section called Configuration Names in the chapter called How to run configure and make). AC_CANONICAL_SYSTEM will also run AC_CANONICAL_HOST to get the host information.
The host and target type will be recorded in the following shell variables:
The canonical configuration name of the host. This will normally be determined by running the config.guess shell script, although the user is permitted to override this by using an explicit --host option.
The canonical configuration name of the target.
The argument to the --host option, if used. Otherwise, the same as the host variable.
The argument to the --target option. If the user did not specify a --target option, this will be the same as host_alias.
The first three parts of the canonical host configuration name.
The first three parts of the canonical target configuration name.
Note that if host and target are the same string, you can assume a native configuration. If they are different, you can assume a cross configuration.
It is possible for host and target to represent the same system, but for the strings to not be identical. For example, if config.guess returns sparc-sun-sunos4.1.4, and somebody configures with --target sparc-sun-sunos4.1, then the slight differences between the two versions of SunOS may be unimportant for your tool. However, in the general case it can be quite difficult to determine whether the differences between two configuration names are significant or not. Therefore, by convention, if the user specifies a --target option without specifying a --host option, it is assumed that the user wants to configure a cross compilation tool.
The target variable should not be handled in the same way as the target_alias variable. In general, whenever the user may actually see a string, target_alias should be used. This includes anything which may appear in the file system, such as a directory name or part of a tool name. It also includes any tool output, unless it is clearly labelled as the canonical target configuration name. This permits the user to use the --target option to specify how the tool will appear to the outside world. On the other hand, when checking for characteristics of the target system, target should be used. This is because a wide variety of --target options may map into the same canonical configuration name. You should not attempt to duplicate the canonicalization done by config.sub in your own code.
By convention, cross tools are installed with a prefix of the argument used with the --target option, also known as target_alias. If the user does not use the --target option, and thus is building a native tool, no prefix is used. For example, if gcc is configured with --target mips-elf, then the installed binary will be named mips-elf-gcc. If gcc is configured without a --target option, then the installed binary will be named gcc.
The Autoconf macro AC_ARG_PROGRAM will handle the names of binaries for you. If you are using Automake, no more need be done; the programs will automatically be installed with the correct prefixes. Otherwise, see the Autoconf documentation for AC_ARG_PROGRAM.
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