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11.1 Specifying the System Type

Like other GNU configure scripts, Autoconf-generated configure scripts can make decisions based on a canonical name for the system type, which has the form: `cpu-vendor-os', where os can be `system' or `kernel-system'

configure can usually guess the canonical name for the type of system it's running on. To do so it runs a script called config.guess, which infers the name using the uname command or symbols predefined by the C preprocessor.

Alternately, the user can specify the system type with command line arguments to configure. Doing so is necessary when cross-compiling. In the most complex case of cross-compiling, three system types are involved. The options to specify them are(3):

the type of system on which the package is being configured and compiled.

the type of system on which the package will run.

the type of system for which any compiler tools in the package will produce code (rarely needed). By default, it is the same as host.

They all default to the result of running config.guess, unless you specify either `--build' or `--host'. In this case, the default becomes the system type you specified. If you specify both, and they're different, configure will enter cross compilation mode, so it won't run any tests that require execution.

Hint: if you mean to override the result of config.guess, prefer `--build' over `--host'. In the future, `--host' will not override the name of the build system type. Also, if you specify `--host', but not `--build', when configure performs the first compiler test it will try to run an executable produced by the compiler. If the execution fails, it will enter cross-compilation mode. Note, however, that it won't guess the build-system type, since this may require running test programs. Moreover, by the time the compiler test is performed, it may be too late to modify the build-system type: other tests may have already been performed. Therefore, whenever you specify --host, be sure to specify --build too.

./configure --build=i686-pc-linux-gnu --host=m68k-coff

will enter cross-compilation mode, but configure will fail if it can't run the code generated by the specified compiler if you configure as follows:

./configure CC=m68k-coff-gcc

configure recognizes short aliases for many system types; for example, `decstation' can be used instead of `mips-dec-ultrix4.2'. configure runs a script called config.sub to canonicalize system type aliases.

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