188.8.131.52 Supporting Building with a Cross Compiler in Makefiles
The main cross compiling issue in a `Makefile' arises when you want
to use a subsidiary program to generate code or data which you will then
include in your real program. If you compile this subsidiary program
using `$(CC)' in the usual way, you will not be able to run it.
This is because `$(CC)' will build a program for the host system,
but the program is being built on the build system. You must instead
use a compiler for the build system, rather than the host system. This
compiler is conventionally called `$(CC_FOR_BUILD)'.
A `configure' script should normally permit the user to define
`CC_FOR_BUILD' explicitly in the environment. Your configure
script should help by selecting a reasonable default value. If the
`configure' script is not being run with a cross compiler (i.e.,
the `cross_compiling' shell variable is `no' after calling
`AC_PROG_CC'), then the proper default for `CC_FOR_BUILD' is
simply `$(CC)'. Otherwise, a reasonable default is simply
Note that you should not include `config.h' in a file you are
compiling with `$(CC_FOR_BUILD)'. The `configure' script will
build `config.h' with information for the host system. However,
you are compiling the file using a compiler for the build system (a
native compiler). Subsidiary programs are normally simple filters which
do no user interaction, and it is often possible to write them in a
highly portable fashion so that the absence of `config.h' is not
gcc `Makefile.in' shows a complex situation in which
certain files, such as `rtl.c', must be compiled into both
subsidiary programs run on the build system and into the final program.
This approach may be of interest for advanced GNU Autotools hackers. Note
that, at least in GCC 2.95, the build system compiler is rather
confusingly called `HOST_CC'.