The fundamental principle of the build system
Assume that we need to build the program or alienated package for working on the three devices with names ci20, bt01 and dm64. The first two devices (ci20, bt01) are based on the MIPS architecture, and the third device (dm64) is built on ARM-based processor. Toolchains for building our program, for simplicity, let's call mips and arm, respectively.
The build script of the source program is the same for each of our devices and is written on GNU Make.
If we present all available combinations of command line calls, required for building the program for our devices, we get:
$ TOOLCHAIN=mips HARDWARE=ci20 make
$ TOOLCHAIN=mips HARDWARE=bt01 make
$ TOOLCHAIN=arm HARDWARE=dm64 make
or (in case when the TOOLCHAIN-HARDWARE pairs are transmitted as arguments):
$ make TOOLCHAIN=mips HARDWARE=ci20
$ make TOOLCHAIN=mips HARDWARE=bt01
$ make TOOLCHAIN=arm HARDWARE=dm64
Thus, the build system must receive a TOOLCHAIN-HARDWARE pair, and then the build system has to determine which toolchain must be used for a particular device.
Let us now consider how to organize the sequence of command calls (on the build system level) in such way that the user can do these actions by applying only one call:
$ make
without specifying additional arguments which are responsible for selection of the target device and applicable toolchain.
If we describe the list of valid terget devices at the beginning of our script, for example, as follows:
COMPONENT_TARGETS = $(HARDWARE_CI20)
COMPONENT_TARGETS += $(HARDWARE_BT01)
COMPONENT_TARGETS += $(HARDWARE_DM64)
then the build system can automatically construct a list of possible TOOLCHAIN-HARDWARE combinations for a given build script, which will looks like following:
targets = target_mips_ci20 target_mips_bt01 target_arm_dm64
With such list, the build system can restore arguments which are needed for each of three our calls. It is very simple to do. On the GNU Make language we can do it as shown by following lines:
target_%: TOOLCHAIN = $(shell echo $(word 2, $(subst _, , $@)))
target_%: HARDWARE = $(shell echo $(word 3, $(subst _, , $@)))
target_%:
$(MAKE) TOOLCHAIN=$(TOOLCHAIN) HARDWARE=$(HARDWARE)
Thus, if we call the Make utility without arguments then TOOLCHAIN and HARDWARE variables will be undefined. In this case the build system starts to collect the targets list. When the targets list will be complete the build system can do the call
$(MAKE) TOOLCHAIN=$(TOOLCHAIN) HARDWARE=$(HARDWARE)
with valid arguments.
When (at the next call) the system will make sure that the TOOLCHAIN and HARDWARE variables are defined, the control of the build process will be passed to our build script without additional calculations.
The described mechanism is directly derived from the GNU Make documentation.