I am developing my packages in buildroot with kernel modules: A and B. The source code from B requires include <A/header.h>. How can I access <A/header.h> from package B?
A/Config.in
config BR2_PACKAGE_A
bool "A"
depends on BR2_LINUX_KERNEL
help
Linux Kernel Module A.
A/a.mk
################################################################################
#
# B
#
################################################################################
A_VERSION = 1.0
A_SITE = ./package/av/a/src
A_SITE_METHOD = local
A_MODULE_SUBDIRS = linux_kernel
$(eval $(kernel-module))
$(eval $(generic-package))
B/Config.in
config BR2_PACKAGE_B
bool "B"
depends on BR2_LINUX_KERNEL
depends on BR2_PACKAGE_A
help
Linux Kernel Module B.
B/b.mk
################################################################################
#
# B
#
################################################################################
B_VERSION = 1.0
B_SITE = ./package/av/b
B_SITE_METHOD = local
B_DEPENDENCIES += a
$(eval $(kernel-module))
$(eval $(generic-package))
B/Makefile
EXTRA_CFLAGS := -I$(BR2_PACKAGE_A)/include/ -I$(src)
KBUILD_EXTRA_SYMBOLS:= $(BR2_PACKAGE_A)/Module.symvers
obj-m += b.o
all:
make -C $(LINUX_DIR) M=$(PWD) modules
Of course, BR2_PACKAGE_A does not work. I have not found a valid option
In general, the best option is to make sure that package A installs its header file(s) in $(STAGING_DIR).
In a/a.mk:
A_INSTALL_STAGING = YES
define A_INSTALL_STAGING_CMDS
install -m 0644 -D $(A_BUILDDIR)/a.h $(STAGING_DIR)/usr/include/a.h
endef
Then you don't need to add any extra -I in B's Makefile, it is in the standard include path.
Unfortunately, this approach won't work here because B is a kernel module, and these don't use the standard include path. You could still use the same approach by installing in e.g. $(STAGING_DIR)/usr/src/linux/include - kernel headers should not be installed in /usr/include.
Alternatively, you can use -I$(A_SRCDIR)/include or -I$(A_BUILDDIR)/include in b.mk (SRCDIR if it is a source file, BUILDDIR if it is a generated file). There are two examples of this approach in Buildroot: sysdig which looks into falcosecurity-libs source, and external kernel modules which look into the kernel's build directory. Since B is in fact a kernel module, it's probably best to use the same approach here.
A third, even more complex alternative is to not make A a separate package, but instead include it in the build of B using the EXTRA_DOWNLOADS and pre/post extract and build hooks. But that makes it impossible to build A without B, so probably not what you need either.