3. Makefiles

This chapter discusses the Makefiles associated with a BSP. It does not describe the process of configuring, building, and installing RTEMS. This chapter will not provide detailed information about this process. Nonetheless, it is important to remember that the general process consists of four phases as shown here:

  • bootstrap
  • configure
  • build
  • install

During the bootstrap phase, you are using the configure.ac and Makefile.am files as input to GNU autoconf and automake to generate a variety of files. This is done by running the bootstrap script found at the top of the RTEMS source tree.

During the configure phase, a number of files are generated. These generated files are tailored for the specific host/target combination by the configure script. This set of files includes the Makefiles used to actually compile and install RTEMS.

During the build phase, the source files are compiled into object files and libraries are built.

During the install phase, the libraries, header files, and other support files are copied to the BSP specific installation point. After installation is successfully completed, the files generated by the configure and build phases may be removed.

3.1. Makefiles Used During The BSP Building Process

RTEMS uses the GNU automake and GNU autoconf automatic configuration package. Consequently, there are a number of automatically generated files in each directory in the RTEMS source tree. The bootstrap script found in the top level directory of the RTEMS source tree is executed to produce the automatically generated files. That script must be run from a directory with a configure.ac file in it. The bootstrap command is usually invoked in one of the following manners:

  • bootstrap to regenerate all files that are generated by autoconf and automake.
  • bootstrap -c to remove all files generated by autoconf and automake.
  • bootstrap -p to regenerate preinstall.am files.

There is a file named Makefile.am in each directory of a BSP. This file is used by automake to produce the file named Makefile.in which is also found in each directory of a BSP. When modifying a Makefile.am, you can probably find examples of anything you need to do in one of the BSPs.

The configure process specializes the Makefile.in files at the time that RTEMS is configured for a specific development host and target. Makefiles are automatically generated from the Makefile.in files. It is necessary for the BSP developer to provide the Makefile.am files and generate the Makefile.in files. Most of the time, it is possible to copy the Makefile.am from another similar directory and edit it.

The Makefile files generated are processed when configuring and building RTEMS for a given BSP.

The BSP developer is responsible for generating Makefile.am files which properly build all the files associated with their BSP. Most BSPs will only have a single Makefile.am which details the set of source files to build to compose the BSP support library along with the set of include files that are to be installed.

This single Makefile.am at the top of the BSP tree specifies the set of header files to install. This fragment from the SPARC/ERC32 BSP results in four header files being installed.

include_HEADERS = include/bsp.h
include_HEADERS += include/tm27.h
include_HEADERS += include/erc32.h
include_HEADERS += include/coverhd.h

When adding new include files, you will be adding to the set of include_HEADERS. When you finish editing the Makefile.am file, do not forget to run bootstrap -p to regenerate the preinstall.am.

The Makefile.am also specifies which source files to build. By convention, logical components within the BSP each assign their source files to a unique variable. These variables which define the source files are collected into a single variable which instructs the GNU autotools that we are building libbsp.a. This fragment from the SPARC/ERC32 BSP shows how the startup related, miscellaneous support code, and the console device driver source is managed in the Makefile.am.

startup_SOURCES = ../../sparc/shared/bspclean.c ../../shared/bsplibc.c \
../../shared/bsppredriverhook.c \
../../shared/bsppost.c ../../sparc/shared/bspstart.c \
../../shared/bootcard.c ../../shared/sbrk.c startup/setvec.c \
startup/spurious.c startup/erc32mec.c startup/boardinit.S
clock_SOURCES = clock/ckinit.c
...
noinst_LIBRARIES = libbsp.a
libbsp_a_SOURCES = $(startup_SOURCES) $(console_SOURCES) ...

When adding new files to an existing directory, do not forget to add the new files to the list of files to be built in the corresponding XXX_SOURCES variable in the Makefile.am and run``bootstrap``.

Some BSPs use code that is built in libcpu. If you BSP does this, then you will need to make sure the objects are pulled into your BSP library. The following from the SPARC/ERC32 BSP pulls in the cache, register window management and system call support code from the directory corresponding to its RTEMS_CPU model.

libbsp_a_LIBADD  = ../../../libcpu/@RTEMS_CPU@/cache.rel \
../../../libcpu/@RTEMS_CPU@/reg_win.rel \
../../../libcpu/@RTEMS_CPU@/syscall.rel

The Makefile.am files are ONLY processed by bootstrap and the resulting Makefile.in files are only processed during the configure process of a RTEMS build. Therefore, when developing a BSP and adding a new file to a Makefile.am, the already generated Makefile will not automatically include the new references unless you configured RTEMS with the --enable-maintainer-mode option. Otherwise, the new file will not being be taken into account!

3.2. Creating a New BSP Make Customization File

When building a BSP or an application using that BSP, it is necessary to tailor the compilation arguments to account for compiler flags, use custom linker scripts, include the RTEMS libraries, etc.. The BSP must be built using this information. Later, once the BSP is installed with the toolset, this same information must be used when building the application. So a BSP must include a build configuration file. The configuration file is make/custom/BSP.cfg.

The configuration file is taken into account when building one’s application using the RTEMS template Makefiles (make/templates). These application template Makefiles have been included with the RTEMS source distribution since the early 1990’s. However there is a desire in the RTEMS user community to move all provided examples to GNU autoconf. They are included in the 4.9 release series and used for all examples provided with RTEMS. There is no definite time table for obsoleting them. You are free to use these but be warned they have fallen out of favor with many in the RTEMS community and may disappear in the future.

The following is a slightly shortened version of the make customization file for the gen68340 BSP. The original source for this file can be found in the make/custom directory.

# The RTEMS CPU Family and Model
RTEMS_CPU=m68k
RTEMS_CPU_MODEL=m68340
include $(RTEMS_ROOT)/make/custom/default.cfg
# This is the actual bsp directory used during the build process.
RTEMS_BSP_FAMILY=gen68340
# This contains the compiler options necessary to select the CPU model
# and (hopefully) optimize for it.
CPU_CFLAGS = -mcpu=cpu32
# optimize flag: typically -O2
CFLAGS_OPTIMIZE_V = -O2 -g -fomit-frame-pointer

The make customization files have generally grown simpler and simpler with each RTEMS release. Beginning in the 4.9 release series, the rules for linking an RTEMS application are shared by all BSPs. Only BSPs which need to perform a transformation from linked ELF file to a downloadable format have any additional actions for program link time. In 4.8 and older, every BSP specified the “make executable” or make-exe rule and duplicated the same actions.

It is generally easier to copy a make/custom file from a BSP similar to the one being developed.