[CMake] Linking object files into executable target

[Colin D Bennett]

I'm using CMake and SDCC to build a project for the 8051
microcontroller.  CMake has been working great for this so far, but now
I need to link in an object file that I've assembled with SDCC's
"asx8051" assembler, and I am not able to get CMake to explicitly add
an object file to the list of objects to be linked into a target.

The problem is that CMake is *silently* ignoring the request to link the
object file into the executable target with ADD_EXECUTABLE().

First, I am using ADD_CUSTOM_COMMAND to assemble the assembly code into
an object file (".rel" extension, as are the object files produced by
SDCC for compiled C code).  The custom command is resulting in the
object file being produced properly.

Next, my ADD_EXECUTABLE() call simply names the object file in addition
to the C source files that I already am specifying.  However, while the
C source files are compiled and linked with into the target executable,
the '.rel' object file is not being added into the list of objects to
link into the executable.

I think that the GENERATED=TRUE and EXTERNAL_OBJECT=TRUE source
properties on the generated object file should cause CMake to link it.

Is there something I'm doing wrong?
Here's my CMakeLists.txt:

-----------------------------------------------------------------
CMAKE_MINIMUM_REQUIRED(VERSION 2.6)
PROJECT(timertest C)
SET(CMAKE_ASM_OUTPUT_EXTENSION ${CMAKE_C_OUTPUT_EXTENSION})

SET(MY_SRCS main.c)
SET(MY_ASM_SRCS sleep_timer_get.asm)

FOREACH(ASMFILE ${MY_ASM_SRCS})
   GET_FILENAME_COMPONENT(BASENAME ${ASMFILE} NAME_WE)
   SET(SRC ${CMAKE_CURRENT_SOURCE_DIR}/${ASMFILE})
   SET(OBJ
${CMAKE_CURRENT_BINARY_DIR}/${BASENAME}${CMAKE_ASM_OUTPUT_EXTENSION})
SET_SOURCE_FILES_PROPERTIES(${OBJ} PROPERTIES
                               GENERATED TRUE
                               EXTERNAL_OBJECT TRUE)
   ADD_CUSTOM_COMMAND(OUTPUT ${OBJ}
                      MAIN_DEPENDENCY ${SRC}
                      COMMAND asx8051-wrapper
                      ARGS -I${CMAKE_CURRENT_SOURCE_DIR} 
                           -o ${OBJ} ${SRC})
   LIST(APPEND MY_ASM_OBJS ${OBJ})
ENDFOREACH(ASMFILE ${MY_ASM_SRCS})

MESSAGE(STATUS "MY_ASM_OBJS is ${MY_ASM_OBJS}")

ADD_EXECUTABLE(timertest main.c ${MY_ASM_OBJS})
-----------------------------------------------------------------

I have attached the output I get when I build using this file.

I have spent all day and most of the night trying to get this to work,
and I've already started working on a Makefile to replace CMake for
this, though I really would hate to give up CMake; however, this silent
failure by CMake is creating an impossible-to-track-down problem.

Thanks for any help.
Regards,
Colin
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