cmake dependency not trigged - cmake

Question

I try to use cmake in my project (actually move from make to cmake). I bump into a problem that I can't solve. I try to streamline my architecture like this (hope this is not too streamlined). I use ld -r here as an example custom command that produce files and intermediate files.
Here is my file structure.
.
`-- libs
|-- CMakeLists.txt
|-- l1
| |-- CMakeLists.txt
| |-- l1_f1.c
| `-- l1_f2.c
`-- l2
|-- CMakeLists.txt
|-- l2_f1.c
`-- l2_f2.c
The .c files
./libs/l1/l1_f1.c::
#include <stdio.h>
int v1;
int l1_f1(){ printf("In l1_f1()\n");}
./libs/l1/l1_f2.c::
#include <stdio.h>
int l1_f2(){ printf("In l1_f2()\n");}
./libs/l2/l2_f1.c::
#include <stdio.h>
int l2_f1(){ printf("In l2_f1()\n");}
./libs/l2/l2_f2.c::
#include <stdio.h>
int l2_f2(){ printf("In l2_f2()\n");}
And my CMakeLists.txt files
./libs/CMakeLists.txt::
cmake_minimum_required(VERSION 3.10)
project(PL VERSION 1.0)
add_subdirectory(../libs/l1 libs/l1)
add_subdirectory(../libs/l2 libs/l2)
add_custom_command(
OUTPUT l.a
COMMAND ld -r libs/l1/l1.o libs/l2/l2.o -o l.o
COMMAND rm -f l.a
COMMAND ar r l.a l.o
COMMAND rm -f l.o
DEPENDS L1 L2
)
add_custom_target(
LA
ALL
DEPENDS l.a
)
./libs/l1/CMakeLists.txt::
add_library(l1 OBJECT l1_f1.c l1_f2.c)
add_custom_command(
OUTPUT l1.o
COMMAND ld -r $<TARGET_OBJECTS:l1> -o l1.o
DEPENDS $<TARGET_OBJECTS:l1>
COMMAND_EXPAND_LISTS
)
add_custom_target(
L1
ALL
DEPENDS l1.o
)
./libs/l2/CMakeLists.txt::
add_library(l2 OBJECT l2_f1.c l2_f2.c)
add_custom_command(
OUTPUT l2.o
COMMAND ld -r $<TARGET_OBJECTS:l2> -o l2.o
DEPENDS $<TARGET_OBJECTS:l2>
COMMAND_EXPAND_LISTS
)
add_custom_target(
L2
ALL
DEPENDS l2.o
)
Now the question
I run the original cmake like this
mkdir build && cd build && cmake -G Ninja ../libs && ninja
-- The C compiler identification is GNU 7.4.0
-- The CXX compiler identification is GNU 7.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/g++
-- Check for working CXX compiler: /usr/bin/g++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/phi/p2/build
[7/7] Generating l.a
ar: creating l.a
I am happy with that l.a is created and doing a nm(1) on it shows the v1 global variable.
nm l.a
l.o:
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
0000000000000013 T l1_f2
0000000000000026 T l2_f1
0000000000000039 T l2_f2
U puts
0000000000000004 C v1
Now the problem, I touch 1 file l1_f1.c and rebuild
VY$ sed -i 's/v1/v2/' ../libs/l1/l1_f1.c
VY$ ninja
[2/2] Generating l1.o
As we see here l.a is not rebuild, despite I thought I made a dep for regen it. Obvioulsy I must be wrong, I am new to cmake.
The nm(1) confirm l.a is not regened.
VY$ nm l.a libs/l1/l1.o
l.a:
l.o:
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
0000000000000013 T l1_f2
0000000000000026 T l2_f1
0000000000000039 T l2_f2
U puts
0000000000000004 C v1
libs/l1/l1.o:
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
0000000000000013 T l1_f2
U puts
0000000000000004 C v2
Can someone enlight me how I could get l.a regened ?
Thanx in advance, cheers.
The following is added after Tsyvarev answer proposition.
1) Adding file-level dependencies in main CMakeLists.txt prevent initial build. Note that your initial line DEPENDS libs/l1/l1.o libs/l1/l2.o generate an error as it look file in the source tree instead of the build tree so I changed it to DEPENDS ../build/libs/l1/l1.o ../build/libs/l2/l2.o
cmake_minimum_required(VERSION 3.10)
project(PL VERSION 1.0)
add_subdirectory(../libs/l1 libs/l1)
add_subdirectory(../libs/l2 libs/l2)
add_custom_command(
OUTPUT l.a
COMMAND ld -r libs/l1/l1.o libs/l2/l2.o -o l.o
COMMAND rm -f l.a
COMMAND ar r l.a l.o
COMMAND rm -f l.o
DEPENDS ../build/libs/l1/l1.o ../build/libs/l2/l2.o
DEPENDS L1 L2
)
add_custom_target(
LA
ALL
DEPENDS l.a
)
This give the following output on initial build
VY$ cd .. && rm -rf build && mkdir build && cd build && cmake -G Ninja ../libs && ninja
-- The C compiler identification is GNU 7.4.0
-- The CXX compiler identification is GNU 7.4.0
... [removed output]
-- Configuring done
-- Generating done
-- Build files have been written to: /home/phi/p2/build
ninja: error: '/home/phi/p2/build/libs/l1/l1.o', needed by 'l.a', missing and no known rule to make it
2) rebuilding the project without the file-level dep.
VY$ cd .. && rm -rf build && mkdir build && cd build && cmake -G Ninja ../libs && ninja
-- The C compiler identification is GNU 7.4.0
-- The CXX compiler identification is GNU 7.4.0
... [removed output]
-- Configuring done
-- Generating done
-- Build files have been written to: /home/phi/p2/build
[7/7] Generating l.a
ar: creating l.a
3) With this initial build and your file-dep added again in the CMakethen ask for a partial rebuild give this
Initial setup after project build.
VY$ ninja;nm libs/l1/CMakeFiles/l1.dir/l1_f1.c.o ; nm l.a
ninja: no work to do.
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
U puts
0000000000000004 C v1
l.o:
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
0000000000000013 T l1_f2
0000000000000026 T l2_f1
0000000000000039 T l2_f2
U puts
0000000000000004 C v1
So this is v1, touching 1 file to make v2 and rebuild with your file-level dep
cmake_minimum_required(VERSION 3.10)
project(PL VERSION 1.0)
add_subdirectory(../libs/l1 libs/l1)
add_subdirectory(../libs/l2 libs/l2)
add_custom_command(
OUTPUT l.a
COMMAND ld -r libs/l1/l1.o libs/l2/l2.o -o l.o
COMMAND rm -f l.a
COMMAND ar r l.a l.o
COMMAND rm -f l.o
DEPENDS ../build/libs/l1/l1.o ../build/libs/l2/l2.o
DEPENDS L1 L2
)
add_custom_target(
LA
ALL
DEPENDS l.a
)
give this
VY$ ninja
[0/1] Re-running CMake...
-- Configuring done
-- Generating done
-- Build files have been written to: /home/phi/p2/build
[2/2] Generating l1.o
VY$ nm libs/l1/CMakeFiles/l1.dir/l1_f1.c.o ; nm l.a
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
U puts
0000000000000004 C v2
l.o:
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
0000000000000013 T l1_f2
0000000000000026 T l2_f1
0000000000000039 T l2_f2
U puts
0000000000000004 C v1
VY$ ninja
[1/1] Generating l.a
ar: creating l.a
VY$ nm libs/l1/CMakeFiles/l1.dir/l1_f1.c.o ; nm l.a
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
U puts
0000000000000004 C v2
l.o:
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
0000000000000013 T l1_f2
0000000000000026 T l2_f1
0000000000000039 T l2_f2
U puts
0000000000000004 C v2
As we see here, on first ninja we got a cmake rebuild because I added back the file-level dep. Then only the l1.o is rebuild not l.o then l.a.
Second ninja trig the l.a rebuild.
It sounds like make basic dependencies actiopn rule is pretty magic with cmake, hope someone could help me here.
Cheers.
The following is added after Tsyvarev latest comment.
The final fix from Tsyvarev make it working, need the set_source_files_propoerties()
The top CMakeLists.txt now look like this
cmake_minimum_required(VERSION 3.10)
project(PL VERSION 1.0)
add_subdirectory(../libs/l1 libs/l1)
add_subdirectory(../libs/l2 libs/l2)
add_custom_command(
OUTPUT l.a
COMMAND ld -r ${CMAKE_CURRENT_BINARY_DIR}/libs/l1/l1.o
${CMAKE_CURRENT_BINARY_DIR}/libs/l2/l2.o -o l.o
COMMAND rm -f l.a
COMMAND ar r l.a l.o
COMMAND rm -f l.o
DEPENDS # Use absolute paths for files for prevent confusion
${CMAKE_CURRENT_BINARY_DIR}/libs/l1/l1.o
${CMAKE_CURRENT_BINARY_DIR}/libs/l2/l2.o
# Target-level dependencies are needed too
L1 L2
)
set_source_files_properties(
${CMAKE_CURRENT_BINARY_DIR}/libs/l1/l1.o
${CMAKE_CURRENT_BINARY_DIR}/libs/l2/l2.o
PROPERTIES GENERATED TRUE
)
add_custom_target(
LA
ALL
DEPENDS l.a
)
And the builds, full and partial, gives the correct answer.
VY$ cd .. && rm -rf build && mkdir build && cd build && cmake -G Ninja ../libs && ninja
-- The C compiler identification is GNU 7.4.0
-- The CXX compiler identification is GNU 7.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/g++
-- Check for working CXX compiler: /usr/bin/g++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/phi/p2/build
[7/7] Generating l.a
ar: creating l.a
VY$ ninja;nm libs/l1/CMakeFiles/l1.dir/l1_f1.c.o ; nm l.a
ninja: no work to do.
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
U puts
0000000000000004 C v1
l.o:
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
0000000000000013 T l1_f2
0000000000000026 T l2_f1
0000000000000039 T l2_f2
U puts
0000000000000004 C v1
VY$ sed -i 's/v1/v2/' ../libs/l1/l1_f1.c
VY$ ninja;nm libs/l1/CMakeFiles/l1.dir/l1_f1.c.o ; nm l.a
[3/3] Generating l.a
ar: creating l.a
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
U puts
0000000000000004 C v2
l.o:
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T l1_f1
0000000000000013 T l1_f2
0000000000000026 T l2_f1
0000000000000039 T l2_f2
U puts
0000000000000004 C v2

Answer 1

As we see here l.a is not rebuild, despite I thought I made a dep for regen it.
You don't actually make l.a dependent from l1.o and other object files.
The line
DEPENDS L1 L2
in the custom command which creates l.a means only dependencies between the target LA (the target which depends from l.a) and the targets L1 and L2. This is described in the documentation for the add_custom_command. But your custom command itself doesn't have a dependency from l1.o file which you need.
The dependency between the targets forces only LA target to be built after L1 and L2.
Correct way is to specify for add_custom_command file-level dependencies.
But, as dependency files are created in other CMakeLists.txt, additional steps are required:
Target-level dependencies. It involves targets which produces dependency files.
Marking the dependency files as GENERATED. Otherwise CMake would treat these files as already existed on configuration stage.
In result:
add_custom_command(
OUTPUT l.a
COMMAND ld -r libs/l1/l1.o libs/l2/l2.o -o l.o
COMMAND rm -f l.a
COMMAND ar r l.a l.o
COMMAND rm -f l.o
DEPENDS
# Use absolute paths for files for prevent confusion
${CMAKE_CURRENT_BINARY_DIR}/libs/l1/l1.o
${CMAKE_CURRENT_BINARY_DIR}/libs/l2/l2.o
# Target-level dependencies are needed too
L1 L2
)
set_source_files_properties(
${CMAKE_CURRENT_BINARY_DIR}/libs/l1/l1.o
${CMAKE_CURRENT_BINARY_DIR}/libs/l2/l2.o
PROPERTIES GENERATED TRUE
)
The rationale behind needs to specify file-level dependencies even if you have target-level ones is that generally CMake has no notion about OUTPUT file for the target. Such notion exists only for targets create by add_executable or add_library, and in this case DEPENDS from such target actually enriches custom command with both target-level and file-level dependencies.