Hi,
Thanks for the suggestion about OBJECT libraries, I've learnt one more way to
design my build process.
Nevertheless, the effect also appears when linking dynamic libraries. For
example, a slightly modified CMakeLists.txt:
cmake_minimum_required(VERSION 3.5)
file(WRITE a.cpp "")
file(WRITE b.cpp "")
file(WRITE lib.cpp "")
file(WRITE main.cpp "int main(){return 0;}")
add_library(A SHARED a.cpp)
add_library(B SHARED b.cpp)
add_library(L SHARED lib.cpp)
target_link_libraries(L
-Wl,-as-needed A -Wl,-no-as-needed
-Wl,-as-needed B -Wl,-no-as-needed
)
add_executable(main main.cpp)
target_link_libraries(main L)
In this example, I only want libA and libB linked into the main executable iff
their symbols are used somewhere in the linking chain (-as-needed). However,
libB appears as a needed library because the -Wl,-as-needed flag has no effect
on it (it's not in front of libB in CMakeFiles/main.dir/link.txt)
$> objdump -p main | grep NEEDED
...
NEEDED libL.so
NEEDED libB.so
...
Furthermore, if I link 'main' manually from the command line, then 'main'
doesn't link libB anymore:
$> g++ -o main ../main.cpp -lL -L.
$> objdump -p main | grep NEEDED
...
NEEDED libL.so
...
The reason why I think it's a bug is that the linkage behaves differently
between cmake and raw command line, due to some conflicts between flags when
constructing the linking command.
Here is yet another weird scenario:
.---> X ---> -as-needed A -no-as-needed
|
main ----|
|
'---> Y ---> -as-needed B -no-as-needed
CMakeLists.txt to reproduce it:
cmake_minimum_required(VERSION 3.5)
file(WRITE a.cpp "")
file(WRITE b.cpp "")
file(WRITE x.cpp "")
file(WRITE y.cpp "")
file(WRITE main.cpp "int main(){return 0;}")
add_library(A SHARED a.cpp)
add_library(B SHARED b.cpp)
add_library(X SHARED x.cpp)
add_library(Y SHARED y.cpp)
target_link_libraries(X
-Wl,-as-needed A -Wl,-no-as-needed
)
target_link_libraries(Y
-Wl,-as-needed B -Wl,-no-as-needed
)
add_executable(main main.cpp)
target_link_libraries(main X Y)
The result if that both libA and libB are linked in the final executable
'main'. The content of CMakeFiles/main.d/link.txt:
/usr/bin/c++ CMakeFiles/main.dir/main.cpp.o -o main -rdynamic libX.so
libY.so libA.so -Wl,-as-needed -Wl,-no-as-needed libB.so -Wl,-rpath,/tmp/build
Best regards,
Sergio
On 14/03/16 15:58, Brad King wrote:
On 03/14/2016 09:33 AM, Sergio Checa wrote:
target_link_libraries(L
-Wl,-whole-archive A -Wl,-no-whole-archive
-Wl,-whole-archive B -Wl,-no-whole-archive
)
add_executable(main main.cpp)
target_link_libraries(main L)
Specifying flags anywhere except the final executable's target_link_libraries
is not very well defined. I'd rather not try to define behavior different
than has been there for a long time.
It looks like you're trying to achieve what OBJECT libraries are for:
https://cmake.org/cmake/help/v3.5/command/add_library.html#object-libraries
Please try that approach instead.
-Brad
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