spdlog for different targets in one CMake project

[rroguski]

Hello,

I've been using spdlog for quite some time now. I added it as a submodule to my project over 2 years ago, when in the README it still said to copy src/spdlog.cpp to project's source directory for faster compile times.

Recently I decided to update the submodule and after that had lots of linker errors. To make the long story short it turned out that templates are not initialized, because more files appeared next to src/spdlog.cpp. Then I noticed that the info on adding src/spdlog.cpp to my source directory is gone from the README and only the precompiled info remains.

Here is my problem - my project is cross-compiled for ARM with unit tests for x86_64 and both binaries use spdlog (since some tested modules require spdlog). The problem is that I would have to use 2 separately built spdlog libraries, but I use spdlog as a submodule in my project already so I'd rather have the old option with adding spdlog files to the rest of my compiled files.

I didn't want to add all files from spdlog/src by hand since with later updates when more files are added I'd have to make changes, so I did the following:

1. In my main CMakeLists.txt file I added the following line: AUX_SOURCE_DIRECTORY(${CMAKE_SOURCE_DIR}/spdlog/src SPDLOG_COMPILE_FILES)
2. In each CMakeLists.txt file which has an ADD_EXECUTABLE I added to the end of the file list ${SPDLOG_COMPILE_FILES}

What AUX_SOURCE_DIRECTORY does is it saves the file names located in spdlog/src directory to SPDLOG_COMPILE_FILES variable.

My question is - is this ok? It seems to be update-proof since if new files are added the list of files updates automatically, but I have an impression that I'm doing something completely wrong and nobody is doing it this way since I can't find any information about it...

Thank you in advance for any insight on this problem.

[tt4g]

Usually, it is enough to call add_subdirectory() to include third party libraries.
CMake targets (e.g. spdlog) declared in CMakeLists.txt loaded by add_subdirectory() can be linked to targets in your project.

[rroguski]

Thanks for answering.

The thing is I have my main CMakeLists.txt which has many ADD_SUBDIRECTORY entries, which also include:

ADD_SUBDIRECTORY(spdlog)
ADD_SUBDIRECTORY(tests)

in tests I have another CMakeLists.txt which also requires ADD_SUBDIRECTORY(spdlog) and this causes a cmake error saying add_library cannot create target "spdlog" because another target with the same name already exists. (and the target exists because of the add_subdirectory(spdlog) in my main CMakeLists.txt).

I have a different library which is built normally for the main project and is additionally mocked for tests, but there, instead of a regular library name in add_library, I have a cmake variable set in main CMakeLists.txt to its regular library name and in the tests CMakeLists.txt the variable is set to a mock name.

Maybe an example will be more clear:
my library CMakeLists.txt:

ADD_LIBRARY(${LIBRARY_NAME}
   # cpp files...
)

main CMakeLists.txt:

# compiler for arm
SET(CMAKE_CXX_COMPILER "${MY_PATH_TO_COMPILER}/bin/arm-linux-gnueabihf-g++")
PROJECT(myproject)
# setting library name so there is no name clash
SET(LIBRARY_NAME mylib)
ADD_SUBDIRECTORY(mylib)
ADD_SUBDIRECTORY(spdlog)
ADD_SUBDIRECTORY(tests)
# ...

tests CMakeLists.txt:

# compiler for x86_64
SET(CMAKE_CXX_COMPILER "/usr/bin/g++")
PROJECT(tests)
# setting library name so there is no name clash
SET(LIBRARY_NAME mylibMock)
ADD_SUBDIRECTORY(mylib)
ADD_SUBDIRECTORY(spdlog)
# ...

No name clash on mylib since for the main project it is linked as mylib and for tests it is linked as mylibMock (mylib (linked with ARM-specific stuff, mylibMock linked with dummy ARM-specific stuff, not relevant but just for clarification). With spdlog there are 2 same names, hence the error.

I think my main problem is that I'm building 2 separate binaries for 2 separate targets in one go. This might be the reason why no one does it this way 😉

[tt4g]

Right. If you import the same project twice with add_subdirectory(), the CMake target names will conflict and should be avoided.
Remove add_subdirectory(spdlog) in tests because spdlog is already defined in main.

Use this small sample project as a reference: https://github.com/tt4g/spdlog-discussions-2213

[rroguski]

Thank you for the example. It works great when spdlog_discussions_2213 and spdlog_discussions_2213_test are compiled for the same target. spdlog is built as a static library and linked to both binaries.

But if I change the compiler for spdlog_discussions_2213 to a cross-compiler for ARM and set a compiler for spdlog_discussions_2213_test to my default compiler on my PC then the statically built spdlog library (built using the cross-compiler for ARM) is being linked corerctly to spdlog_discussions_2213, but there in a linker error when it's trying to link to the spdlog_discussions_2213_test binary built for a different target.

I think you're missing the fact that I'm creating one binary for ARM and another for x86_64. Adding spdlog as a subdirectory in one place results in the spdlog static library being built for only one of those targets. I can't add spdlog twice using add_subdirectory because of the name clash. That is why I had to go back to the option of adding spdlog sources to my build tree (this option was deleted in commit f0f4499).

So the question is why was the option with adding cpp files to the build tree removed from the description?

[tt4g]

So the question is why was the option with adding cpp files to the build tree removed from the description?

I can't answer this question. @gabime Could you please answer it?