Combining two third party libraries into a single imported target

Usage
1

Hi

My need as described in the topic subject is occurring with the GMP library (Gnu Multiple Precision).
On my system it does not seem to be detectable by find_package so I tried to make it detectable through a Findgmp.cmake.

The issue is that it is actually made fo two libraries gmp (C API) and gmpxx (C++ API, which depend on the first one).

I can link both using two different find modules, and generating two targets (gmp::gmp and gmpxx::gmpxx) but I’d like to have only one target (say gmp::gmp) that allows me to link both libraries.

So far I cannot write my findmodule accordingly (example attached).

Can someone tell me how to fix it?

By the way, I’m utterly confused with the difference between _LIBRARY and _LIBRARIES, _INCLUDE_DIR and _INCLUDE_DIRS. I’d appreciate some clarifications on this topic.

Regards
A.
Findgmp.cmake (1.2 KB)

2

The singular ones tend to be find_* cache variables. Once the package is considered found, the plural ones are set as needed as local variables.

Right, a find module would need to be written. You can write one that finds both targets. Something like:

add_library(gmp::gmp UNKNOWN IMPORTED)
# skipping over location settings

# component conditional?
add_library(gmp::gmpxx UNKNOWN IMPORTED)
set_target_properties(gmp::gmpxx PROPERTIES LINK_INTERFACE_LIBRARIES gmp::gmp)
# skipping over location settings

add_library(gmp::gmpall INTERFACE)
set_target_properties(gmp::gmpxx PROPERTIES LINK_INTERFACE_LIBRARIES "gmp::gmp;gmp::gmpxx")
3

Thanks for your answer.

Here is my failed try
Findgmp.cmake (1.4 KB)

in my CMakeList:

  IF(gmp_FOUND)
  	MESSAGE("gmp package found")
  	IF (NOT TARGET gmp::gmp)
  		MESSAGE(WARNING "gmp::gmp target not found")
  	ENDIF()
  ENDIF()

and the configuration output:

gmp package found
gmp::gmp target not found

What did I do wrong?

Thanks again
A.

4

AND gmpxx_FOUND

Nothing ever sets gmpxx_FOUND. Additionally, you should guard target creation with if (NOT TARGET …) because find_package(gmp) running twice within a scope is otherwise a hard error on the target re-creation.

5

Thanks for having pinpointed a first error.

Here is the update

Now the configuration seems OK.

I find_package(gmp) then target_link_libraries(MyTargetName PRIVATE gmp::gmp) but on link, only libgmpxx is passed to the linker whereas libgmp is also expected and needed.

where is the next mistake?
Findgmp.cmake (1.9 KB)

A.

6

You still need to wrap all of the post-FPHSA stuff in if (gmp_FOUND). But the gmp::gmp target is completely wrong. It needs to be an INTERFACE, not an UNKNOWN library. You’re setting the same property twice which means that the first setting is completely ignored.

7

Sorry but I’m still lost.

I fixed the INTERFACE issue but I have the same behaviour, nothing is linked, as if gmp::gmp would be empty.

GMPTest.7z (1.4 KB)

Here is a minimal example to reproduce

on the other hand targets gmp::gmpc and gmp::gmpxx as, if I replace

target_link_libraries(${TARGET_NAME} PRIVATE gmp::gmp)

by

target_link_libraries(${TARGET_NAME} PRIVATE gmp::gmpc)
target_link_libraries(${TARGET_NAME} PRIVATE gmp::gmpxx)

it compiles and runs fine

another fix is to replace

set_target_properties(gmp::gmp PROPERTIES LINK_INTERFACE_LIBRARIES "gmp::gmpc;gmp::gmpxx")```
by
```cmake
target_link_libraries(gmp::gmp INTERFACE gmp::gmpc gmp::gmpxx)

whereas both commands seem equivalent to me…
then

target_link_libraries(${TARGET_NAME} PRIVATE gmp::gmp)

works properly
But anyway, whatever the commands used, variables gmp_LIBRARY and gmp_INCLUDE_DIR with incorrect values are created. How to set them correctly?

thanks again for the help

8

I have another issue with my Findgmp module.
on some random machine, I seem to find a gmp install as _INCLUDE_DIR and _LIBRARY paths are set

/usr/lib/x86_64-linux-gnu/libgmp.so
/usr/include/x86_64-linux-gnu
/usr/lib/x86_64-linux-gnu/libgmpxx.so
/usr/include

yet, when running CMake, I have the following error:
CMake Error at …/Findgmp.cmake:50 (target_link_libraries):
Cannot specify link libraries for target “gmp::gmp” which is not built by this project.

What does it mean? How can I fix it.
Thanks
Findgmp.cmake (1.6 KB)

9

Issue on cmake 3.10.2, no issue on 3.19.2

mere curiosity, what has changed between both version that may explained the error.
from what version should I consider that my Findgmp is OK?

I will just issue an error message if client cmake version is too old.

Thx,
A.

10

Probably CMP0079 in v3.13:
target_link_libraries() allows use with targets in other directories.

If that’s not the one: You can play around with different cmake_minimum_required until the problem also occurs in your newer version.

11

Thx, it sounds correct.
Regards
A.