Re: [cmake] Problem with dependencies for configure checks

From: Peter Dimov (pdimov_at_[hidden])
Date: 2021-06-03 15:43:06

Andrey Semashev wrote:
> Hi,
> 
> I've been having this problem with CMake in Boost.Filesystem, where I need to
> have the Boost::library targets available before Boost::filesystem target is
> defined. This is needed in order to perform configuration checks that will
> affect how Boost::filesystem will be defined. For example, see the
> check_cxx_source_compiles calls:
> 
> https://github.com/boostorg/filesystem/blob/af6ac28b5785820b93a2af7dd6
> e9b801232fec19/CMakeLists.txt#L23-L50
> 
> For the check_cxx_source_compiles calls, I would have liked to add targets like
> Boost::config and Boost::winapi to CMAKE_REQUIRED_LIBRARIES so that the
> checks are compiled with include directories of those dependencies added in
> the command line. This doesn't work because these targets may not be
> defined at the point when the configuration checks are performed.
> 
> My current workaround, as you can see, is to add the Boost root to
> CMAKE_REQUIRED_INCLUDES, where the boost directory with the common
> headers tree is located. However, this will not work if we're going to make that
> directory optional or remove it. Also, you can see that I'm reusing checks
> implemented in Boost.Config, for which I also had to hard code its relative
> location from Boost.Filesystem. If we're going to allow different libraries
> located in separate directory trees, this will also break.
> 
> My question is, can this be fixed and how?

I add ../config/include to CMAKE_REQUIRED_INCLUDES.

https://github.com/boostorg/stacktrace/blob/79aff77771a8f3a3d62852de6619a955fd4c2634/CMakeLists.txt#L75

I don't think we should mandate add_subdirectory calls to come in a specific
order. Even if we can hack something in the superproject to do it, users who use
add_subdirectory manually will need to get the order right.

Incidentally,

#include <${CMAKE_CURRENT_SOURCE_DIR}/config/has_stat_st_blksize.cpp>

doesn't work on Cygwin with the Windows CMake; this ends up generating

#include <C:/boost-dir/libs/filesystem/config/ has_stat_st_blksize.cpp>

which the Cygwin compiler doesn't like and the test always fails. So I've had to use

https://github.com/boostorg/stacktrace/blob/79aff77771a8f3a3d62852de6619a955fd4c2634/CMakeLists.txt#L59

instead.