features.h: allow building without CMake-generated feature header #4346
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In commit a390a84 (cmake: move defines into "features.h" header, 2017-07-01), we have introduced a new "features.h" header. This file is being generated by the CMake build system based on how the libgit2 build has been configured, replacing the preexisting method of simply setting the defines inside of the CMake build system. This was done to help splitting up the build instructions into multiple separate subdirectories. An overlooked shortcoming of this approach is that some projects making use of libgit2 build the library with custom build systems, without making use of CMake. For those users, the introduction of the "features.h" file makes their life harder as they would have to also generate this file. Fix this issue by guarding all inclusions of the generated header file by the `LIBGIT2_NO_FEATURES_H` define. Like this, other build systems can skip the feature header and simply define all used features by specifying `-D` flags for the compiler again.
pks-t
referenced
this pull request
Sep 12, 2017
In a future commit, we will split out the build instructions for our library directory and move them into a subdirectory. One of the benefits is fixing scoping issues, where e.g. defines do not leak to build targets where they do not belong to. But unfortunately, this does also pose the problem of how to propagate some defines which are required by both the library and the test suite. One way would be to create another variable keeping track of all added defines and declare it inside of the parent scope. While this is the most obvious and simplest way of going ahead, it is kind of unfortunate. The main reason to not use this is that these defines become implicit dependencies between the build targets. By simply observing a define inside of the CMakeLists.txt file, one cannot reason whether this define is only required by the current target or whether it is required by different targets, as well. Another approach would be to use an internal header file keeping track of all defines shared between targets. While configuring the library, we will set various variables and let CMake configure the file, adding or removing defines based on what has been configured. Like this, one can easily keep track of the current environment by simply inspecting the header file. Furthermore, these dependencies are becoming clear inside the CMakeLists.txt, as instead of simply adding a define, we now call e.g. `SET(GIT_THREADSAFE 1)`. Having this header file though requires us to make sure it is always included before any "#ifdef"-preprocessor checks are executed. As we have already refactored code to always include the "common.h" header file before any statement inside of a file, this becomes easy: just make sure "common.h" includes the new "features.h" header file first.
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Thanks @pks-t ! |
This was referenced Aug 12, 2022
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In commit a390a84 (cmake: move defines into "features.h" header,
2017-07-01), we have introduced a new "features.h" header. This file is
being generated by the CMake build system based on how the libgit2 build
has been configured, replacing the preexisting method of simply setting
the defines inside of the CMake build system. This was done to help
splitting up the build instructions into multiple separate
subdirectories.
An overlooked shortcoming of this approach is that some projects making
use of libgit2 build the library with custom build systems, without
making use of CMake. For those users, the introduction of the
"features.h" file makes their life harder as they would have to also
generate this file.
Fix this issue by guarding all inclusions of the generated header file
by the
LIBGIT2_NO_FEATURES_Hdefine. Like this, other build systemscan skip the feature header and simply define all used features by
specifying
-Dflags for the compiler again.