我正在处理一个使用ExternalProject的项目，BUILD_IN_SOURCE=TRUE，每次都会重建整个项目(所有源和目标(，就好像make无法看到文件已经存在一样。我的实现看起来是这样的：
# Need to explicitly enable ExternalProject functionality
include(ExternalProject)
# Download or update library as an external project
ExternalProject_Add(project_bdsg
GIT_REPOSITORY https://github.com/vgteam/libbdsg-easy.git
PREFIX ${CMAKE_CURRENT_BINARY_DIR}/external/
CONFIGURE_COMMAND ""
UPDATE_DISCONNECTED True
BUILD_ALWAYS False
BUILD_IN_SOURCE True
INSTALL_DIR ${CMAKE_SOURCE_DIR}/external/bdsg/
INSTALL_COMMAND make INSTALL_PREFIX=${CMAKE_SOURCE_DIR}/external/bdsg/ install
)
# Define INSTALL_DIR as the install directory for external library
ExternalProject_Get_Property(project_bdsg INSTALL_DIR)
# Create new library for external project (so it can be linked with main library)
add_library(bdsg STATIC IMPORTED)
set_property(TARGET bdsg
PROPERTY IMPORTED_LOCATION ${INSTALL_DIR}/lib/libbdsg.a)
add_library(divsufsort STATIC IMPORTED)
set_property(TARGET divsufsort
PROPERTY IMPORTED_LOCATION ${INSTALL_DIR}/lib/libdivsufsort64.a)
add_library(libhandlegraph STATIC IMPORTED)
set_property(TARGET libhandlegraph
PROPERTY IMPORTED_LOCATION ${INSTALL_DIR}/lib/libhandlegraph.a)
add_library(libsdsl STATIC IMPORTED)
set_property(TARGET libsdsl
PROPERTY IMPORTED_LOCATION ${INSTALL_DIR}/lib/libsdsl.a)
# Define library as dependent on the downloaded project
add_dependencies(bdsg
project_bdsg
libsdsl
libhandlegraph
divsufsort)
# Define main library as dependent on the downloaded project (transitively)
add_dependencies(Bluntifier bdsg)
# Ensure that main library has access to primary dependencies' and secondary dependencies' headers
include_directories(external/bdsg/include/)
我有这样定义的目标：
add_executable(${FILENAME_PREFIX} src/test/${FILENAME_PREFIX}.cpp)
set_property(TARGET ${FILENAME_PREFIX} PROPERTY INSTALL_RPATH "$ORIGIN")
target_link_libraries(${FILENAME_PREFIX}
Bluntifier
Threads::Threads
bdsg
divsufsort
libhandlegraph
libsdsl)
运行带有一些调试输出的make会给我这样的消息：
Updating goal targets....
Considering target file 'CMakeFiles/Bluntifier.dir/build'.
File 'CMakeFiles/Bluntifier.dir/build' does not exist.
Considering target file 'Bluntifier.a'.
Considering target file 'CMakeFiles/Bluntifier.dir/link.txt'.
Finished prerequisites of target file 'CMakeFiles/Bluntifier.dir/link.txt'.
No need to remake target 'CMakeFiles/Bluntifier.dir/link.txt'.
Considering target file 'CMakeFiles/Bluntifier.dir/src/adjacency_components.cpp.o'.
File 'CMakeFiles/Bluntifier.dir/src/adjacency_components.cpp.o' does not exist.
Considering target file '../src/adjacency_components.cpp'.
Finished prerequisites of target file '../src/adjacency_components.cpp'.
No need to remake target '../src/adjacency_components.cpp'.
Considering target file '../external/bdsg/include/bdsg/internal/packed_structs.hpp'.
Finished prerequisites of target file '../external/bdsg/include/bdsg/internal/packed_structs.hpp'.
No need to remake target '../external/bdsg/include/bdsg/internal/packed_structs.hpp'.
Considering target file '../external/bdsg/include/handlegraph/expanding_overlay_graph.hpp'.
Finished prerequisites of target file '../external/bdsg/include/handlegraph/expanding_overlay_graph.hpp'.
No need to remake target '../external/bdsg/include/handlegraph/expanding_overlay_graph.hpp'.
Considering target file '../external/bdsg/include/handlegraph/handle_graph.hpp'.
Finished prerequisites of target file '../external/bdsg/include/handlegraph/handle_graph.hpp'.
No need to remake target '../external/bdsg/include/handlegraph/handle_graph.hpp'.
Considering target file '../external/bdsg/include/handlegraph/iteratee.hpp'.
Finished prerequisites of target file '../external/bdsg/include/handlegraph/iteratee.hpp'.
No need to remake target '../external/bdsg/include/handlegraph/iteratee.hpp'.
Considering target file '../external/bdsg/include/handlegraph/types.hpp'.
Finished prerequisites of target file '../external/bdsg/include/handlegraph/types.hpp'.
No need to remake target '../external/bdsg/include/handlegraph/types.hpp'.
Considering target file '../external/bdsg/include/handlegraph/util.hpp'.
Finished prerequisites of target file '../external/bdsg/include/handlegraph/util.hpp'.
No need to remake target '../external/bdsg/include/handlegraph/util.hpp'.
Considering target file '../external/bdsg/include/sdsl/bits.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/bits.hpp'.
No need to remake target '../external/bdsg/include/sdsl/bits.hpp'.
Considering target file '../external/bdsg/include/sdsl/config.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/config.hpp'.
No need to remake target '../external/bdsg/include/sdsl/config.hpp'.
Considering target file '../external/bdsg/include/sdsl/int_vector.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/int_vector.hpp'.
No need to remake target '../external/bdsg/include/sdsl/int_vector.hpp'.
Considering target file '../external/bdsg/include/sdsl/int_vector_buffer.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/int_vector_buffer.hpp'.
No need to remake target '../external/bdsg/include/sdsl/int_vector_buffer.hpp'.
Considering target file '../external/bdsg/include/sdsl/io.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/io.hpp'.
No need to remake target '../external/bdsg/include/sdsl/io.hpp'.
Considering target file '../external/bdsg/include/sdsl/iterators.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/iterators.hpp'.
No need to remake target '../external/bdsg/include/sdsl/iterators.hpp'.
Considering target file '../external/bdsg/include/sdsl/memory_management.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/memory_management.hpp'.
No need to remake target '../external/bdsg/include/sdsl/memory_management.hpp'.
Considering target file '../external/bdsg/include/sdsl/ram_filebuf.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/ram_filebuf.hpp'.
No need to remake target '../external/bdsg/include/sdsl/ram_filebuf.hpp'.
Considering target file '../external/bdsg/include/sdsl/ram_fs.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/ram_fs.hpp'.
No need to remake target '../external/bdsg/include/sdsl/ram_fs.hpp'.
Considering target file '../external/bdsg/include/sdsl/sdsl_concepts.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/sdsl_concepts.hpp'.
No need to remake target '../external/bdsg/include/sdsl/sdsl_concepts.hpp'.
Considering target file '../external/bdsg/include/sdsl/sfstream.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/sfstream.hpp'.
No need to remake target '../external/bdsg/include/sdsl/sfstream.hpp'.
Considering target file '../external/bdsg/include/sdsl/structure_tree.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/structure_tree.hpp'.
No need to remake target '../external/bdsg/include/sdsl/structure_tree.hpp'.
Considering target file '../external/bdsg/include/sdsl/uintx_t.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/uintx_t.hpp'.
No need to remake target '../external/bdsg/include/sdsl/uintx_t.hpp'.
Considering target file '../external/bdsg/include/sdsl/util.hpp'.
Finished prerequisites of target file '../external/bdsg/include/sdsl/util.hpp'.
No need to remake target '../external/bdsg/include/sdsl/util.hpp'.
Considering target file '../inc/adjacency_components.hpp'.
Finished prerequisites of target file '../inc/adjacency_components.hpp'.
No need to remake target '../inc/adjacency_components.hpp'.
Considering target file '../inc/subtractive_graph.hpp'.
Finished prerequisites of target file '../inc/subtractive_graph.hpp'.
No need to remake target '../inc/subtractive_graph.hpp'.
Considering target file '../inc/utility.hpp'.
Finished prerequisites of target file '../inc/utility.hpp'.
No need to remake target '../inc/utility.hpp'.
Pruning file '../src/adjacency_components.cpp'.
Pruning file 'CMakeFiles/Bluntifier.dir/flags.make'.
Finished prerequisites of target file 'CMakeFiles/Bluntifier.dir/src/adjacency_components.cpp.o'.
Must remake target 'CMakeFiles/Bluntifier.dir/src/adjacency_components.cpp.o'.
[ 38%] Building CXX object CMakeFiles/Bluntifier.dir/src/adjacency_components.cpp.o
Successfully remade target file 'CMakeFiles/Bluntifier.dir/src/adjacency_components.cpp.o'.
其中adjacency_components.cpp是每次运行make时重新生成的几个源文件之一。为什么make得出结论说它需要重建这些目标？
在进一步查看我的调试输出后，很明显cmake正在跟踪ExternalProject下载内容中的文件时间戳。由于每次重新编译时都会重新下载，所以总是有一个新的时间戳会导致所有依赖项在相对方面都显得过时，从而触发重新编译所有内容。为了暂时停止这种情况，在开发时，添加以下行就足够了：
DOWNLOAD_COMMAND ""
UPDATE_COMMAND ""
ExternalProject参数。理想情况下，这将与开发人员可以调用的标志绑定，因此这不是默认行为。