添加从 CMake 中的模板构建 linker 脚本文件的步骤？

Question

Basically, before linking is performed, I would like to convert a GCC linker script template file into a final linker script, as discussed in Can I use Preprocessor Directives in.ld file - and I'd like that step to be performed by Cmake.

我想我的问题类似于add_custom_command is not generate a target中的讨论； 但我仍然看不到如何解决它。 这是一个最小的例子，我用 empty.ld linker 脚本模板文件伪造； 首先，让我们使用bash创建文件：

mkdir /tmp/cmake_test && cd /tmp/cmake_test

touch my_linkerscript_template.ld

cat > main.c <<'EOF'
#include <stdio.h>

const char greeting[] = "hello world";

int main() {
    printf("%s!\n", greeting);
    return 0;
}
EOF

cat > CMakeLists.txt <<'EOF'
cmake_minimum_required(VERSION 3.13)
SET(CMAKE_INCLUDE_CURRENT_DIR ON)

project(foobar C)
message("PROJECT_NAME is '${PROJECT_NAME}'")

add_executable(${PROJECT_NAME} 
               main.c
              )

add_compile_options(-Wall
                   )

target_link_options(${PROJECT_NAME} PRIVATE
                    "LINKER:--script=${CMAKE_SOURCE_DIR}/my_linkerscript.ld"
                   )
add_custom_command(OUTPUT ${CMAKE_SOURCE_DIR}/my_linkerscript.ld
                   DEPENDS ${CMAKE_SOURCE_DIR}/my_linkerscript_template.ld
                   COMMAND bash ARGS -c "gcc -E -x c -CC -I/usr/include my_linkerscript_template.ld | grep -v '^#' > my_linkerscript.ld"
                   WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
                   COMMENT "build my_linkerscript_template.ld into my_linkerscript.ld (${CMAKE_CURRENT_SOURCE_DIR})"
                   VERBATIM
                   )
set_target_properties(${PROJECT_NAME} PROPERTIES LINK_DEPENDS "${CMAKE_SOURCE_DIR}/my_linkerscript.ld")
EOF

请注意，上面没有创建my_linkerscript.ld ； 因为我打算将my_linkerscript_template.ld放入 git，然后按需生成最终的my_linkerscript.ld 。

如果在这种情况下，我会：

mkdir /tmp/cmake_test/build && cd /tmp/cmake_test/build
cmake ../ -DCMAKE_BUILD_TYPE=Debug
make

我得到：

$ make
make[2]: *** No rule to make target 'D:/msys64/tmp/cmake_test/my_linkerscript.ld', needed by 'foobar.exe'.  Stop.
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/foobar.dir/all] Error 2
make: *** [Makefile:91: all] Error 2

$ grep -r linkerscript .
./CMakeFiles/foobar.dir/build.make:foobar.exe: D:/msys64/tmp/cmake_test/my_linkerscript.ld
./CMakeFiles/foobar.dir/build.make:     /D/msys64/mingw64/bin/cc.exe -g -Wl,--script=D:/msys64/tmp/cmake_test/my_linkerscript.ld -Wl,--whole-archive CMakeFiles/foobar.dir/objects.a -Wl,--no-whole-archive -o foobar.exe -Wl,--out-implib,libfoobar.dll.a -Wl,--major-image-version,0,--minor-image-version,0  -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32

所以，我确实得到了对my_linkerscript.ld的依赖； 它确实被添加到 linker 选项中 - 但是add_custom_command没有生成构建步骤。

如何生成正确的依赖项，以便当my_linkerscript.ld需要链接但它不存在时，运行命令（在上面的add_custom_command中给出）以从模板生成此文件？

Answer 1

add_custom_command 的add_custom_command需要被某个目标“消耗”，否则自定义命令无效。

LINK_DEPENDS属性不提供消费者，因此您需要自己创建消费者目标：

# ... in addition to your code

# Create a consumer target for the linker script.
# So previous `add_custom_command` will have an effect.
add_custom_target(my_linkerscript
  DEPENDS ${CMAKE_SOURCE_DIR}/my_linkerscript.ld)

# Make executable to depend on that target.
# So, the check whether to relink the executable will be performed
# after possible rebuilding the linker script.
add_dependencies(${PROJECT_NAME} my_linkerscript)