add_custom_target在CMake中的作用是什么？

Question

从CMake Cookbook中可以看到，我们可以使用命令add_custom_command和add_custom_target在构建时运行自定义命令。有一个玩具示例，我希望提取子目录中的压缩文件，并将其链接到最终的可执行文件。我们有两个CMakeLists.txt文件，下面一个在子目录中。

find_package(BLAS REQUIRED)
find_package(LAPACK REQUIRED)

set(MATH_SRCS
  ${CMAKE_CURRENT_BINARY_DIR}/wrap_BLAS_LAPACK/CxxBLAS.cpp
  ${CMAKE_CURRENT_BINARY_DIR}/wrap_BLAS_LAPACK/CxxLAPACK.cpp
  ${CMAKE_CURRENT_BINARY_DIR}/wrap_BLAS_LAPACK/CxxBLAS.hpp
  ${CMAKE_CURRENT_BINARY_DIR}/wrap_BLAS_LAPACK/CxxLAPACK.hpp
  )

add_custom_target(BLAS_LAPACK_wrappers
  WORKING_DIRECTORY
    ${CMAKE_CURRENT_BINARY_DIR}
  DEPENDS
    ${MATH_SRCS}
  COMMENT
    "Intermediate BLAS_LAPACK_wrappers target"
  VERBATIM
  )

add_custom_command(
  OUTPUT
    ${MATH_SRCS}
  COMMAND
    ${CMAKE_COMMAND} -E tar xzf ${CMAKE_CURRENT_SOURCE_DIR}/wrap_BLAS_LAPACK.tar.gz
  WORKING_DIRECTORY
    ${CMAKE_CURRENT_BINARY_DIR}
  DEPENDS
    ${CMAKE_CURRENT_SOURCE_DIR}/wrap_BLAS_LAPACK.tar.gz
  COMMENT
    "Unpacking C++ wrappers for BLAS/LAPACK"
  )

add_library(math "")

target_sources(math
  PRIVATE
    ${MATH_SRCS}
  )

target_include_directories(math
  INTERFACE
    ${CMAKE_CURRENT_BINARY_DIR}/wrap_BLAS_LAPACK
  )

# BLAS_LIBRARIES are included in LAPACK_LIBRARIES
target_link_libraries(math
  PUBLIC
    ${LAPACK_LIBRARIES}
  )

下面的CMakeLists.txt位于主目录中。

cmake_minimum_required(VERSION 3.5 FATAL_ERROR)

# Fortran needed to discover LAPACK with some compilers
project(recipe-04 LANGUAGES CXX Fortran)

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_EXTENSIONS OFF)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

add_subdirectory(deps)

add_executable(linear-algebra linear-algebra.cpp)

target_link_libraries(linear-algebra
  PRIVATE
    math
  )

我不知道为什么在这个例子中我们需要add_custom_target。

Answer 1

如果不生成任何目标，则无法执行命令add_custom_command。这就是为什么我们需要自定义目标来运行这个命令。此外，它只能在配置时执行。如果我们想在配置CMakeLists.txt文件之后运行该命令，我们可以创建一个自定义目标，如下所示。

add_custom_target(BLAS_LAPACK_wrappers
  WORKING_DIRECTORY
    ${CMAKE_CURRENT_BINARY_DIR}
  DEPENDS
    ${MATH_SRCS}
  COMMENT
    "Intermediate BLAS_LAPACK_wrappers target"
  VERBATIM
  )

add_custom_command(
  OUTPUT
    ${MATH_SRCS}
  COMMAND
    ${CMAKE_COMMAND} -E tar xzf ${CMAKE_CURRENT_SOURCE_DIR}/wrap_BLAS_LAPACK.tar.gz
  WORKING_DIRECTORY
    ${CMAKE_CURRENT_BINARY_DIR}
  DEPENDS
    ${CMAKE_CURRENT_SOURCE_DIR}/wrap_BLAS_LAPACK.tar.gz
  COMMENT
    "Unpacking C++ wrappers for BLAS/LAPACK"
  )

配置CMakeLists.txt文件之后，在构建时使用Cmake --build . --target=BLAS_LAPACK_wrappers执行它。