add_custom_target在CMake中的作用是什么？

Question

我从 CMake Cookbook 中了解到，我们可以使用 add_custom_command 和 add_custom_target 命令来在构建时运行自定义命令。以下是一个玩具示例，我想要在子目录中提取压缩文件并将其链接到最终可执行文件。我们有两个 CMakeLists.txt 文件，下面的这个在子目录中。

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}
  )

以下的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
  )

我不知道在这个例子中为什么需要使用add_custom_target。

- Sean

Answer 1

如果没有生成任何目标，则无法执行add_custom_command命令。这就是为什么我们需要自定义目标来运行此命令的原因。此外，它只能在配置时执行。如果我们想在配置完CMakeLists.txt文件后运行该命令，可以创建自定义目标来实现如下。

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"
  )

在配置完CMakeLists.txt文件后，我们使用Cmake --build . --target=BLAS_LAPACK_wrappers 在构建时执行它。

- Sean