在某些情况下CMake找不到Boost的可能原因? [英] Possible causes for Boost not being found by CMake in certain situations?

问题描述

我使用 CMake (3.4.1)根据 Boost 库构建C ++项目.主机平台是 Linux ，目标是主机和 cross-build Android NDK.

I build a C++ project depending on the Boost library using CMake (3.4.1). Host platform is Linux, targets are that host and cross-build Android NDK.

我只使用Boost头文件，我只是下载/提取了boost文件夹(而且我没有/usr/include/boost目录).

I'm only using Boost header files and I just downloaded/extracted the boost folder (and I don't have a /usr/include/boost directory).

在我的CMakeLists.txt文件中，我这样声明对Boost的依赖关系:

In my CMakeLists.txt file I declare the dependency to Boost like this:

find_package(Boost 1.57 REQUIRED)

我将构建配置如下:

BOOST_ROOT=/path/to/boost cmake ../src

实际可以达到我的 native 构建预期的水平.

Which actually works as expected for my native build.

当我现在以完全相同的方式配置构建时(仅指定更多环境变量和CMAKE_TOOLCHAIN_FILE)，CMake给了我:

When I now configure a build exactly the same way (only specifying some more environment variables and a CMAKE_TOOLCHAIN_FILE) CMake gives me:

BOOST_ROOT=/path/to/boost JAVA_HOME=/bla/bla/bla \
ANDROID_NDK=/bla/bla/bla \
ANDROID_SDK=/bla/bla/bla \
ANT=/usr/bin/ant \
cmake ../src -DCMAKE_TOOLCHAIN_FILE=/bla/bla/android.toolchain.cmake

CMake Error at /usr/share/cmake/Modules/FindBoost.cmake:1247 (message):
  Unable to find the requested Boost libraries.

  Unable to find the Boost header files.  Please set BOOST_ROOT to the root
  directory containing Boost or BOOST_INCLUDEDIR to the directory containing
  Boost's headers.
Call Stack (most recent call first):
  CMakeLists.txt:4 (find_package)

因此，我相信我为Android目标进行了几乎相同的构建，但在此处无法找到用于主机构建的Boost的完全相同的方法.

So I believe I did almost the same to build for the Android target but the very same method that finds Boost for the host-build doesn't work here.

我尝试将Boost_DIR，BOOSTROOT和BOOST_INCLUDEDIR设置为相同的效果.另外，在尝试任何新操作之前，我已经删除了构建目录中的所有 内容.

I tried to set Boost_DIR, BOOSTROOT and BOOST_INCLUDEDIR all with the same effect. Also I've deleted all content in the build directory before trying anything new.

此行为的可能原因是什么?我已经尝试像这样直接在FindBoost.cmake脚本中打印BOOST_ROOT:

What can be possible reasons for this behavior? I've already tried to print BOOST_ROOT directly in the FindBoost.cmake script like this:

message("BOOST_ROOT: $ENV{BOOST_ROOT}")

具有预期的行为(编写BOOST_ROOT: /path/to/boost).

With the expected behavior (writing BOOST_ROOT: /path/to/boost).

当然我现在可以作弊，只需将boost文件夹链接到交叉编译器的include文件夹中，但这当然不好，我想知道发生了什么.

Of course I can cheat now and just link the boost folder into the include folder of the cross compiler but that's not nice of course and I want to find out what's going on.

推荐答案

交叉编译时，工具链文件通常会设置变量 CMAKE_FIND_ROOT_PATH_MODE_LIBRARY 变量设置为ONLY，CMAKE_FIND_ROOT_PATH变量用作find_library调用的有效 chroot ，因此仅搜索给定前缀下的库.

When cross-compile, toolchain file normally sets variable CMAKE_FIND_ROOT_PATH. Combined with CMAKE_FIND_ROOT_PATH_MODE_LIBRARY variable set to ONLY, CMAKE_FIND_ROOT_PATH variable is used as effective chroot for find_library call, so only libraries under given prefix(es) are searched.

存在模拟变量，用于调整find_path(用于搜索包含路径)和find_program的行为.

Analogue variables exist for adjust behavior for find_path (used for search include paths) and find_program.

您使用的工具链文件实际上将CMAKE_FIND_ROOT_PATH设置为第1521行:

Toolchain file you use actually sets CMAKE_FIND_ROOT_PATH at line 1521:

set( CMAKE_FIND_ROOT_PATH "${ANDROID_TOOLCHAIN_ROOT}/bin"
    "${ANDROID_TOOLCHAIN_ROOT}/${ANDROID_TOOLCHAIN_MACHINE_NAME}"
    "${ANDROID_SYSROOT}"
    "${CMAKE_INSTALL_PREFIX}"
    "${CMAKE_INSTALL_PREFIX}/share" )

及以下将CMAKE_FIND_ROOT_PATH_MODE_*变量设置为ONLY.因此，您需要在这些目录之一下安装Boost，并提供相对于它的提示(例如BOOST_ROOT).

and below sets CMAKE_FIND_ROOT_PATH_MODE_* variables to ONLY. So you need to have Boost installed under one of these directory, and give hints(like BOOST_ROOT) relative to it.

请注意，Boost应该为目标平台(在您的情况下为Android NDK)而不是在交叉编译的平台(Linux)上构建.

Note, that Boost should be built for target platform (Android NDK in you case), not for the platform where you cross-compile (Linux).

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