与Cmake一起查找Protobuf包装

Question

我似乎无法始终找到我的 protobuf package package。我想包括Protobuf来源，并将其作为我项目的一部分构建。我正在使用 protobuf 3.15.8 和 protobuf-c-c 1.3.3 （用作控制示例）。

这是示例的目录设置：

- CMakeLists.txt
- deps/
       - CMakeLists.txt
       - protobuf/            # The protobuf 3.15.8 repository
       - protobuf-c/          # The protobuf-c 1.3.3 repository
- target/
         - CMakeLists.txt

root cmakelists.txt：

cmake_minimum_required(VERSION 3.15)
project(PROTOBUF_IMPORT_FAIL)
add_subdirectory(deps)
add_subdirectory(target)

dep/cmakelists.txt：

add_subdirectory(protobuf/cmake)
set(Protobuf_LIBRARIES "${CMAKE_CURRENT_BINARY_DIR}/protobuf/cmake/lib")
set(Protobuf_INCLUDE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/protobuf/src")
set(Protobuf_PROTOC_LIBRARY "${CMAKE_CURRENT_BINARY_DIR}/protobuf/cmake/libprotoc.a")
add_subdirectory(protobuf-c/build-cmake)

target/cmakelists.txt：

set(Protobuf_LIBRARIES "${CMAKE_CURRENT_BINARY_DIR}/../deps/protobuf/cmake/lib")
set(Protobuf_INCLUDE_DIR "${CMAKE_CURRENT_SOURCE_DIR}../deps/protobuf/src")
message("This is the problem!")
FIND_PACKAGE(Protobuf REQUIRED)

一旦一切都是设置我然后可以尝试运行cmake：

mkdir build
cd build
cmake ..

不幸的是，这最终给了我以下错误：

CMake Error at /usr/local/Cellar/cmake/3.23.0/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find Protobuf (missing: Protobuf_INCLUDE_DIR)
Call Stack (most recent call first):
  /usr/local/Cellar/cmake/3.23.0/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:594 (_FPHSA_FAILURE_MESSAGE)
  /usr/local/Cellar/cmake/3.23.0/share/cmake/Modules/FindProtobuf.cmake:650 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  target/CMakeLists.txt:7 (FIND_PACKAGE)

如果它有用，以下是一些环境信息：

$ uname -a
Darwin coop-mac 21.4.0 Darwin Kernel Version 21.4.0: Fri Mar 18 00:45:05 PDT 2022; root:xnu-8020.101.4~15/RELEASE_X86_64 x86_64
$ cmake --version
cmake version 3.23.0

protobuf-c也试图找到<代码> Protobuf 软件包，但实际上是成功的。为什么从目标搜索它找不到它？如何修复它？

Answer 1

find_package用于查找安装库。通常是由装有CMAKE（所谓的Find-Modules）的脚本找到的安装库，或者是因为库提供了一个配置脚本（例如Protobuf-config.cmake.cmake或Profobuf-targets.cmake）。一旦库构建并安装了库后，就会生成后者。这可能就是为什么您的第二个配置步骤成功的原因。

您不想链接到已安装的库，而是要自己构建库。因此，您需要做的就是使用add_subDirectory添加子文件夹（不要添加Cmake subfolder，根据Protobuf将其弃用）。之后，您可以使用target_link_libraries无需使用find_package将Protobuf链接到目标。

Answer 2

我最近遇到了精确问题：两个子模块呼叫find_package（protobuf），其中一个因“缺失：ProtoBuf_include_dir”而失败（在第一次运行中）。

启用CMAKE TRACE后，我发现了一些警告：

• 第1 find_package（ProtoBuf）的警告：

cmake-3.28/Modules/FindProtobuf.cmake(559):  find_path(Protobuf_INCLUDE_DIR google/protobuf/service.h PATHS ${Protobuf_SRC_ROOT_FOLDER}/src )
/root/bin/cmake/share/cmake-3.28/Modules/FindProtobuf.cmake(563):  mark_as_advanced(Protobuf_INCLUDE_DIR )
CMake Warning (dev) at /root/bin/cmake/share/cmake-3.28/Modules/FindProtobuf.cmake:563 (mark_as_advanced):
  Policy CMP0102 is not set: The variable named "Protobuf_INCLUDE_DIR" is not
  in the cache.  This results in an empty cache entry which is no longer
  created when policy CMP0102 is set to NEW.  Run "cmake --help-policy
  CMP0102" for policy details.  Use the cmake_policy command to set the
  policy and suppress this warning.
Call Stack (most recent call first):
  3rdparty/sentencepiece/src/CMakeLists.txt:75 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

• 第二find_package（protobuf）的警告：

cmake-3.28/Modules/FindProtobuf.cmake(559):  find_path(Protobuf_INCLUDE_DIR google/protobuf/service.h PATHS ${Protobuf_SRC_ROOT_FOLDER}/src )
CMake Warning (dev) at /root/bin/cmake/share/cmake-3.28/Modules/FindProtobuf.cmake:559 (find_path):
  Policy CMP0126 is not set: set(CACHE) does not remove a normal variable of
  the same name.  Run "cmake --help-policy CMP0126" for policy details.  Use
  the cmake_policy command to set the policy and suppress this warning.

  For compatibility with older versions of CMake, normal variable
  "Protobuf_INCLUDE_DIR" will be removed from the current scope.
Call Stack (most recent call first):
  3rdparty/deprecated/sami/lightseq/CMakeLists.txt:36 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

“ ProtoBuf_include_dir”将从当前范围中删除。”第二次跑步的警告是可疑的。

解决方案

将所有ProtoBuf_*变量设置为缓存变量，例如：

set(Protobuf_INCLUDE_DIR "${CMAKE_CURRENT_SOURCE_DIR}../deps/protobuf/src" CACHE INTERNAL "Protobuf Include Dir")