package-153360_960_720.png

先前分析了find_package()的原理，也分析了find_package()查找系統(tǒng)Protobuf（apt安裝）的具體細(xì)節(jié)。這次來(lái)分析自行編譯安裝的Protobuf是如何被（沒(méi)）找到、如何配置使得能被找到。

環(huán)境：

• ubuntu 16.04；
• 執(zhí)行了sudo apt remove libprotobuf-dev卸載protobuf；
• 自行編譯安裝了protobuf到/home/zz/soft/protobuf-3.8.0

1. Protobuf的頭文件目錄

首先我們知道cmake安裝目錄下提供了FindProtobuf.cmake，因此find_package(Protobuf)一定是在MODULE模式下而不是CONFIG模式下被搜索到的。（題外話：現(xiàn)代的cmake推薦用XXXConfig.cmake也就是CONFIG模式來(lái)找依賴(lài)包，這方面OpenCV可以作為典范寫(xiě)的確實(shí)越來(lái)越好）。

在CMakeLists.txt中做查找：

find_package(Protobuf REQUIRED)

提示報(bào)錯(cuò)：

-- The C compiler identification is GNU 5.4.0
-- The CXX compiler identification is GNU 5.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc - works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ - works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Error at /home/zz/soft/cmake/share/cmake-3.17/Modules/FindPackageHandleStandardArgs.cmake:164 (message):
  Could NOT find Protobuf (missing: Protobuf_INCLUDE_DIR)
Call Stack (most recent call first):
  /home/zz/soft/cmake/share/cmake-3.17/Modules/FindPackageHandleStandardArgs.cmake:445 (_FPHSA_FAILURE_MESSAGE)
  /home/zz/soft/cmake/share/cmake-3.17/Modules/FindProtobuf.cmake:626 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  CMakeLists.txt:11 (find_package)


-- Configuring incomplete, errors occurred!
See also "/home/zz/work/oh-my-cmake/build/CMakeFiles/CMakeOutput.log".
See also "/home/zz/work/oh-my-cmake/build/CMakeFiles/CMakeError.log".

可以看出是Protobuf_INCLUDE_DIR變量為空，而這是由于/home/zz/soft/cmake/share/cmake-3.17/Modules/FindProtobuf.cmake沒(méi)找到protobuf的頭文件搜索目錄。具體來(lái)說(shuō)是這段調(diào)用：

# Find the include directory
find_path(Protobuf_INCLUDE_DIR
    google/protobuf/service.h
    PATHS ${Protobuf_SRC_ROOT_FOLDER}/src
)
mark_as_advanced(Protobuf_INCLUDE_DIR)

find_path()并沒(méi)有找到包含google/protobuf/service.h的目錄，因?yàn)椋?）我們用apt卸載了（或者說(shuō)沒(méi)有安裝）apt倉(cāng)庫(kù)里的libprotobuf-dev；2）給find_path()傳入的搜索參數(shù)也不能讓它找到這樣的目錄。注意到FindProtobuf.cmake開(kāi)頭的多行注釋中提到，可以設(shè)置(set)或使用(use)如下緩存變量（cache variable）：

``Protobuf_LIBRARY``
  The protobuf library
``Protobuf_PROTOC_LIBRARY``
  The protoc library
``Protobuf_INCLUDE_DIR``
  The include directory for protocol buffers
``Protobuf_PROTOC_EXECUTABLE``
  The protoc compiler
``Protobuf_LIBRARY_DEBUG``
  The protobuf library (debug)
``Protobuf_PROTOC_LIBRARY_DEBUG``
  The protoc library (debug)
``Protobuf_LITE_LIBRARY``
  The protobuf lite library
``Protobuf_LITE_LIBRARY_DEBUG``
  The protobuf lite library (debug)

因此，可以通過(guò)指定Protobuf_INCLUDE_DIR變量，來(lái)讓find_package(Protobuf REQUIRED)正確的找到頭文件目錄（真是“多此一舉”）。

而根據(jù)前一篇對(duì)find_path()的第一條規(guī)則的了解，只要設(shè)定CMAKE_SYSTEM_PREFIX_PATH追加一個(gè)能找到google/protobuf/service.h的目錄，就可以正確的產(chǎn)生Protobuf_INCLUDE_DIR變量。

實(shí)測(cè)發(fā)現(xiàn)，CMAKE_SYSTEM_PREFIX_PATH和CMAKE_PREFIX_PATH的設(shè)定，都可以影響find_path()。在本文的分析場(chǎng)景中，以下兩種設(shè)定都可以讓Protobuf_INCLUDE_PATH產(chǎn)生正確的值（但是庫(kù)文件還是找不到的，暫時(shí)忽略）：

list(APPEND CMAKE_SYSTEM_PREFIX_PATH "/home/zz/soft/protobuf-3.8.0/include")
message(STATUS "==== CMAKE_SYSTEM_PREFIX_PATH: ${CMAKE_SYSTEM_PREFIX_PATH}")
find_package(Protobuf REQUIRED)

list(APPEND CMAKE_PREFIX_PATH "/home/zz/soft/protobuf-3.8.0/include")
message(STATUS "=== CMAKE_PREFIX_PATH is: ${CMAKE_PREFIX_PATH}")
find_package(Protobuf REQUIRED)

翻看了CMAKE_SYSTEM_PREFIX_PATH的文檔頁(yè)面，此變量是若干其它變量取值的拼接，不建議修改；鼓勵(lì)修改CMAKE_PREFIX_PATH。
而CMAKE_PREFIX_PATH的文檔頁(yè)面，則表明了它是用來(lái)在find_package(), find_library(), find_program(), find_file(), find_path()等命令中執(zhí)行查找時(shí)提供prefix的選擇。

2. Protobuf的庫(kù)文件

本小結(jié)探究Protobuf的庫(kù)文件被搜索到的過(guò)程。我們首先確保Protobuf的頭文件搜索能被找到，這次選擇在CMAKE_PREFIX_PATH里進(jìn)行設(shè)定，對(duì)應(yīng)的輸出：

-- The C compiler identification is GNU 5.4.0
-- The CXX compiler identification is GNU 5.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc - works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ - works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- === CMAKE_PREFIX_PATH is: /home/zz/soft/protobuf-3.8.0/include
-- Found Protobuf: Protobuf_LIBRARY-NOTFOUND;-lpthread (found version "3.8.0")
-- Configuring done
-- Generating done
-- Build files have been written to: /home/zz/work/oh-my-cmake/build

注意其中Protobuf_LIBRARY-NOTFOUND;-lpthread的NOTFOUND字樣，其實(shí)是protobuf庫(kù)文件沒(méi)找到的造成的。

根據(jù)前面分析，以及文檔中對(duì)CMAKE_PREFIX_PATH的說(shuō)明，我們讓CMAKE_PREFIX_PATH再增加一項(xiàng)，也就是protobuf的庫(kù)文件所在目錄

list(APPEND CMAKE_PREFIX_PATH "/home/zz/soft/protobuf-3.8.0/include;/home/zz/soft/protobuf-3.8.0/lib")

清理CMakeCache.txt后重新執(zhí)行cmake，protobuf的庫(kù)文件就能被正確的找到了，find_package(Protobuf REQUIRED)因而不再報(bào)錯(cuò):

-- Found Protobuf: /home/zz/soft/protobuf-3.8.0/lib/libprotobuf.a;-lpthread (found version "3.8.0")

3. Protobuf可執(zhí)行文件

大多數(shù)用到Protobuf的C/C++工程，只需要find_protobuf(Protobuf)能提供頭文件搜索目錄、庫(kù)文件絕對(duì)路徑。

但也有那么一小撮C/C++程序，還需要調(diào)用protobuf的編譯器，也就是名為protoc的可執(zhí)行文件。對(duì)于本文討論的情況，我們并沒(méi)有假設(shè)~/soft/protobuf-3.8.0/bin放在了PATH環(huán)境變量中。你可以放，但既然已經(jīng)是手動(dòng)編譯的Protobuf了，也應(yīng)該知道不在PATH里添加protoc所在目錄的情況下，在CMakeLists.txt中進(jìn)行設(shè)定的方式。

依然是翻看FindProtobuf.cmake，發(fā)現(xiàn)可以手動(dòng)指定Protobuf_PROTOC_EXECUTABLE這一緩存變量，不過(guò)這讓人覺(jué)得多此一舉。

而在CMAKE_PREFIX_PATH的文檔頁(yè)中提到，它里面的值作為prefix可以用于find_program()，而FindProtobuf.cmake中對(duì)于protoc的查找正是基于find_program()實(shí)現(xiàn)的。因此仍然是在CMAKE_PREFIX_PATH中添加一項(xiàng)，來(lái)找到protoc。然而一次性塞了include目錄、庫(kù)目錄、bin目錄，比較臃腫，考慮用變量：

set(Protobuf_PREFIX_PATH
    "/home/zz/soft/protobuf-3.8.0/include"
    "/home/zz/soft/protobuf-3.8.0/lib"
    "/home/zz/soft/protobuf-3.8.0/bin"
)
list(APPEND CMAKE_PREFIX_PATH "${Protobuf_PREFIX_PATH}")

當(dāng)然，實(shí)際的例子可能還需要額外的設(shè)定。來(lái)看具體的例子吧。

4. Protobuf的一個(gè)實(shí)際例子

這里例子中，既需要Protobuf的include目錄、庫(kù)文件路徑，也需要protoc的可執(zhí)行路徑；而因?yàn)橛昧藀rotobuf3.8，還需要開(kāi)啟C++11。

目錄結(jié)構(gòu)：

(base) arcsoft-43% tree
.
├── CMakeLists.txt
├── proto
│ └── addressbook.proto
├── src
│ ├── protobuf_example_read.cpp
│ └── protobuf_example_write.cpp
└── utils.cmake

CMakeLists.txt

cmake_minimum_required(VERSION 3.15)

project(oh-my-cmake)

set(CMAKE_CXX_STANDARD 11)

set(Protobuf_PREFIX_PATH
    "/home/zz/soft/protobuf-3.8.0/include"
    "/home/zz/soft/protobuf-3.8.0/lib"
    "/home/zz/soft/protobuf-3.8.0/bin"
)
list(APPEND CMAKE_PREFIX_PATH "${Protobuf_PREFIX_PATH}")
message(STATUS "=== CMAKE_PREFIX_PATH is: ${CMAKE_PREFIX_PATH}")

set(protobuf_MODULE_COMPATIBLE ON CACHE BOOL "")
find_package(Protobuf REQUIRED)
#message(STATUS "=== Protobuf_PROTOC_EXECUTABLE: ${Protobuf_PROTOC_EXECUTABLE}")


message(STATUS "=== Protobuf_INCLUDE_DIR is: ${Protobuf_INCLUDE_DIR}")
message(STATUS "=== Protobuf_INCLUDE_DIRS is: ${Protobuf_INCLUDE_DIRS}")
include_directories(${Protobuf_INCLUDE_DIRS})
include_directories(${CMAKE_CURRENT_BINARY_DIR})

protobuf_generate_cpp(AddressBook_PROTO_SRCS AddressBook_PROTO_HDRS proto/addressbook.proto)

add_executable(protobuf_example_write src/protobuf_example_write.cpp ${AddressBook_PROTO_SRCS} ${AddressBook_PROTO_HDRS})
add_executable(protobuf_example_read  src/protobuf_example_read.cpp  ${AddressBook_PROTO_SRCS} ${AddressBook_PROTO_HDRS})

target_link_libraries(protobuf_example_write ${Protobuf_LIBRARIES})
target_link_libraries(protobuf_example_read  ${Protobuf_LIBRARIES})

src/protobuf_example_read.cpp

#include <iostream>
#include <fstream>
#include <string>
#include "addressbook.pb.h"
using namespace std;

// Iterates though all people in the AddressBook and prints info about them.
void ListPeople(const tutorial::AddressBook& address_book) {
  for (int i = 0; i < address_book.people_size(); i++) {
    const tutorial::Person& person = address_book.people(i);

    cout << "Person ID: " << person.id() << endl;
    cout << "  Name: " << person.name() << endl;
    if (person.has_email()) {
      cout << "  E-mail address: " << person.email() << endl;
    }

    for (int j = 0; j < person.phones_size(); j++) {
      const tutorial::Person::PhoneNumber& phone_number = person.phones(j);

      switch (phone_number.type()) {
        case tutorial::Person::MOBILE:
          cout << "  Mobile phone #: ";
          break;
        case tutorial::Person::HOME:
          cout << "  Home phone #: ";
          break;
        case tutorial::Person::WORK:
          cout << "  Work phone #: ";
          break;
      }
      cout << phone_number.number() << endl;
    }
  }
}

// Main function:  Reads the entire address book from a file and prints all
//   the information inside.
int main(int argc, char* argv[]) {
  // Verify that the version of the library that we linked against is
  // compatible with the version of the headers we compiled against.
  GOOGLE_PROTOBUF_VERIFY_VERSION;

  if (argc != 2) {
    cerr << "Usage:  " << argv[0] << " ADDRESS_BOOK_FILE" << endl;
    return -1;
  }

  tutorial::AddressBook address_book;

  {
    // Read the existing address book.
    fstream input(argv[1], ios::in | ios::binary);
    if (!address_book.ParseFromIstream(&input)) {
      cerr << "Failed to parse address book." << endl;
      return -1;
    }
  }

  ListPeople(address_book);

  // Optional:  Delete all global objects allocated by libprotobuf.
  google::protobuf::ShutdownProtobufLibrary();

  return 0;
}

src/protobuf_example_write.cpp

#include <iostream>
#include <fstream>
#include <string>
#include "addressbook.pb.h"
using namespace std;

// This function fills in a Person message based on user input.
void PromptForAddress(tutorial::Person* person) {
  cout << "Enter person ID number: ";
  int id;
  cin >> id;
  person->set_id(id);
  cin.ignore(256, '\n');

  cout << "Enter name: ";
  getline(cin, *person->mutable_name());

  cout << "Enter email address (blank for none): ";
  string email;
  getline(cin, email);
  if (!email.empty()) {
    person->set_email(email);
  }

  while (true) {
    cout << "Enter a phone number (or leave blank to finish): ";
    string number;
    getline(cin, number);
    if (number.empty()) {
      break;
    }

    tutorial::Person::PhoneNumber* phone_number = person->add_phones();
    phone_number->set_number(number);

    cout << "Is this a mobile, home, or work phone? ";
    string type;
    getline(cin, type);
    if (type == "mobile") {
      phone_number->set_type(tutorial::Person::MOBILE);
    } else if (type == "home") {
      phone_number->set_type(tutorial::Person::HOME);
    } else if (type == "work") {
      phone_number->set_type(tutorial::Person::WORK);
    } else {
      cout << "Unknown phone type.  Using default." << endl;
    }
  }
}

// Main function:  Reads the entire address book from a file,
//   adds one person based on user input, then writes it back out to the same
//   file.
int main(int argc, char* argv[]) {
  // Verify that the version of the library that we linked against is
  // compatible with the version of the headers we compiled against.
  GOOGLE_PROTOBUF_VERIFY_VERSION;

  if (argc != 2) {
    cerr << "Usage:  " << argv[0] << " ADDRESS_BOOK_FILE" << endl;
    return -1;
  }

  tutorial::AddressBook address_book;

  {
    // Read the existing address book.
    fstream input(argv[1], ios::in | ios::binary);
    if (!input) {
      cout << argv[1] << ": File not found.  Creating a new file." << endl;
    } else if (!address_book.ParseFromIstream(&input)) {
      cerr << "Failed to parse address book." << endl;
      return -1;
    }
  }

  // Add an address.
  PromptForAddress(address_book.add_people());

  {
    // Write the new address book back to disk.
    fstream output(argv[1], ios::out | ios::trunc | ios::binary);
    if (!address_book.SerializeToOstream(&output)) {
      cerr << "Failed to write address book." << endl;
      return -1;
    }
  }

  // Optional:  Delete all global objects allocated by libprotobuf.
  google::protobuf::ShutdownProtobufLibrary();

  return 0;
}

proto/addressbook.proto

syntax = "proto2";

package tutorial;

message Person {
  required string name = 1;
  required int32 id = 2;
  optional string email = 3;

  enum PhoneType {
    MOBILE = 0;
    HOME = 1;
    WORK = 2;
  }

  message PhoneNumber {
    required string number = 1;
    optional PhoneType type = 2 [default = HOME];
  }

  repeated PhoneNumber phones = 4;
}

message AddressBook {
  repeated Person people = 1;
}

具體的構(gòu)建過(guò)程：

mkdir build
cd build
cmake ..
make

5. 總結(jié)

通常，使用Protobuf作為依賴(lài)庫(kù)的C/C++程序，并且Protobuf是自行編譯安裝的版本；設(shè)定CMAKE_PREFIX_PATH為同時(shí)包含protobuf的include目錄、庫(kù)目錄，然后執(zhí)行find_package(Protobuf)即可。

個(gè)別復(fù)雜的，還需要添加可執(zhí)行文件的目錄到CMAKE_PREFIX_PATH。如果還是不夠用（例如cmake正常而make階段報(bào)錯(cuò))，則翻看FindProtobuf.cmake并結(jié)合CMake官方文檔查閱即可。