Google的Protobuf是一个高效的二进制编解码机制，应用很广，很多RPC库都使用了它，包括gRPC。

为了让大家更好的理解它，今天我来用一个简单的例子和大家介绍下它的使用方式：

示例程序介绍

示例程序由三部分组成：

1. person.proto：定义通信用的协议数据
2. client.cc：客户端实现，生成通信二进制数据发送给server
3. server.cc：服务端时间，解析客户端发送来的二进制数据，简单输出

下面分别说明：

person.proto

syntax = "proto2";

package tutorial;  // 会生成namespace tutorial

message Person {   // 会生成class 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;
}

调用protoc工具，生成类代码：

ligang@vm-xubuntu ~/devspace/hogwarts/cppsimple/src/protobuf $ protoc -I ./ --cpp_out=./ person.proto 
ligang@vm-xubuntu ~/devspace/hogwarts/cppsimple/src/protobuf $ ll
total 80
-rw-rw-r-- 1 ligang ligang  1992 11月 10 13:55 client.cc
-rw-rw-r-- 1 ligang ligang   590 11月 10 13:55 CMakeLists.txt
-rw-rw-r-- 1 ligang ligang 33288 11月 10 16:55 person.pb.cc         // 生成的person类的实现代码
-rw-rw-r-- 1 ligang ligang 25548 11月 10 16:55 person.pb.h          // 生成的person类的header文件
-rw-rw-r-- 1 ligang ligang   354 11月 10 13:55 person.proto         // 定义协议
-rw-rw-r-- 1 ligang ligang  2749 11月 10 13:55 server.cc

client.cc

#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>

#include <iostream>
#include <cstring>

#include "person.pb.h"

const int kRwBufLen = 1024;

int ConnectToServer(uint16_t port) {
  int connfd = socket(AF_INET, SOCK_STREAM, 0);
  if (connfd == -1) {
    perror("connfd fail");
    return -1;
  }

  struct sockaddr_in serverAddr;
  memset(&serverAddr, 0, sizeof(struct sockaddr_in));

  serverAddr.sin_family = AF_INET;
  if (inet_pton(AF_INET, "127.0.0.1", &(serverAddr.sin_addr)) != 1) {
    std::cout << "inet_pton fail" << std::endl;
    return -1;
  }

  serverAddr.sin_port = htons(port);

  if (connect(connfd, (struct sockaddr *) &serverAddr, sizeof(serverAddr)) == -1) {
    perror("connect failed");
    return -1;
  }

  return connfd;
}

std::string ProtobufPersonSerialize() {
  tutorial::Person person;
  person.set_name("person1");
  person.set_id(1);
  person.set_email("p1@protobuf.com");

  tutorial::Person_PhoneNumber *phone = person.add_phones();
  phone->set_number("123");
  phone->set_type(tutorial::Person_PhoneType::Person_PhoneType_MOBILE);

  phone = person.add_phones();
  phone->set_number("234");
  phone->set_type(tutorial::Person_PhoneType::Person_PhoneType_HOME);

  phone = person.add_phones();
  phone->set_number("345");
  phone->set_type(tutorial::Person_PhoneType::Person_PhoneType_WORK);

  std::string person_serialize = person.SerializeAsString();
  std::cout << person_serialize.size() << std::endl;

  return person_serialize;
}

int main() {
  GOOGLE_PROTOBUF_VERIFY_VERSION;

  int connfd = ConnectToServer(9008);     // 获得通信用socket

  char buf[kRwBufLen];
  memset(buf, 0, kRwBufLen);

  std::string person_serialize = ProtobufPersonSerialize();   // 生成通信用的person的二进制数据
  uint32_t dl = htonl(person_serialize.size());
  size_t dlsize = sizeof dl;

  memcpy(buf, &dl, dlsize);   // 二进制数据的长度
  memcpy(buf + dlsize, (char *) person_serialize.c_str(), person_serialize.size());     // 编码protobuf二进制数据
  write(connfd, buf, dlsize + person_serialize.size());    // 组成数据后一起发送

  return 0;
}

客户端运行后，输出编码发送的二进制数据长度为：55

server.cc

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

#include <iostream>
#include <cstring>

#include "person.pb.h"

const int kRwBufLen = 1024;

int TcpBindAndListen(uint16_t port) {
  int listenfd = socket(AF_INET, SOCK_STREAM, 0);
  if (listenfd == -1) {
    perror("listenfd fail");
    return -1;
  }

  struct sockaddr_in serverAddr;
  memset(&serverAddr, 0, sizeof(struct sockaddr_in));

  serverAddr.sin_family = AF_INET;
  if (inet_pton(AF_INET, "127.0.0.1", &(serverAddr.sin_addr)) != 1) {
    std::cout << "inet_pton fail" << std::endl;
    return -1;
  }

  serverAddr.sin_port = htons(port);

  if (bind(listenfd, (struct sockaddr *) &serverAddr, sizeof(serverAddr)) == -1) {
    perror("bind fail");
    return -1;
  }

  if (listen(listenfd, 1024) == -1) {
    perror("listen failed");
    return -1;
  }

  return listenfd;
}

int ReadConn(int connfd, size_t len, char *buf) {
  size_t readed = 0;
  size_t remain = len - readed;
  while (remain > 0) {
    auto n = read(connfd, buf + readed, remain);

    if (n < 0) {
      if (errno == EINTR) {
        continue;
      }
      perror("read error");
      return -1;
    }

    readed += n;
    remain = len - readed;
  }

  return 0;
}

void ReadProtobufPerson(int connfd, size_t dl) {
  char buf[kRwBufLen];

  int r = ReadConn(connfd, dl, buf);
  if (r == -1) {
    perror("read conn error");
    return;
  }

  tutorial::Person person;
  std::cout << "dl = " << dl << std::endl;
  person.ParseFromArray(buf, dl);            // 解码protobuf二进制数据

  std::cout << person.id() << std::endl
            << person.name() << std::endl
            << person.email() << std::endl;

  for (auto phone : person.phones()) {
    std::cout << phone.number() << std::endl;
    std::cout << phone.type() << std::endl;
  }
}

void ProtobufServe(int connfd) {
  char buf[kRwBufLen];
  size_t dl_len = 4;

  int r = ReadConn(connfd, dl_len, buf);      // 先解析二进制数据的长度
  if (r == -1) {
    perror("read conn error");
    return;
  }

  uint32_t dl;
  memcpy(&dl, buf, dl_len);
  std::cout << "before ntoh: " << dl << std::endl;

  dl = ntohl(dl);
  std::cout << "after ntoh: " << dl << std::endl;

  ReadProtobufPerson(connfd, dl);      // 解析二进制数据
}

int main() {
  GOOGLE_PROTOBUF_VERIFY_VERSION;

  int listenfd = TcpBindAndListen(9008);    // listen用socket
  if (listenfd < 0) {
    return 1;
  }

  bool running = true;
  while (running) {
    struct sockaddr_in clientAddr;
    memset(&clientAddr, 0, sizeof(struct sockaddr_in));
    socklen_t clientLen;

    int connfd = accept(listenfd, (struct sockaddr *) &clientAddr, &clientLen);  // 客户端连接后获得通信用socket
    if (connfd == -1) {
      running = false;
      perror("accept failed");
    } else {
      ProtobufServe(connfd);  // 读取客户端发送来的二进制数据后解析输出
    }

    close(connfd);
  }

  return 0;
}

运行后，服务端输出客户端发送来的数据为：

dl = 55  // 数据长度
1        // id
person1  // name
p1@protobuf.com  // email
123  // 这里开始是地址薄
0
234
1
345
2

结束语

这里只是为了示例protobuf的基本使用方法，更多的protobuf语法，请查看官方手册，谢谢大家！