摘要

本文先簡(jiǎn)單介紹MessagePack的基本概念。

然后，介紹一個(gè)MessagePack C API - MPack的通常使用。

接著嘗試對(duì)MPack截?cái)鄶?shù)據(jù)的讀取。

注：本文完整代碼見倉(cāng)庫(kù)。

MessagePack簡(jiǎn)介

如果你使用過C/C++的json庫(kù)，那么上手MessagePack是比較容易的。關(guān)于C/C++ Json庫(kù)的使用可見：C++ JSON庫(kù)的一般使用方法-CSDN博客。

這里，我先說下結(jié)論，對(duì)于用戶層面而言，MessagePack相對(duì)于json節(jié)省空間，但犧牲了可讀性(對(duì)于人類而言)。（更多區(qū)別見：Why Not Just Use JSON?）

下面我們來看兩個(gè)示例，了解下MessagPack是如何壓縮json內(nèi)容。這個(gè)壓縮過程，遵循MessagePack specification(MessagePack規(guī)范)

第一個(gè)示例，來自MessagePack首頁(yè)。它將27個(gè)字節(jié)的JSON內(nèi)容，壓縮到18個(gè)字節(jié)。

我們根據(jù)MessagePack規(guī)范來看下上面是如何壓縮的。

# 82的含義
## 對(duì)于元素個(gè)數(shù)不超過15個(gè)的map存儲(chǔ)，遵循如下格式
## 82 == 1000 0010 表示該結(jié)構(gòu)為map，有兩個(gè)元素
+--------+~~~~~~~~~~~~~~~~~+
|1000XXXX|   N*2 objects   |
+--------+~~~~~~~~~~~~~~~~~+

# A7的含義
## 對(duì)于長(zhǎng)度不超過31的固定長(zhǎng)度字符串存儲(chǔ)，遵循如下格式
## A7 == 1010 0111 表示該結(jié)構(gòu)為字符串，有7個(gè)字符
+--------+========+
|101XXXXX|  data  |
+--------+========+

# C3含義
## false:
+--------+
|  0xc2  |
+--------+
## true:
+--------+
|  0xc3  |
+--------+

# A6的含義--略，查詢過程同上面A7

# 00的含義
## 可以使用7-bit存儲(chǔ)的正整數(shù)，遵循如下格式
## 00 = 0000 0000
+--------+
|0XXXXXXX|
+--------+

我們?cè)賮砜匆粋€(gè)示例，練習(xí)下。這個(gè)示例來自： mpack/docs/expect.md at develop · ludocode/mpack

[
  "hello",
  "world!",
  [
    1,
    2,
    3,
    4
  ]
]

上面這個(gè)json，使用MessagPack編碼如下。

93                     # an array containing three elements
  a5 68 65 6c 6c 6f    # "hello"
  a6 77 6f 72 6c 64 21 # "world!"
  94                   # an array containing four elements
    01                 # 1
    02                 # 2
    03                 # 3
    04                 # 4

MPACK的簡(jiǎn)單使用

了解了MessagePack的概念之后，我們看下它的C api的使用。MPACK是MessagePack C語(yǔ)言實(shí)現(xiàn)的API之一。mpack的stars沒有msgpack-c多。但是mpack對(duì)libc的版本沒有要求。下面是對(duì)mpack的使用教程。

JSON的使用包含兩個(gè)部分：將json數(shù)據(jù)寫入內(nèi)存/文件；從已有的json數(shù)據(jù)中讀取內(nèi)容。

mapck在使用結(jié)構(gòu)上，和json類似，分為寫和讀。MPack api是mpack的api文檔，接口使用的詳細(xì)介紹見文檔。寫數(shù)據(jù)的部分調(diào)用Write API。如果沒有內(nèi)存限制，讀取的時(shí)候使用Node API。如果有內(nèi)存限制，則使用Reader API+Expect API。(這里說的內(nèi)存限制是指，程序運(yùn)行的內(nèi)存限制，而不是數(shù)據(jù)存儲(chǔ)的內(nèi)存限制)

這個(gè)參考示例不錯(cuò)，在開始之前，可以一讀：一個(gè)C語(yǔ)言MessagePack庫(kù)：mpack

下面我們開始寫demo。寫一個(gè)最簡(jiǎn)單，也是最常用的demo：存儲(chǔ)數(shù)據(jù)的內(nèi)存無(wú)限制時(shí)，使用mpack進(jìn)行數(shù)據(jù)的寫入和讀取。示例參考自官方的README:ludocode/mpack: MPack - A C encoder/decoder for the MessagePack serialization format / msgpack.org[C]. 這里對(duì)這些API進(jìn)行簡(jiǎn)單的介紹，詳細(xì)介紹見官方文檔。

• mpack_writer_init_growable(mpack_writer_t* writer, char** target_data, size_t* target_size)使用一個(gè)會(huì)增長(zhǎng)的buffer。
• 在調(diào)用mpack_writer_destroy后，將上面target_data指向?qū)懭霐?shù)據(jù)的地址。
• 最后必須調(diào)用MPACK_FREE()釋放申請(qǐng)的數(shù)據(jù)。
• 使用mpack_build_map開始構(gòu)建一個(gè)map，我們此時(shí)不知道m(xù)ap中元素的個(gè)數(shù)。如果知道元素個(gè)數(shù)，使用mpack_start_map()替代。
• 使用mpack_build_map后，后面必須跟偶數(shù)個(gè)數(shù)的元素，在結(jié)束的時(shí)候調(diào)用 mpack_complete_map()。
• 使用Node API讀取數(shù)據(jù)。

#include "mpack.h"
#include <stdio.h>

/*
{
    "name": "3-1",
    "number": 56,
    "students": [
        {
            "name": "zhangsan",
            "score": 76.8
        },
        {
            "name": "lisi",
            "score": 77
        }
    ]
}
*/

mpack_error_t class_information_serialize(char **data, size_t *size) {
  mpack_writer_t writer;
  mpack_writer_init_growable(&writer, data, size);

  mpack_build_map(&writer);

  mpack_write_cstr(&writer, "name");
  mpack_write_cstr(&writer, "3-1");

  mpack_write_cstr(&writer, "number");
  mpack_write_u8(&writer, 56);

  mpack_write_cstr(&writer, "students");
  mpack_build_array(&writer);

  mpack_start_map(&writer, 2);
  mpack_write_cstr(&writer, "name");
  mpack_write_cstr(&writer, "zhangsan");
  mpack_write_cstr(&writer, "score");
  mpack_write_float(&writer, 76.8);
  mpack_finish_map(&writer);

  mpack_start_map(&writer, 2);
  mpack_write_cstr(&writer, "name");
  mpack_write_cstr(&writer, "lisi");
  mpack_write_cstr(&writer, "score");
  mpack_write_float(&writer, 77);
  mpack_finish_map(&writer);

  mpack_complete_array(&writer);

  mpack_complete_map(&writer);

  mpack_error_t ret = mpack_writer_destroy(&writer);

  return ret;
}

mpack_error_t class_information_deserialize(const char *data, size_t length) {
  mpack_tree_t tree;
  mpack_tree_init_data(&tree, data, length);
  mpack_tree_parse(&tree);

  mpack_node_t root = mpack_tree_root(&tree);
  const char *name = mpack_node_str(mpack_node_map_cstr(root, "name"));
  size_t name_len = mpack_node_strlen(mpack_node_map_cstr(root, "name"));
  uint8_t number = mpack_node_u8(mpack_node_map_cstr(root, "number"));
  printf("name:%.*s\n", name_len, name);
  printf("number:%u\n", number);

  printf("students:\n");
  mpack_node_t students = mpack_node_map_cstr(root, "students");
  size_t student_num = mpack_node_array_length(students);
  for (unsigned int i = 0; i < student_num; i++) {
    mpack_node_t student = mpack_node_array_at(students, i);
    const char *name = mpack_node_str(mpack_node_map_cstr(student, "name"));
    size_t name_len = mpack_node_strlen(mpack_node_map_cstr(student, "name"));
    float score = mpack_node_float(mpack_node_map_cstr(student, "score"));
    printf("  name:%.*s\n", name_len, name);
    printf("  score:%.2f\n", score);
  }

  mpack_error_t ret = mpack_tree_destroy(&tree);
  return ret;
}

int main(int argc, char *argv[]) {
  char *data = NULL;
  size_t size = 0;
  class_information_serialize(&data, &size);
  class_information_deserialize(data, size);
  MPACK_FREE(data);
}

程序輸出如下。

name:3-1
number:56
students:
  name:zhangsan
  score:76.80
  name:lisi
  score:77.00

在定長(zhǎng)的buffer存儲(chǔ)不定長(zhǎng)的數(shù)據(jù)

工作的時(shí)候，會(huì)想使用比較奇怪的調(diào)用。首先，所有的數(shù)據(jù)都要存儲(chǔ)在一個(gè)定長(zhǎng)的buffer里面。因?yàn)檫@個(gè)buffer是從一個(gè)內(nèi)存池中取出的，所以它的長(zhǎng)度是定長(zhǎng)的。但是，往里面寫入數(shù)據(jù)的時(shí)候，會(huì)寫多次，長(zhǎng)度不一定。

修改上面的示例：現(xiàn)在要寫入不定個(gè)數(shù)的student到數(shù)組中；允許截?cái)啵?/p>

截?cái)嗟臅r(shí)候發(fā)生了什么？截?cái)嗪筮€能否讀??？

遇到截?cái)?，writer會(huì)設(shè)置錯(cuò)誤的標(biāo)記位。正常編碼的時(shí)候，這個(gè)writer數(shù)據(jù)不應(yīng)該在后續(xù)進(jìn)行讀取。因?yàn)樗呀?jīng)被標(biāo)記為錯(cuò)誤。

但是，總有些頭鐵的需求，想用發(fā)生截?cái)鄷r(shí)，已經(jīng)寫入內(nèi)存的數(shù)據(jù)。這個(gè)從API文檔里面是看不出來的，要看mpack的源碼。

我先說結(jié)論：

• 從前往后，不斷將build中的內(nèi)容，復(fù)制寫入buffer。寫之前檢查空間是否足夠，不夠則停止寫入，并標(biāo)記錯(cuò)誤。
• 使用 node api 無(wú)法讀取。因?yàn)閿?shù)據(jù)被截?cái)啵缓戏ā?/li>

大體結(jié)構(gòu)圖如下。

示例代碼如下。

#include "mpack.h"
#include <stdio.h>

/*
{
    "name": "3-1",
    "number": 56,
    "students": [
        {
            "name": "zhangsan",
            "score": 76.8
        },
        {
            "name": "lisi",
            "score": 77
        }
    ]
}
*/

mpack_error_t class_information_serialize(char *data, size_t *size) {
  mpack_writer_t writer;
  mpack_writer_init(&writer, data, *size);

  mpack_build_map(&writer);

  mpack_write_cstr(&writer, "name");
  mpack_write_cstr(&writer, "3-1");

  mpack_write_cstr(&writer, "number");
  mpack_write_u8(&writer, 56);

  mpack_write_cstr(&writer, "students");

  mpack_build_array(&writer);

  for (unsigned int i = 0; i < 56; i++) {
    mpack_start_map(&writer, 2);
    mpack_write_cstr(&writer, "name");
    mpack_write_cstr(&writer, "zhangsan");
    mpack_write_cstr(&writer, "score");
    mpack_write_float(&writer, 76.8);

    if (mpack_writer_error(&writer) != mpack_ok) {
      printf("error_%u: %d\n", i, mpack_writer_error(&writer));
    }
    mpack_finish_map(&writer);
  }

  mpack_complete_array(&writer);

  mpack_complete_map(&writer);

  if (mpack_writer_error(&writer) != mpack_ok) {
    printf("after write all students error: %d\n", mpack_writer_error(&writer));
  }

  *size = mpack_writer_buffer_used(&writer);
  mpack_error_t ret = mpack_writer_destroy(&writer);

  return ret;
}

mpack_error_t class_information_deserialize(const char *data, size_t length) {
  mpack_tree_t tree;
  mpack_tree_init_data(&tree, data, length);
  mpack_tree_parse(&tree);

  if (mpack_tree_error(&tree) != mpack_ok) {
    printf("parse tree error: %d\n", mpack_tree_error(&tree));
  }

  mpack_node_t root = mpack_tree_root(&tree);
  const char *name = mpack_node_str(mpack_node_map_cstr(root, "name"));
  size_t name_len = mpack_node_strlen(mpack_node_map_cstr(root, "name"));
  uint8_t number = mpack_node_u8(mpack_node_map_cstr(root, "number"));
  printf("name:%.*s\n", name_len, name);
  printf("number:%u\n", number);

  printf("students:\n");
  mpack_node_t students = mpack_node_map_cstr(root, "students");
  size_t student_num = mpack_node_array_length(students);
  for (unsigned int i = 0; i < student_num; i++) {
    mpack_node_t student = mpack_node_array_at(students, i);
    const char *name = mpack_node_str(mpack_node_map_cstr(student, "name"));
    size_t name_len = mpack_node_strlen(mpack_node_map_cstr(student, "name"));
    float score = mpack_node_float(mpack_node_map_cstr(student, "score"));
    printf("  name:%.*s\n", name_len, name);
    printf("  score:%.2f\n", score);
  }

  mpack_error_t ret = mpack_tree_destroy(&tree);
  return ret;
}

int main(int argc, char *argv[]) {
#define DATA_BUFFER_SIZE 35
  char data[DATA_BUFFER_SIZE] = {0};
  size_t size = DATA_BUFFER_SIZE;
  class_information_serialize(data, &size);
  class_information_deserialize(data, size);
}

輸出如下。

after write all students error: 6
parse tree error: 3
name:
number:0
students:

讀取截?cái)嗟臄?shù)據(jù)

既然使用node api解析數(shù)據(jù)會(huì)失敗。那不要解析，順序讀取，一直讀取到異常。

參考自：Using the Expect API。代碼如下。

#include "mpack.h"
#include <stdio.h>

/*
[
  {
    "name": "zhangsan",
    "score": 76.8
  },
  {
    "name": "lisi",
    "score": 77
  }
  ...
]

*/

mpack_error_t class_information_serialize(char *data, size_t *size) {
  mpack_writer_t writer;
  mpack_writer_init(&writer, data, *size);

  mpack_build_array(&writer);

  for (unsigned int i = 0; i < 56; i++) {
    mpack_build_map(&writer);
    mpack_write_cstr(&writer, "name");
    mpack_write_cstr(&writer, "zhangsan");
    mpack_write_cstr(&writer, "score");
    mpack_write_float(&writer, 76.8);
    mpack_complete_map(&writer);
  }

  mpack_complete_array(&writer);

  if (mpack_writer_error(&writer) != mpack_ok) {
    printf("after write all students error: %d\n", mpack_writer_error(&writer));
  }

  *size = mpack_writer_buffer_used(&writer);
  mpack_error_t ret = mpack_writer_destroy(&writer);

  return ret;
}

mpack_error_t class_information_deserialize(const char *data, size_t length) {
  mpack_reader_t reader;
  mpack_reader_init_data(&reader, data, length);

  uint32_t students_num = mpack_expect_array(&reader);
  printf("students num: %u\n", students_num);
  for (unsigned int i = 0; i < students_num; i++) {
    uint32_t elem_cnt = mpack_expect_map(&reader);
    mpack_expect_cstr_match(&reader, "name");
    char name[20];
    size_t name_len = mpack_expect_str_buf(&reader, name, 20);
    mpack_expect_cstr_match(&reader, "score");
    float score = mpack_expect_float(&reader);

    if (mpack_reader_error(&reader) != mpack_ok) {
      break;
    }

    printf("name:%.*s\n", name_len, name);
    printf("score:%.2f\n", score);
    mpack_done_map(&reader);
  }

  return mpack_ok;
}

int main(int argc, char *argv[]) {
#define DATA_BUFFER_SIZE 100
  char data[DATA_BUFFER_SIZE] = {0};
  size_t size = DATA_BUFFER_SIZE;
  class_information_serialize(data, &size);
  class_information_deserialize(data, size);
}

輸出如下。文章來源地址http://www.zghlxwxcb.cn/news/detail-830380.html

after write all students error: 6
students num: 56
name:zhangsan
score:76.80
name:zhangsan
score:76.80
name:zhangsan
score:76.80

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