Python uses the struct module to convert C language structures, pack and unpack binary data

Reference for this article: https://docs.python.org/2/library/struct.html#struct-format-strings

This article is the message header I wrote using the socket test network interface, which contains the following contents:
(1) Convert the C structure into Python language (struct.Struct) through Python's struct module
(2) Packing and unpacking (pack_into and unpack_from )
(3) Serialization and deserialization (SerializeToString and ParseFromString)

The message header of the C language is as follows, which is a C structure:

#pragma pack(1)
struct PduHead{
         unsigned int   flag; 
         unsigned short packet_len;
         unsigned int   cmd;
         unsigned char  version;
         unsigned char  reserve[1];
         unsigned char  body[0];
};
#pragma pack()

Convert the message header of the C language into a Python class, where reserve[1] is a reserved word, don't care, body[0] is an array of length 0, save the first address of the body, don't care. After conversion as follows

import struct
import ctypes

class PduHead(object):
    def __init__(self, cmdtype=0):
        self.flag = GLOBAL_SIGNAL_FLAG
        self.packet_len = 0
        self.cmd = cmdtype
        self.version = VERSION
        # struct.Struct返回一个新的Struct对象，根据传入的字节顺序字符串写入和读取二进制数据
        # “!IHIcc”表示二进制数据的字节顺序
        # “!”表示字节顺序为network
        # “IHIcc”分别是根据以上几个参数的类型得到的格式字符
        # 二进制流会根据这个字节顺序去解析          
        self.struct = struct.Struct('!IHIcc')

    # 序列化函数
    def SerializeToStringWithMsg(self, req_msg):
        if not req_msg:
            logger.error('No request message set!')
            return ''
        body_str = req_msg.SerializeToString()  # 序列化，将对象转化为可传输的二进制流
        values = (self.flag, len(body_str) + self.struct.size, self.cmd, self.version, '\0')
        buffer = ctypes.create_string_buffer(self.struct.size) # 创建一个buffer
        self.struct.pack_into(buffer, 0, *values)  # 根据字节序打包，将打包的字节写入到buffer中，values元组表示要写入的值
        # buffer对象提供了raw属性访问当前buffer内容
        # 该方法返回一个要传输的消息头和消息体的二进制流
        return buffer.raw + body_str 

    # 反序列化函数
    def ParseFromStringWithMsg(self, data):
        # 根据字节序解析二进制流，返回一个元素，形似SerializeToStringWithMsg方法中的value，所以self.cmd是元组的第三个元素
        self.cmd = self.struct.unpack_from(data[:self.struct.size])[2]  
        resp_msg = dict_message.get(self.cmd)  # 根据self.cmd得到响应数据的二进制流（这是个不用关心，跟业务有关）
        if not resp_msg:
            logger.error('Message [%d] Not Found!' % self.cmd)
            return None
        resp_msg = resp_msg()
        if resp_msg:
            resp_msg.ParseFromString(data[self.struct.size:])  # 反序列化，将二进制流转化为实际的对象
        return [self.cmd, resp_msg]

Byte order:
1. The first character of the byte order can be used to indicate the byte order, size and way of the packed data

2. Type conversion between format characters C and Python, the combined string represents the byte order of binary data, such as "!IHIcc" in the above program