问题:如何在代码层面封装协定细节?如何将接收缓冲区中的数据解析为 Message ?
深度思考
数据是否可能解析成为 Message ?
数据量足够
- 如果数据量足够,是否可能解析不止一个 Message?
- 如何解决残余数据 (属于下一个 Message)
数据量有余
- 是否达到协定最小长度(12 字节)?
- 如何解决数据量超过最小长度,但不足以创立一个 Message 的状况?
初步的解决方案
- 定义一个模块用于从字节流解析 Message
- 可 从指定内存 或 从指定文件描述符 读取并解析
当至多存在 12 个字节时开始解析
- 首先解析协定中的头信息和数据区长度(length)
- 依据数据区长度持续从字节流读取数据(payload)
- 当协定数据解析实现时,创立 Message 并返回,否则,返回 NULL
协定解析模块的初步设计
解析器接口定义
typedef void MParser;MParser *MParser_New();Message *MParser_ReadMem(MParser *parser, unsigned char *mem, unsigned int length);Message *MParser_ReadFd(MParser *parser, int fd);void MParser_Reset(MParser *parser);void MParser_Del(MParser *parser);
解析器数据结构
typedef struct msg_parser { Message cache; // 缓存已解析的音讯头 int header; // 标识音讯头是否解析胜利 int need; // 标识还剩多少字节能力实现解析 Message *msg; // 解析中的协定音讯(半成品)}MsgParser;
条件:内存长度至多间断 12 个字节
memcpy(&p->cache, mem, p->need);p->cache.type = ntohs(p->cache.type); // 从网络字节序转换为本机字节序p->cache.cmd = ntohs(p->cache.cmd);p->cache.index = ntohs(p->cache.index);p->cache.total = ntohs(p->cache.total);p->cache.length = ntohs(p->cache.length);mem += p->need;length -= p->need;p->header = 1;p->need = p->cache.length;
从内存中读取 payload 中的数据(可读取屡次)
if (!p->msg) { // 胜利创立音讯头之后, 创立 Message p->msg = malloc(sizeof(p->cache) + p->need); if (p->msg) { *p->msg = p->cache; }}if (p->msg) { unsigned int len = (p->need < length) > p->need : length; unsigned int offset = p->msg->length - p->need; memcpy(p->msg->payload + offset, mem, len); p->need -= len;}
编程试验:协定解析模块初步设计
msg_parser.h
#ifndef MSG_PARSER_H#define MSG_PARSER_H#include "message.h"typedef void MParser;MParser *MParser_New();Message *MParser_ReadMem(MParser *parser, unsigned char *mem, unsigned int length);Message *MParser_ReadFd(MParser *parser, int fd);void MParser_Reset(MParser *parse);void MParser_Del(MParser *parse);#endif
msg_parser.c
#include <malloc.h>#include <string.h>#include <arpa/inet.h>#include <unistd.h>#include "msg_parser.h"typedef struct msg_parser { Message cache; int header; int need; Message *msg;}MsgParser;MParser *MParser_New(){ MsgParser *ret = calloc(1, sizeof(MsgParser)); MParser_Reset(ret); return ret;}Message *MParser_ReadMem(MParser *parser, unsigned char *mem, unsigned int length){ Message *ret = NULL; MsgParser *p = (MsgParser*)parser; if (!p || !mem || !length) { return ret; } if (!p->header) { if (p->need <= length) { memcpy(&p->cache, mem, p->need); p->cache.type = ntohs(p->cache.type); p->cache.cmd = ntohs(p->cache.cmd); p->cache.index = ntohs(p->cache.index); p->cache.total = ntohs(p->cache.total); p->cache.length = ntohl(p->cache.length); mem += p->need; length -= p->need; p->header = 1; p->need = p->cache.length; ret = MParser_ReadMem(parser, mem, length); } } else { if (!p->msg) { p->msg = malloc(sizeof(p->cache) + p->need); if (p->msg) { *p->msg = p->cache; } } if (p->msg) { unsigned int len = (p->need < length) ? p->need : length; unsigned int offset = p->msg->length - p->need; memcpy(p->msg->payload, mem, len); p->need -= len; } if (!p->need) { ret = p->msg; p->msg = NULL; MParser_Reset(p); } } return ret;}Message *MParser_ReadFd(MParser *parser, int fd){ Message *ret = NULL; return ret;}void MParser_Reset(MParser *parse){ MsgParser *p = (MsgParser*)parse; if (p) { p->header = 0; p->need = sizeof(p->cache); free(p->msg); p->msg = NULL; }}void MParser_Del(MParser *parse){ MsgParser *p = (MsgParser*)parse; if (p) { free(p->msg); free(p); }}
test.c
#include <stdio.h>#include <unistd.h>#include <stdlib.h>#include "msg_parser.h"int main(){ MParser *p = MParser_New(); char buf[] = {0x00, 0x01, 0x00, 0x02, 0x00, 0x03, 0x00, 0x04, 0x00, 0x00, 0x00, 0x04}; char data[] = {0x11, 0x12, 0x13, 0x14}; Message *m = MParser_ReadMem(p, buf, sizeof(buf)); int i = 0; if (!m) { printf("parse again...\n"); m = MParser_ReadMem(p, data, sizeof(data)); } printf("m = %p\n", m); if (m) { printf("type = %d\n", m->type); printf("cmd = %d\n", m->cmd); printf("index = %d\n", m->index); printf("total = %d\n", m->total); printf("length = %d\n", m->length); for (i=0; i<m->length; ++i) { printf("0x%02x ", m->payload[i]); } printf("\n"); free(m); } MParser_Del(p); return 0; }
思考:如何通过 socket 文件描述符实时解析协定数据?