初学者通过下面几个c语言，大家可以提高自己的编程水平，
1. 打印任意一段内存的数据
void print_array(char *title, unsigned char *data, int len) { printf("%s:\n", title); for (int i = 0; i < len; i++) { // 每行开头打印当前字节的地址 if (i % 16 == 0) { printf("0x%08X: ", (unsigned int)(data + i)); } // 打印当前字节的十六进制形式 printf("%02X ", data[i]); // 每行打印16个字节后换行，并打印ASCII字符 if ((i + 1) % 16 == 0 || i == len - 1) { // 对齐填充 for (int j = 0; j < 16 - (i % 16) - 1; j++) { printf(" "); } printf(" | "); // 打印ASCII字符 for (int j = i - (i % 16); j <= i; j++) { if (data[j] >= 32 && data[j] <= 126) { printf("%c", data[j]); } else { printf("."); } } printf("\n"); } } printf("----------------------------------------\n");}2. 实现下面信令的封装和解析，只写出结构即可
名称字段个数说明起始字节1只能为0x7e命令1参数2高字节在低位，低字节在高位长度2包括子命令和数据区长度，高字节在低位，低字节在高位子命令1数据区N可变长度，最大长度为20校验字1从命令开始累计到数据区结束符1只能为0x7e
数据帧封装
int msg_send(u8 cmd, u16 param, u8 subcmd, u8 *data, u8 len){ int pos = 0; u8 crc = 0; int i,j; int status = 0; int ret = 0; u8 rawdata[MAX_FRM_LEN] = {0}; //填充7e pos = 0; rawdata[pos] = 0x7e; pos+=1; rawdata[pos] = cmd; pos+=1; setdata16(&rawdata[pos],cmd); pos+=2; setdata16(&rawdata[pos],len+1); pos+=2; rawdata[pos] = subcmd; pos+=1; for(i=0;i
数据帧解析
用上一个程序执行结果，作为解析函数的测试数据
u8 buf[]={0x7E,0x01 ,0x00 ,0x01 ,0x00 ,0x05 ,0x01 ,0x01 ,0x02 ,0x03 ,0x04 ,0x12 ,0x7E };
int my_check_crc(char *data){ int i; u8 crc=0; u16 datalen=0; getdata16(&datalen,&data[4]); for(i=1;i<1+6+datalen-1;i++) { crc+=data[i]; } printf("crc=%x %x\n",crc&0xff,data[1+6+datalen-1]);//datalen包括子命令1个字节 return (crc&0xff)==(data[1+6+datalen-1]&0xff);}int frm_parse(PENG_FRM_MSG_S *pmsg,u8 data[],int len){ int ret = -1; int pos = 0; if(len<9) { printf("invalid frm len %d\n",len); return -1; } //check crc ret = my_check_crc(data); if(ret != 1) { printf("crc check error\n"); return -1; } pos = 0; pmsg->startCode = data[pos]; pos++; pmsg->cmd = data[pos]; pos++; getdata16(&pmsg->param,&data[pos]); pos += 2; getdata16(&pmsg->len,&data[pos]); pos += 2; pmsg->subcmd = data[pos]; pos++; if(pmsg->len+8 != len) { printf("err invalid frm len=%d\n",pmsg->len); return -1; } memcpy(pmsg->data,&data[pos],pmsg->len-1); return 1;}int main(){ int len; int ret; PENG_FRM_MSG_S msg; PENG_FRM_MSG_S *pmsg = &msg; u8 buf[]={0x7E,0x01 ,0x00 ,0x01 ,0x00 ,0x05 ,0x01 ,0x01 ,0x02 ,0x03 ,0x04 ,0x12 ,0x7E }; ret = frm_parse(pmsg,buf,sizeof(buf)); printf("cmd:%x param:%x,len:%x,subcmd:%x\n", pmsg->cmd,pmsg->param,pmsg->len,pmsg->subcmd); print_array("[data]<<<",pmsg->data,pmsg->len-1); return 1;}
思考：如果起始字符和结束符之间的数据如果有0x7e/0x7d需要转义为0x7d5e/0x7d5d，应该如何处理？
参考下面一篇文章 《7E头解析的那些事儿（帧格式分析实例）》
3. 编写基于udp C/S架构的服务器客户端程序
- udp_server.c
#include #include #include #include #include #include #include #include #include #include #define SERVER_PORT 8888 #define MAX_MSG_SIZE 1024 void udps_respon(int sockfd) { struct sockaddr_in addr; int addrlen,n; char msg[MAX_MSG_SIZE]; while(1) { /* 从网络上读,并写到网络上 */ bzero(msg,sizeof(msg)); // 初始化,清零 addrlen = sizeof(struct sockaddr_in); //addr存放客户端的port ip信息 n=recvfrom(sockfd,msg,MAX_MSG_SIZE,0,(struct sockaddr*)&addr,&addrlen); // 从客户端接收消息 msg[n]=0; /* 显示服务端已经收到了信息 */ fprintf(stdout,"Server have received %s %s\n",msg,inet_ntoa(addr.sin_addr)); // 显示消息 sendto(sockfd,msg,strlen(msg),0,(struct sockaddr*)&addr,addrlen); } } int main(void) { int sockfd; struct sockaddr_in addr; /* 服务器端开始建立socket描述符 */ sockfd=socket(AF_INET,SOCK_DGRAM,0); if(sockfd<0) { fprintf(stderr,"Socket Error:%s\n",strerror(errno)); exit(1); } /* 服务器端填充 sockaddr结构 */ bzero(&addr,sizeof(struct sockaddr_in)); addr.sin_family=AF_INET; addr.sin_addr.s_addr=htonl(INADDR_ANY); addr.sin_port=htons(SERVER_PORT); /* 捆绑sockfd描述符 */ if(bind(sockfd,(struct sockaddr *)&addr,sizeof(struct sockaddr_in))<0) { fprintf(stderr,"Bind Error:%s\n",strerror(errno)); exit(1); } udps_respon(sockfd); // 进行读写操作 close(sockfd); }
- udp_client.c
#include #include #include #include #include #include #include #include #include #include #define SERVER_PORT 8888 #define MAX_BUF_SIZE 1024 void udpc_requ(int sockfd,const struct sockaddr_in *addr,int len) { char buffer[MAX_BUF_SIZE]; int addrlen,n; struct sockaddr_in addr_server; while(1) { /* 从键盘读入,写到服务端 */ printf("Please input char:\n"); fgets(buffer,MAX_BUF_SIZE,stdin); sendto(sockfd,buffer,strlen(buffer),0,addr,len); bzero(buffer,MAX_BUF_SIZE); addrlen = sizeof(struct sockaddr); n=recvfrom(sockfd,buffer,MAX_BUF_SIZE,0,(struct sockaddr*)&addr_server,&addrlen); buffer[n]=0; printf("recv:%s\n",buffer); } } int main(int argc,char **argv) { int sockfd; struct sockaddr_in addr; if(argc!=2) { fprintf(stderr,"Usage:%s server_ip\n",argv[0]); exit(1); } /* 建立 sockfd描述符 */ sockfd=socket(AF_INET,SOCK_DGRAM,0); if(sockfd<0) { fprintf(stderr,"Socket Error:%s\n",strerror(errno)); exit(1); } /* 填充服务端的资料 */ bzero(&addr,sizeof(struct sockaddr_in)); addr.sin_family=AF_INET; addr.sin_port=htons(SERVER_PORT); if(inet_aton(argv[1],&addr.sin_addr)<0) /*inet_aton函数用于把字符串型的IP地址转化成网络2进制数字*/ { fprintf(stderr,"Ip error:%s\n",strerror(errno)); exit(1); } udpc_requ(sockfd,&addr,sizeof(struct sockaddr_in)); // 进行读写操作 close(sockfd); }
编译：
gcc udp_server.c -o sgcc udp_client.c -o c
先运行服务器
./s
再打开一个终端，运行客户端
./c
客户端输入任意字符串会直接返回给服务器。
4. 基于步骤2/3通过udp实现信令在client/server之间传输，只写出结构即可
clien.c
#define SERVER_PORT 8888 #define MAX_BUF_SIZE 1024 typedef struct sockaddr SA;void sendfrm_thread(int sockfd,const struct sockaddr_in *addr,int len) { u8 data[FRM_DATA_MAX_LEN]={0x1,0x2,0x3,0x4}; int addrlen,frmlen; struct sockaddr_in addr_server; u16 param = 0x9527; u8 rawdata[MAX_FRM_LEN] = {0}; while(1) { frmlen = msg_send(rawdata,MSG_TYPE_QUERY, param, 1, data, 4); sendto(sockfd,rawdata,frmlen,0,(SA *)addr,len); sleep(2); } } int main(int argc,char **argv) { int sockfd; struct sockaddr_in addr; if(argc!=2) { fprintf(stderr,"Usage:%s server_ip\n",argv[0]); exit(1); } sockfd=socket(AF_INET,SOCK_DGRAM,0); if(sockfd<0) { fprintf(stderr,"Socket Error:%s\n",strerror(errno)); exit(1); } bzero(&addr,sizeof(struct sockaddr_in)); addr.sin_family=AF_INET; addr.sin_port=htons(SERVER_PORT); if(inet_aton(argv[1],&addr.sin_addr)<0) { fprintf(stderr,"Ip error:%s\n",strerror(errno)); exit(1); } sendfrm_thread(sockfd,&addr,sizeof(struct sockaddr_in)); close(sockfd); }
server.c
void rcvfrm_thread(int sockfd) { int ret; struct sockaddr_in addr; int addrlen,len; PENG_FRM_MSG_S msg; PENG_FRM_MSG_S *pmsg = &msg; char buf[MAX_MSG_SIZE]; addrlen = sizeof(struct sockaddr_in); while(1) { bzero(buf,sizeof(buf)); len=recvfrom(sockfd,buf,MAX_MSG_SIZE,0,(struct sockaddr*)&addr,&addrlen); buf[len]=0; ret = frm_parse(pmsg,buf,len); printf("\n[msg]<<< cmd:%x param:%x,len:%x,subcmd:%x\n", pmsg->cmd,pmsg->param,pmsg->len,pmsg->subcmd); //print_array("[data]<<<",pmsg->data,pmsg->len-1); } } int main(void) { int sockfd; struct sockaddr_in addr; sockfd=socket(AF_INET,SOCK_DGRAM,0); if(sockfd<0) { fprintf(stderr,"Socket Error:%s\n",strerror(errno)); exit(1); } bzero(&addr,sizeof(struct sockaddr_in)); addr.sin_family=AF_INET; addr.sin_addr.s_addr=htonl(INADDR_ANY); addr.sin_port=htons(SERVER_PORT); if(bind(sockfd,(struct sockaddr *)&addr,sizeof(struct sockaddr_in))<0) { fprintf(stderr,"Bind Error:%s\n",strerror(errno)); exit(1); } rcvfrm_thread(sockfd); close(sockfd); }
5. 使用数据库sqlite，将步骤4的信令insert到数据库中，
qslite操作可以参考下面文章：
《嵌入式数据库sqlite3【基础篇】-基本命令操作，小白一看就懂》
6. 用c语言执行步骤5的数据库操作指令，并将代码合并到步骤4的架构中
C语言操作数据库，可以参考：《如何用C语言操作sqlite3，一文搞懂》
7. 用任意一款抓包工具抓取一个ip数据包,保存到数组frm[]
拷贝出数据：
80 8F 1D C7 A6 07 D8 BB C1 C8 51 C3 08 00 45 00 00 34 FC 6E 40 00 80 06 00 00 C0 A8 00 6B 8E FA C4 CA 21 AD 01 BB 25 C1 79 D5 00 00 00 00 80 02 FA F0 14 FF 00 00 02 04 05 B4 01 03 03 08 01 01 04 02
转换成数组：
unsigned char frm[]={0x80,0x8F,0x1D,0xC7,0xA6,0x07,0xD8,0xBB,0xC1,0xC8,0x51,0xC3,0x08,0x00,0x45,0x00,0x00,0x34,0xFC,0x6E,0x40,0x00,0x80,0x06,0x00,0x00,0xC0,0xA8,0x00,0x6B,0x8E,0xFA,0xC4,0xCA,0x21,0xAD,0x01,0xBB,0x25,0xC1,0x79,0xD5,0x00,0x00,0x00,0x00,0x80,0x02,0xFA,0xF0,0x14,0xFF,0x00,0x00,0x02,0x04,0x05,0xB4,0x01,0x03,0x03,0x08,0x01,0x01,0x04,0x02};
8. 提取出ip数据包的mac头,ip数据包的ip头
注意：字节序问题
mact头提取
#define MAC_FMG "%02x:%02x:%02x:%02x:%02x:%02x\n"#define printfmac(x) printf(MAC_FMG,x[0],x[1],x[2],x[3],x[4],x[5])struct mac_h{ u8 dst[6]; u8 src[6]; u16 pro;};int parse_mac_h(u8 *data,struct mac_h *pmac_h,int len){ int pos = 0; if(len<6+6+2) { return -1; } pos = 0; memcpy(pmac_h->dst,&data[pos],6); pos+=6; memcpy(pmac_h->src,&data[pos],6); pos+=6; pmac_h->pro = data[pos]<<8 | data[pos+1]; pos+=2;}int main(){ struct mac_h mac_header; ...... parse_mac_h(frm,&mac_header,sizeof(frm)); ...... return 1;}
执行结果如下：
ip头
#define IPADDR_FMG "%d.%d.%d.%d\n"#define printipaddr(x) printf(IPADDR_FMG,((u8*)&(x))[0],((u8*)&(x))[1],((u8*)&(x))[2],((u8*)&(x))[3])void dump_iphdr(struct iphdr *iph){ printf("ihl:%d\nversion:%d\ntos:%d\ntot_len:%x\nid:%x\nfrag_off:%x\nttl:%d\nprotocol:%d\ncheck:%x\n", iph->ihl, iph->version, iph->tos, iph->tot_len, iph->id, iph->frag_off, iph->ttl, iph->protocol, iph->check ); printipaddr(iph->saddr); printipaddr(iph->daddr); }int parse_iphdr(u8 *data,struct iphdr *iph,int len){ int pos = 0; iph->ihl = data[pos]&0xf; iph->version = data[pos]>>4; pos+=1; iph->tos = data[pos]; pos+=1; getdata16(&iph->tot_len,&data[pos]); pos+=2; getdata16(&iph->id,&data[pos]); pos+=2; getdata16(&iph->frag_off,&data[pos]); pos+=2; iph->ttl = data[pos]; pos+=1; iph->protocol = data[pos]; pos+=1; getdata16(&iph->check,&data[pos]); pos+=2; getdata32(&iph->saddr,&data[pos]); pos+=4; getdata32(&iph->daddr,&data[pos]); pos+=4; }
在这里插入图片描述
思考：为什么不可以直接用下面代码提取数据信息：
memcpy(&mac_header,&frm[0],sizeof(struct mac_h)); memcpy(&ip_header,&frm[14],sizeof(struct iphdr));
9. 将步骤8的结构体变量赋一些初始值，并按照mac头IP头格式将结构体内容填充到一段buf中
void send_pkt(struct mac_h *mach,struct iphdr *iph,char *pdata,int datalen){ int pos = 0; char buf[14+20+1500]={0}; /*add mac头*/ pos =0 ; memcpy(&buf[pos],mach->dst,6); pos+=6; memcpy(&buf[pos],mach->src,6); pos+=6; setdata16(&buf[pos],mach->pro); pos+=2; /*add ip头*/ buf[pos] = (char)(iph->ihl | iph->version<<4); pos+=1; buf[pos] = iph->tos; pos+=1; setdata16(&buf[pos],iph->tot_len); pos+=2; setdata16(&buf[pos],iph->id); pos+=2; setdata16(&buf[pos],iph->frag_off); pos+=2; buf[pos] = iph->ttl; pos+=1; buf[pos] = iph->protocol; pos+=1; setdata16(&buf[pos],iph->check); pos+=2; setdata32(&buf[pos],iph->saddr); pos+=4; setdata32(&buf[pos],iph->daddr); pos+=4; memcpy(&buf[pos],pdata,datalen); pos+=datalen; print_array("\nfrm",buf,pos);}main(){... datalen = ip_header.tot_len; memcpy(data,frm + sizeof(struct mac_h)+sizeof(struct iphdr),datalen-20); send_pkt(&mac_header,&ip_header,data,datalen-20);...}
10. 将3步骤的结构体变量保存到文件，并读取出来,
void save_head(struct mac_h *mach,struct iphdr *iph,char *pdata,int datalen){ int fd =-1; int len = 0; struct mac_h mac_head_test; struct iphdr ip_head_test; char buf[1500]; fd =open(FAILE_NAME,O_RDWR|O_CREAT); if(fd < 0) { perror("open fail\n"); return; } write(fd,mach,sizeof(struct mac_h)); write(fd,iph,sizeof(struct iphdr)); write(fd,pdata,datalen); lseek(fd,0,SEEK_SET); len = read(fd,&mac_head_test,sizeof(struct mac_h)); len = read(fd,&ip_head_test,sizeof(struct iphdr)); len = read(fd,buf,1500); buf[len] = '\0'; printf("------recover from file--------------\n"); dump_machdr(&mac_head_test); dump_iphdr(&ip_head_test); print_array("\ndata",buf,len);}save_head(&mac_header,&ip_header,data,datalen-20);
完整代码可从附件中获取。