TCP网络通讯如何解决分包粘包问题

TCP作为常用的网络传输协议，数据流解析是网络应用开发人员永远绕不开的一个问题。

TCP数据传输是以无边界的数据流传输形式，所谓无边界是指数据发送端发送的字节数，在数据接收端接受时并不一定等于发送的字节数，可能会出现粘包情况。

一、TCP粘包情况：

1. 发送端发送了数量比较的数据，接收端读取数据时候数据分批到达，造成一次发送多次读取；通常网络路由的缓存大小有关系，一个数据段大小超过缓存大小，那么就要拆包发送。

2. 发送端发送了几次数据，接收端一次性读取了所有数据，造成多次发送一次读取；通常是网络流量优化，把多个小的数据段集满达到一定的数据量，从而减少网络链路中的传输次数。

TCP粘包的解决方案有很多种方法，最简单的一种就是发送的数据协议定义发送的数据包的结构：

1. 数据头：数据包的大小，固定长度。

2. 数据内容：数据内容，长度为数据头定义的长度大小。

实际操作如下：

a）发送端：先发送数据包的大小，再发送数据内容。

b）接收端：先解析本次数据包的大小N，在读取N个字节，这N个字节就是一个完整的数据内容。

具体流程如下：

实现源码

/**  * read size of len from sock into buf.  */  bool readPack(int sock, char* buf, size_t len) {      if (NULL == buf || len < 1) {          return false;      }      memset(buf, 0, len); // only reset buffer len.      ssize_t read_len = 0, readsum = 0;      do {          read_len = read(sock, buf + readsum, len - readsum);          if (-1 == read_len) { // ignore error case              return false;          }          printf("receive data: %s\n", buf + readsum);          readsum += read_len;      } while (readsum < len && 0 != read_len);      return true;  } 

二、测试用例介绍

本篇提供的demo主要流程如下：

1. 客户端负责模拟发送数据，服务端负责接受数据，处理粘包问题

a）emulate_subpackage

模拟情况1，一个长数据经过多次才到达目的地，

在客户端字符串“This is a test case for client send subpackage data. data is not send complete at once.”每次只发送6个字节长度。服务端要把字符串集满才能处理数据（打印字符串）

b）emulate_adheringpackage

模拟情况2，多个数据在一次性到达目的地

在客户端将字符串“Hello I'm lucky. Nice too me you”切成三个数据段（都包含数据头和数据内容），然后一次性发送，服务端读取数据时对三个数据段逐个处理。

三、源码实现

server.cpp

#include <cstdio>#include <cstdlib>#include <cstring>#include <errno.h>#include <sys/socket.h>#include <sys/types.h>#include <arpa/inet.h>#include <unistd.h>void newclient(int sock);bool readPack(int sock, char* buf, size_t len);void safe_close(int &sock);int main(int argc, char *argv[]) {int sockfd = -1, newsockfd = -1;socklen_t c = 0;struct sockaddr_in serv_addr, cli_addr;// Create socketsockfd = socket(AF_INET, SOCK_STREAM, 0);if (-1 == sockfd) {printf("new socket failed. errno: %d, error: %s\n", errno, strerror(errno));exit(-1);}// Prepare the sockaddr_in structureserv_addr.sin_family = AF_INET;serv_addr.sin_addr.s_addr = INADDR_ANY;serv_addr.sin_port = htons(7890);// bindif (bind(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {printf("bind failed. errno: %d, error: %s\n", errno, strerror(errno));exit(-1);}// listenlisten(sockfd, 5);printf("listening...\n");// accept new connection.c = sizeof(struct sockaddr_in);int i = 0;while (i++ < 3) {printf("waiting for new socket accept.\n");    newsockfd = accept(sockfd, (struct sockaddr*)&cli_addr, (socklen_t*)&c);    if (newsockfd < 0) {    printf("accept connect failed. errno: %d, error: %s\n", errno, strerror(errno));    safe_close(sockfd);    exit(-1);    }pid_t pid = fork();if (0 == pid) {newclient(newsockfd);        safe_close(sockfd);break;} else if (pid > 0) {safe_close(newsockfd);}}safe_close(sockfd);return 0;}void newclient(int sock) {printf("newclient sock fd: %d\n", sock);int datasize = 0;const int HEAD_SIZE = 9;char buf[512] = {0};while (true) {memset(buf, 0, sizeof(buf));    if (! readPack(sock, buf, HEAD_SIZE)) {       printf("read head buffer failed.\n");       safe_close(sock);       return;    }    datasize = atoi(buf);    printf("data size: %s, value:%d\n", buf, datasize);    memset(buf, 0, sizeof(buf));    if (! readPack(sock, buf, datasize)) {       printf("read data buffer failed\n");       safe_close(sock);       return;    }    printf("data size: %d, text: %s\n", datasize, buf);if (0 == strcmp(buf, "exit")) {break;}}memset(buf, 0, sizeof(buf));snprintf(buf, sizeof(buf), "from server read complete.");write(sock, buf, strlen(buf) + 1);printf("newclient sockfd: %d, finish.\n", sock);safe_close(sock);}void safe_close(int &sock) {if (sock > 0) {close(sock);sock = -1;}}/** * read size of len from sock into buf. */bool readPack(int sock, char* buf, size_t len) {if (NULL == buf || len < 1) {return false;}memset(buf, 0, len); // only reset buffer len.ssize_t read_len = 0, readsum = 0;do {read_len = read(sock, buf + readsum, len - readsum);if (-1 == read_len) { // ignore error casereturn false;}printf("receive data: %s\n", buf + readsum);readsum += read_len;} while (readsum < len && 0 != read_len);return true;}

client.cpp

#include <cstdio>#include <cstdlib>#include <cstring>#include <time.h>#include <errno.h>#include <sys/socket.h>#include <arpa/inet.h>#include <unistd.h>void safe_close(int &sock);void emulate_subpackage(int sock);void emulate_adheringpackage(int sock);int main(int argc, char *argv[]) {char buf[128] = {0};int sockfd = -1;struct sockaddr_in serv_addr;// Create socksockfd = socket(AF_INET, SOCK_STREAM, 0);if (-1 == sockfd) {printf("new socket failed. errno: %d, error: %s\n", errno, strerror(errno));exit(-1);}serv_addr.sin_addr.s_addr = inet_addr("127.0.0.1");serv_addr.sin_family = AF_INET;serv_addr.sin_port = htons(7890);// Connect to remote serverif (connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {printf("connection failed. errno: %d, error: %s\n", errno, strerror(errno));exit(-1);}emulate_subpackage(sockfd);emulate_adheringpackage(sockfd);const int HEAD_SIZE = 9;const char temp[] = "exit";memset(buf, 0, sizeof(buf));snprintf(buf, sizeof(buf), "%0.*zu", HEAD_SIZE - 1, sizeof(temp));write(sockfd, buf, HEAD_SIZE);write(sockfd, temp, sizeof(temp));printf("send complete.\n");memset(buf, 0, sizeof(buf));read(sockfd, buf, sizeof(buf));printf("receive data: %s\n", buf);printf("client finish.\n");safe_close(sockfd);return 0;}void safe_close(int &sock) {if (sock > 0) {close(sock);sock = -1;}}/** * emulate socket data write multi part. */void emulate_subpackage(int sock) {printf("emulate_subpackage...\n");char text[] = "This is a test case for client send subpackage data. data is not send complete at once.";const size_t TEXTSIZE = sizeof(text);ssize_t len = 0;size_t sendsize = 0, sendsum = 0;const int HEAD_SIZE = 9;char buf[64] = {0};snprintf(buf, HEAD_SIZE, "%08zu", TEXTSIZE);write(sock, buf, HEAD_SIZE);printf("send data size: %s\n", buf);do {sendsize = 6;if (sendsum + sendsize > TEXTSIZE) {sendsize = TEXTSIZE - sendsum;}    len = write(sock, text + sendsum, sendsize);if (-1 == len) {printf("send data failed. errno: %d, error: %s\n", errno, strerror(errno));return;}memset(buf, 0, sizeof(buf));snprintf(buf, len + 1, text + sendsum);printf("send data: %s\n", buf);sendsum += len;sleep(1);} while (sendsum < TEXTSIZE && 0 != len);}/** * emualte socket data write adhering. */void emulate_adheringpackage(int sock) {printf("emulate_adheringpackage...\n");const int HEAD_SIZE = 9;char buf[1024] = {0};char text[128] = {0};char *pstart = buf;// append textmemset(text, 0, sizeof(text));snprintf(text, sizeof(text), "Hello ");snprintf(pstart, HEAD_SIZE, "%08zu", strlen(text) + 1);pstart += HEAD_SIZE;snprintf(pstart, strlen(text) + 1, "%s", text);pstart += strlen(text) + 1;// append textmemset(text, 0, sizeof(text));snprintf(text, sizeof(text), "I'm lucky.");snprintf(pstart, HEAD_SIZE, "%08zu", strlen(text) + 1);pstart += HEAD_SIZE;snprintf(pstart, strlen(text) + 1, "%s", text);pstart += strlen(text) + 1;// append textmemset(text, 0, sizeof(text));snprintf(text, sizeof(text), "Nice too me you");snprintf(pstart, HEAD_SIZE, "%08zu", strlen(text) + 1);pstart += HEAD_SIZE;snprintf(pstart, strlen(text) + 1, "%s", text);pstart += strlen(text) + 1;write(sock, buf, pstart - buf);}

Makefile

CC=g++CFLAGS=-Iall: server.o client.oserver.o: server.cpp$(CC) -o server.o server.cppclient.o: client.cpp$(CC) -o client.o client.cppclean:rm *.o

四、测试结果

编译及运行

$ make

g++ -o server.o server.cpp

g++ -o client.o client.cpp

客户端模拟发送数据

$ ./client.o

emulate_subpackage...

send data size: 00000088

send data: This i

send data: s a te

send data: st cas

send data: e for

send data: client

send data: send

send data: subpac

send data: kage d

send data: ata. d

send data: ata is

send data: not s

send data: end co

send data: mplete

send data: at on

send data: ce.

emulate_adheringpackage...

send complete.

receive data: from server read complete.

client finish.

服务端模拟接受数据

$ ./server.o

listening...

waiting for new socket accept.

waiting for new socket accept.

newclient sock fd: 4

receive data: 00000088

data size: 00000088, value:88

receive data: This i

receive data: s a te

receive data: st cas

receive data: e for

receive data: client

receive data: send

receive data: subpac

receive data: kage d

receive data: ata. d

receive data: ata is

receive data: not s

receive data: end co

receive data: mplete

receive data: at on

receive data: ce.

data size: 88, text: This is a test case for client send subpackage data. data is not send complete at once.

receive data: 00000007

data size: 00000007, value:7

receive data: Hello

data size: 7, text: Hello

receive data: 00000011

data size: 00000011, value:11

receive data: I'm lucky.

data size: 11, text: I'm lucky.

receive data: 00000016

data size: 00000016, value:16

receive data: Nice too me you

data size: 16, text: Nice too me you

receive data: 00000005

data size: 00000005, value:5

receive data: exit

data size: 5, text: exit

newclient sockfd: 4, finish.