Linux 網絡編程詳解四（流協議與粘包）

TCP/IP協議是一種流協議，流協議是字節流，只有開始和結束，包與包之間沒有邊界，所以容易產生粘包，但是不會丟包。
UDP/IP協議是數據報，有邊界，不存在粘包，但是可能丟包。

產生粘包問題的原因
1.SQ_SNDBUF套接字本身有緩沖區（發送緩沖區，接收緩沖區）
2.tcp傳送的網絡數據最大值MSS大小限制
3.鏈路層也有MTU（最大傳輸單元）大小限制，如果數據包大于>MTU要在IP層進行分片，導致消息分割。（可以簡單的認為MTU是MSS加包頭數據）
4.tcp的流量控制和擁塞控制，也可能導致粘包
5.tacp延遲發送機制等等
結論：TCP/IP協議，在傳輸層沒有處理粘包問題，必須由程序員處理

粘包的解決方案--本質上是要在應用層維護消息與消息的邊界
1.定包長
2.包尾加\r\n（比如ftp協議）
3.包頭加上包體長度
4.更復雜的應用層協議

粘包的幾種狀態

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//粘包解決方案--包頭加上包體長度
//服務器
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>typedef struct _packet
{int len; //定義包體長度char buf[1024]; //定義包體
} Packet;/** fd：文件描述符* buf：數據緩存區* count：讀取字符數* */
ssize_t readn(int fd, const void *buf, ssize_t count)
{//定義臨時指針變量char *pbuf = (char *)buf;//定義每次已讀數據ssize_t nread = 0;//定義剩余數據ssize_t lread = count;while (lread > 0){nread = read(fd, pbuf, lread);/** 情況分析：假設b緩沖區buf足夠大* 如果nread==count,說明數據正好被讀完* nread<count，說明數據沒有被讀完，這種情況就是由于粘包產生的* socket只接收了數據的一部分，TCP/IP協議不可能出現丟包情況* nread==0，說明對方關閉文件描述符* nread==-1，說明read函數報錯* nread>count，這種情況不可能存在* */if (nread == -1){//read()屬于可中斷睡眠函數，需要做信號處理if (errno == EINTR)continue;perror("read() err");return -1;} else if (nread == 0){printf("client is closed !\n");//返回已經讀取的字節數return count - lread;}//重新獲取 剩余的 需要讀取的 字節數lread = lread - nread;//指針后移pbuf = pbuf + nread;}return count;
}/* fd：文件描述符* buf：數據緩存區* count：讀取字符數* */
ssize_t writen(int fd, const void *buf, ssize_t count)
{//定義臨時指針變量char *pbuf = (char *)buf;//每次寫入字節數ssize_t nwrite = 0;//剩余未寫字節數ssize_t lwrite = count;while (lwrite > 0){nwrite = write(fd, pbuf, lwrite);if (nwrite == -1){if (errno == EINTR)continue;perror("write() err");return -1;} else if (nwrite == 0){printf("client is closed !\n");//對方關閉文件描述符，返回已經寫完的字節數return count - lwrite;}lwrite -= nwrite;pbuf += nwrite;}return count;
}int main(int arg, char *args[])
{//create socketint listenfd = socket(AF_INET, SOCK_STREAM, 0);if (listenfd == -1){perror("socket() err");return -1;}//reuseaddrint optval = 1;if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(optval))== -1){perror("setsockopt() err");return -1;}//bindstruct sockaddr_in addr;addr.sin_family = AF_INET;addr.sin_port = htons(8080);addr.sin_addr.s_addr = inet_addr("127.0.0.1");if (bind(listenfd, (struct sockaddr *) &addr, sizeof(addr)) == -1){perror("bind() err");return -1;}//listenif(listen(listenfd,SOMAXCONN)==-1){perror("listen() err");return -1;}//acceptstruct sockaddr_in peeraddr;socklen_t peerlen = sizeof(peeraddr);int conn = accept(listenfd, (struct sockaddr *)&peeraddr,&peerlen);if (conn == -1){perror("accept() err");return -1;}Packet _packet;while (1){memset(&_packet, 0, sizeof(_packet));//獲取報文自定義包頭int rc = readn(conn, &_packet.len, 4);if (rc == -1){exit(0);} else if (rc < 4){exit(0);}//把網絡字節序轉化成本地字節序int n = ntohl(_packet.len);//獲取報文自定義包體rc = readn(conn, _packet.buf, n);if (rc == -1){exit(0);} else if (rc < n){exit(0);}//打印報文數據
        fputs(_packet.buf, stdout);//將原來的報文數據發送回去printf("發送報文的長度%d\n", 4 + n);rc = writen(conn, &_packet, 4 + n);if (rc == -1){exit(0);} else if (rc < 4 + n){exit(0);}}return 0;
}

//粘包解決方案--包頭加上包體長度
//客戶端
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>typedef struct _packet
{int len; //定義包體長度char buf[1024]; //定義包體
} Packet;/** fd：文件描述符* buf：數據緩存區* count：讀取字符數* */
ssize_t readn(int fd, const void *buf, ssize_t count)
{//定義臨時指針變量char *pbuf = (char *)buf;//定義每次已讀數據ssize_t nread = 0;//定義剩余數據ssize_t lread = count;while (lread > 0){nread = read(fd, pbuf, lread);/** 情況分析：假設b緩沖區buf足夠大* 如果nread==count,說明數據正好被讀完* nread<count，說明數據沒有被讀完，這種情況就是由于粘包產生的* socket只接收了數據的一部分，TCP/IP協議不可能出現丟包情況* nread==0，說明對方關閉文件描述符* nread==-1，說明read函數報錯* nread>count，這種情況不可能存在* */if (nread == -1){//read()屬于可中斷睡眠函數，需要做信號處理if (errno == EINTR)continue;perror("read() err");return -1;} else if (nread == 0){printf("client is closed !\n");//返回已經讀取的字節數return count - lread;}//重新獲取 剩余的 需要讀取的 字節數lread = lread - nread;//指針后移pbuf = pbuf + nread;}return count;
}/* fd：文件描述符* buf：數據緩存區* count：讀取字符數* */
ssize_t writen(int fd, const void *buf, ssize_t count)
{//定義臨時指針變量char *pbuf = (char *)buf;//每次寫入字節數ssize_t nwrite = 0;//剩余未寫字節數ssize_t lwrite = count;while (lwrite > 0){nwrite = write(fd, pbuf, lwrite);if (nwrite == -1){if (errno == EINTR)continue;perror("write() err");return -1;} else if (nwrite == 0){printf("client is closed !\n");//對方關閉文件描述符，返回已經寫完的字節數return count - lwrite;}lwrite -= nwrite;pbuf += nwrite;}return count;
}int main(int arg, char *args[])
{//create socketint sockfd = socket(AF_INET, SOCK_STREAM, 0);if (sockfd == -1){perror("socket() err");return -1;}//connectstruct sockaddr_in addr;addr.sin_family = AF_INET;addr.sin_port = htons(8080);addr.sin_addr.s_addr = inet_addr("127.0.0.1");if (connect(sockfd, (struct sockaddr *) &addr, sizeof(addr)) == -1){perror("connect() err");return -1;}int rc = 0, numx = 0;Packet _packet;memset(&_packet, 0, sizeof(_packet));while (fgets(_packet.buf, sizeof(_packet.buf), stdin) != NULL){//發送數據numx = strlen(_packet.buf);//將本地字節轉化成網絡字節序_packet.len = htonl(numx);rc = writen(sockfd, &_packet, 4 + numx);if (rc == -1){return -1;} else if (rc < 4 + numx){return -1;}//接收數據memset(&_packet, 0, sizeof(_packet));//獲取包頭rc = readn(sockfd, &_packet.len, 4);if (rc == -1){return -1;} else if (rc < 4){return -1;}//將網絡字節轉化成本地字節numx = ntohl(_packet.len);//printf("接收數據的大小是%d\n",numx);//獲取包體rc = readn(sockfd, &_packet.buf, numx);if (rc == -1){return -1;} else if (rc < numx){return -1;}//打印包體
        fputs(_packet.buf,stdout);memset(&_packet, 0, sizeof(_packet));}return 0;
}

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