網絡編程-TCP粘包

//TCP數據傳輸是以無邊界的數據流傳輸形式，所謂無邊界是指數據發送端發送的字節數，
//在數據接收端接受時並不一定等於發送的字節數，可能會出現粘包情況。

//TCP粘包情況：
    //（1）發送方引起的粘包是由TCP協議本身造成的，TCP爲提高傳輸效率，發送方往往要收集到足夠多的數據後才發送一包數據。
    //若連續幾次發送的數據都很少，通常TCP會根據優化算法把這些數據合成一包後一次發送出去，這樣接收方就收到了粘包數據。
//（2）接收方引起的粘包是由於接收方用戶進程不及時接收數據，從而導致粘包現象。
//這是因爲接收方先把收到的數據放在系統接收緩衝區，用戶進程從該緩衝區取數據，若下一包數據到達時前一包數據尚未被用戶進程取走，
//則下一包數據放到系統接收緩衝區時就接到前一包數據之後，而用戶進程根據預先設定的緩衝區大小從系統接收緩衝區取數據，這樣就一次取到了多包數據。

//解決辦法：
//(1)發送固定長度的消息
//(2)把消息的尺寸與消息一塊發送
//(3)使用特殊標記來區分消息間隔

//下面介紹一種方法：
/*1. 數據頭：數據包的大小，固定長度。
2. 數據內容：數據內容，長度爲數據頭定義的長度大小。
實際操作如下：
a）發送端：先發送數據包的大小，再發送數據內容。
b）接收端：先解析本次數據包的大小N，在讀取N個字節，這N個字節就是一個完整的數據內容。*/

/*demo講解：
1. 客戶端負責模擬發送數據，服務端負責接受數據，處理粘包問題

a）emulate_subpackage

模擬情況1，一個長數據經過多次纔到達目的地，

在客戶端字符串“This is a test case for client send subpackage data. data is not send complete at once.”
每次只發送6個字節長度。服務端要把字符串集滿才能處理數據（打印字符串）

b）emulate_adheringpackage

模擬情況2，多個數據在一次性到達目的地

在客戶端將字符串“Hello I’m lucky. Nice too me you”切成三個數據段（都包含數據頭和數據內容），
然後一次性發送，服務端讀取數據時對三個數據段逐個處理。*/

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;    
}  

案例

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 sock
    sockfd = 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 server
    if (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;   //六個字節多次發送
        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 text
    memset(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 text
    memset(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 text
    memset(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);
}


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 socket
    sockfd = 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 structure
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = INADDR_ANY;
    serv_addr.sin_port = htons(7890);

    // bind
    if (bind(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
        printf("bind failed. errno: %d, error: %s\n", errno, strerror(errno));
        exit(-1);
    }

    // listen
    listen(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 case
            return false;
        }
        printf("receive data: %s\n", buf + readsum);
        readsum += read_len;
    } while (readsum < len && 0 != read_len);
    return true;
}

結果：

客戶端發送數據
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.


服務端模擬接受數據
$ ./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.