c - 未接收来自 TCP 连接的文件传输

Question

我正在为学校项目编写 TCP 客户端和服务器协议(protocol)。客户端发送“GET\r\n”消息，服务器必须传输“+OK\r\n”、文件大小以及文件(如果文件存在于服务器目录中)。我在文件传输中被阻止

我尝试一次一小步地解决它。我建立了连接，发送了来自客户端的请求并收到了来自服务器的“OK”消息。

现在我在服务器中打开文件并尝试一次将 128 个字节发送到客户端。文件的读取工作正常，显然缓冲区的发送也工作正常，但客户端没有收到任何内容......

这是我的服务器.c

#include    <stdlib.h>
#include    <string.h>
#include    <inttypes.h>

#include    "../errlib.h"
#include    "../sockwrap.h"

#define BUFLEN      128 /* Buffer length */
#define TIMEOUT     15  /* TIMEOUT */

/* FUNCTION PROTOTYPES */
void service(int s);

/* GLOBAL VARIABLES */
char *prog_name;

int main(int argc, char *argv[])
{
    int         conn_request_skt;   /* passive socket */
    uint16_t    lport_n, lport_h;   /* port used by server (net/host ord.) */
    int         bklog = 2;          /* listen backlog */
    int         s;                  /* connected socket */

    fd_set      cset;               // waiting for connection
    struct timeval  tval;           // timeout
    size_t      n;

    socklen_t   addrlen;
    struct sockaddr_in  saddr, caddr;   /* server and client addresses */

    prog_name = argv[0];

    if (argc != 2) {
        printf("Usage: %s <port number>\n", prog_name);
        exit(1);
    }

    /* get server port number */
    if (sscanf(argv[1], "%" SCNu16, &lport_h)!=1)
    err_sys("Invalid port number");
    lport_n = htons(lport_h);

    /* create the socket */
    printf("creating socket...\n");
    s = Socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    printf("done, socket number %u\n",s);

    /* bind the socket to any local IP address */
    bzero(&saddr, sizeof(saddr));
    saddr.sin_family      = AF_INET;
    saddr.sin_port        = lport_n;
    saddr.sin_addr.s_addr = INADDR_ANY;
    showAddr("Binding to address", &saddr);
    Bind(s, (struct sockaddr *) &saddr, sizeof(saddr));
    printf("done.\n");

    /* listen */
    printf ("Listening at socket %d with backlog = %d \n",s,bklog);
    Listen(s, bklog);
    printf("done.\n");

    conn_request_skt = s;

    /* main server loop */
    for ( ; ; )
    {
        printf("waiting for connection...\n");
        /* accept next connection */
        FD_ZERO(&cset);
        FD_SET(conn_request_skt, &cset);
        tval.tv_sec = TIMEOUT;
        tval.tv_usec = 0;
        n = Select(FD_SETSIZE, &cset, NULL, NULL, &tval);

        if ( n > 0 ){
            addrlen = sizeof(struct sockaddr_in);
            s = Accept(conn_request_skt, (struct sockaddr *) &caddr, &addrlen);
            showAddr("Accepted connection from", &caddr);
            printf("new socket: %u\n",s);

            /* serve the client on socket s */
            service(s);
        } else {
            printf("No connection request after %d seconds\n",TIMEOUT);

        }
    }
}

void service(int s) {
    char    buf[BUFLEN];        /* reception buffer */
    char    filename[BUFLEN];

    int     n;
    long    filesize;
    uint32_t fsize;

    FILE *fp;

    for ( ; ; )
    {
        n = recv(s, buf, BUFLEN, 0);
        if (n < 0) {
            printf("Read error\n");
            close(s);
            printf("Socket %d closed\n", s);
            break;
        } else if (n == 0) {
            printf("Connection closed by party on socket %d\n",s);
            close(s);
            break;
        } else {
            printf("Received request from socket %03d :\n", s);
            sscanf(buf, "GET %s\r\n", filename);
            strcpy(buf, "+OK\r\n");
            printf("%s",buf);
            if(writen(s, buf, strlen(buf)) != strlen(buf))
                printf("Write error while sending +OK\n");

            // open file
            fp = fopen(filename, "r");
            if( fp == NULL){
                //TODO close connection
            }
            // calculating dim of file
            fseek(fp, 0L, SEEK_END);
            filesize = ftell(fp);
            rewind(fp); // go back at beginning of file

            fsize = htonl(filesize); // size file in network byte order

            // sending file size
            if(writen(s, &fsize, 4) != 4)
                printf("Write error while sending file size\n");

            while(fread(buf, 1, BUFLEN - 1, fp) == BUFLEN - 1){
                printf("%s", buf);
                if(writen(s, buf, strlen(buf)) != strlen(buf))
                    printf("Write error while buf\n");
            }
            printf("%s", buf);
            printf("I am here\n");
        }
    }
}

这是我的 client.c

#include     <stdlib.h>
#include     <string.h>
#include     <inttypes.h>

#include     "../errlib.h"
#include     "../sockwrap.h"

#define BUFLEN  128 /* BUFFER LENGTH */
#define TIMEOUT 15  /* TIMEOUT*/

/* GLOBAL VARIABLES */
char *prog_name;

int main(int argc, char *argv[])
{
    char        request[BUFLEN];        /* request buffer */
    char        rbuf[BUFLEN];       /* reception buffer */

    uint32_t        taddr_n;            /* server IP addr. (net/host ord) */
    uint16_t        tport_n, tport_h;   /* server port number (net/host ord) */

    int        s, len;
    int        result;
    struct sockaddr_in  saddr;      /* server address structure */
    struct in_addr  sIPaddr;        /* server IP addr. structure */

    fd_set      cset;               // variables for timeout
    struct timeval  tval;
    size_t n;

    prog_name = argv[0];

    if(argc < 4)
        err_sys("Wrong number of parameters!\n");

    // read address from first argument
    taddr_n = inet_addr(argv[1]);
    if (taddr_n == INADDR_NONE)
        err_sys("Invalid address");

    // read port number from second argument
    if (sscanf(argv[2], "%" SCNu16, &tport_h)!=1)
    err_sys("Invalid port number");
    tport_n = htons(tport_h);

    /* create the socket */
    printf("Creating socket\n");
    s = Socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    printf("done. Socket fd number: %d\n",s);

    /* prepare address structure */
    bzero(&saddr, sizeof(saddr));
    saddr.sin_family = AF_INET;
    saddr.sin_port   = tport_n;
    saddr.sin_addr   = sIPaddr;


    printf("trying to connect to the server...\n");

    /* connect */
    showAddr("Connecting to target address", &saddr);
    Connect(s, (struct sockaddr *) &saddr, sizeof(saddr));
    printf("done.\n");

    // loop to request files
    for (int i = 3 ; i < argc ; i++ ){ // i = 4 because the first file is the fourth argument
        // check if file name is too big
        if(strlen(argv[i]) >= BUFLEN - 6)
            err_sys("The file name is too big for the buffer request!\n");

        // create the string of bytes for the request
        strcpy(request, "GET ");
        strcat(request, argv[i]);
        strcat(request, "\r\n");

        len = strlen(request);
        if(writen(s, request, len) != len){
            printf("Write error\n");
            break;
        }

        printf("waiting for response...\n");
        // receive file from server
        n = recv(s, rbuf, BUFLEN, 0);
        if (n < 0) {
            printf("Read error\n");
            close(s);
            printf("Socket %d closed\n", s);
            break;
        } else if (n == 0) {
            printf("Connection closed by party on socket %d\n",s);
            close(s);
            break;
        } else {
            printf("Received reply from server\n");

            uint32_t fsize;

            printf("%s",rbuf);
            if(strcmp(rbuf, "+OK\r\n") == 0){
                n = recv(s, &fsize, 4, 0);
                if (n < 0) {
                    printf("Read error\n");
                    close(s);
                    printf("Socket %d closed\n", s);
                    break;
                } else if (n == 0) {
                    printf("Connection closed by party on socket %d\n",s);
                    close(s);
                    break;
                } else {
                    // received file dimension
                    fsize = ntohl(fsize);
                }

                while(fsize > 0){
                    printf("I am here1n\n");
                    // receive file
                    n = recv(s, rbuf, BUFLEN-1, 0);
                    if (n < 0) {
                        printf("Read error\n");
                        close(s);
                        printf("Socket %d closed\n", s);
                        break;
                    } else if (n == 0) {
                        printf("Connection closed by party on socket %d\n",s);
                        close(s);
                        break;
                    } else {
                        printf("I am here");
                        fsize -= n;
                    }
                }

            }
        }
    }

    printf("===========================================================\n");
    close(s);
    exit(0);
}

我应该接收文件的客户端中的recv只是阻塞而没有接收到任何东西。我不明白我错过了什么......

Answer 1

这里的问题是一个常见问题:您没有注意消息边界。

在您的客户端中，您执行一次recv并检查字节数是否大于0。但是您不会进行更多的长度检查。接下来，您对期望接收的特定字符串执行 strcmp (+OK\r\n)。但是您可能收到了 3 个字节 (+OK)，或者您可能收到了 10: (+OK\r\nXXXXX) 或更多 [旁白:还有， recv 不保证您的字节字符串以 null 结尾]。没有什么可以阻止远端的内核将前导码加上后续字节批处理到单个 TCP 数据包中。同样，没有什么可以阻止本地端将多个 TCP 数据包聚合到单个缓冲区中。

您必须提供消息边界。如果您预计下一条消息为 5 个字节，那么您应该正好收到 5 个字节(如果收到的字节数更少，请重试 - 也要小心检查 EOF，以防另一方提前中止)。或者，也可以在接收逻辑前面放置一个缓冲层，以便它将接收大量数据，返回给您所需的字节数，然后保存多余的内容以供后续的“接收”调用。

以不同的方式重申这一点:您的服务器发送 +OK\r\n，然后发送一个四字节长度，然后开始发送文件。但这意味着客户端的第一个 recv 可能会在一次系统调用中接收前导码、长度以及文件的前 N ​​个字节。 p>

TCP 不尊重、提供或强制执行消息边界。