c - 为什么我的 C 套接字文件传输(服务器/客户端)程序只能正确运行一次？

Question

这是我第一次在 Stack Overflow 上发帖。如果我没有正确遵守协议(protocol)，我深表歉意。

我有一个带有客户端和服务器组件的简单 C 套接字程序。该程序将文件从一台 VM 上的客户端发送到另一台 VM 上的服务器。程序运行成功，文件第一次发送成功。

但是，当我再次尝试运行该程序时，文件似乎没有被传输。通过调查，我发现在重新启动虚拟机后，该程序再次运行。为什么会这样？

这是服务器代码:

新文件服务器.c

/* Server code */
/* TODO : Modify to meet your need */
#include <stdio.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/sendfile.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <netinet/in.h>

#define PORT_NUMBER     5000
#define SERVER_ADDRESS  "10.20.20.55"
#define FILENAME    "/home/saul/M2.py"

int main(int argc, char **argv)
{
        int server_socket;
        int peer_socket;
        socklen_t sock_len;
        ssize_t len;
        struct sockaddr_in server_addr;
        struct sockaddr_in peer_addr;
        ssize_t rec;
        int sent_bytes = 0;
        char file_size[512];
        int file_size_A;
        struct stat file_stat;
        off_t offset;
        int remain_data;
        char buffer[BUFSIZ];
        FILE *received_file;
        int closed = 0;
        int fclosed = 0;

        /* Create server socket */
        server_socket = socket(AF_INET, SOCK_STREAM, 0);
        if (server_socket == -1)
        {
                fprintf(stderr, "Error creating socket --> %s", strerror(errno));

                exit(EXIT_FAILURE);
        }

        printf("Socket Created Successfully: %d\n", server_socket);
        /* Zeroing server_addr struct */
        memset(&server_addr, 0, sizeof(server_addr));
        /* Construct server_addr struct */
        server_addr.sin_family = AF_INET;
        //inet_pton(AF_INET, SERVER_ADDRESS, &(server_addr.sin_addr));
        inet_pton(AF_INET, SERVER_ADDRESS, &(server_addr.sin_addr));
        server_addr.sin_port = htons(PORT_NUMBER);

        /* Bind */
        if ((bind(server_socket, (struct sockaddr *)&server_addr, sizeof(struct sockaddr))) == -1)
        {
                fprintf(stderr, "Error on bind --> %s", strerror(errno));

                exit(EXIT_FAILURE);
        }

        printf("Binding to socket done\n");
        /* Listening to incoming connections */
        if ((listen(server_socket, 5)) == -1)
        {
                fprintf(stderr, "Error on listen --> %s", strerror(errno));

                exit(EXIT_FAILURE);
        }

        printf("No error on listen\n");

        sock_len = sizeof(struct sockaddr_in);
        peer_socket = accept(server_socket, (struct sockaddr *)&peer_addr, &sock_len);

        printf("\nAccept has been performed\n");

        if (peer_socket < 0)
        {
                fprintf(stderr, "Error on accept --> %s", strerror(errno));

                exit(EXIT_FAILURE);
        }
        fprintf(stdout, "Accept peer --> %s\n", inet_ntoa(peer_addr.sin_addr));


        /* To receive file */
        /*____________________________________________________*/
        /* Receiving file size */

        len = recv(peer_socket, buffer, BUFSIZ, 0);
        if(len <= 0)
        {
            printf("\nEither an error has occurred or there is no data in recv: %d\n", len);
        }
        printf("\nThe message has been received and the length of it is %d\n", len);
        len = 5;
        file_size_A = atoi(buffer);
        printf("\nThe size of file_size_A is %d\n", file_size_A);
        //fprintf(stdout, "\nFile size : %d\n", file_size);

        received_file = fopen(FILENAME, "w");
        if (received_file == NULL)
        {
                fprintf(stderr, "Failed to open file foo --> %s\n", strerror(errno));

                exit(EXIT_FAILURE);
        }

        remain_data = file_size_A;

        while (((len = recv(peer_socket, buffer, BUFSIZ, 0)) > 0) && (remain_data > 0))
        {
                fwrite(buffer, sizeof(char), len, received_file);
                remain_data -= len;
                fprintf(stdout, "Receive %d bytes and we hope :- %d bytes\n", len, remain_data);
        }

        printf("Len after skipping the while loop is %d\n", len);

        fclosed = fclose(received_file);

        if(fclosed != 0)
        {
            fprintf(stderr, "Failed to close received_file --> %s\n", strerror(errno));

                exit(EXIT_FAILURE);
        }
        else
        {
            printf("\nreceived_file closed\n");
        }

        /*_____________________________________________________*/


        closed = close(peer_socket);
        if(closed == -1)
        {
            fprintf(stderr, "Failed to close peer_socket --> %s\n", strerror(errno));

                exit(EXIT_FAILURE);
        }
        else
        {
            printf("\nPeer Socket closed\n");
        }

        closed = close(server_socket);
        if(closed == -1)
        {
            fprintf(stderr, "Failed to close server_socket --> %s\n", strerror(errno));

                exit(EXIT_FAILURE);
        }
        else
        {
            printf("\nServer Socket closed\n");
        }

        printf("\nProcess ID is: %d\n", getpid());

        return 0;
}

这是客户端代码:

新文件客户端.c

/* Client code */
/* TODO : Modify to meet your need */
#include <stdio.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <netinet/in.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/sendfile.h>

#define PORT_NUMBER     5000
#define SERVER_ADDRESS  "10.20.20.55"
#define FILENAME        "/home/saul2/test.py"

int main(int argc, char **argv)
{
        int client_socket;
        ssize_t len;
        struct sockaddr_in remote_addr;
        char buffer[BUFSIZ];
        char ip[BUFSIZ];
        //int file_size;
        char file_size[512];
        FILE *received_file;
        int remain_data = 0;
        int pNum = 0;
        int fd;
        struct stat file_stat;
        socklen_t       sock_len;
        struct sockaddr_in      peer_addr;
        off_t offset;
        int sent_bytes = 0;
        int closed = 0;
        int closeFd = 0;

        /* Zeroing remote_addr struct */
        memset(&remote_addr, 0, sizeof(remote_addr));

        /* Construct remote_addr struct */
        remote_addr.sin_family = AF_INET;
        inet_pton(AF_INET, SERVER_ADDRESS, &(remote_addr.sin_addr));
        remote_addr.sin_port = htons(PORT_NUMBER);

        /* Create client socket */
        client_socket = socket(AF_INET, SOCK_STREAM, 0);
        if (client_socket == -1)
        {
                fprintf(stderr, "Error creating socket --> %s\n", strerror(errno));

                exit(EXIT_FAILURE);
        }

        fprintf(stderr, "Socket created successfully %d\n", client_socket);
        inet_ntop(AF_INET, &(remote_addr.sin_addr), ip, BUFSIZ);
        fprintf(stderr, "\nThe ip address is converted back is %s \n", ip);
        pNum = ntohs(remote_addr.sin_port);
        fprintf(stderr, "\nThe re-converted port number is, %d\n", pNum);


        /* Connect to the server */
        if (connect(client_socket, (struct sockaddr *)&remote_addr, sizeof(struct sockaddr)) == -1)
        {
                fprintf(stderr, "Error on connect --> %s\n", strerror(errno));

                exit(EXIT_FAILURE);
        }




        /* To send file */
        /*______________________________________________*/
        fd = open(FILENAME, O_RDONLY);
        if (fd == -1)
        {
                fprintf(stderr, "Error opening file --> %s", strerror(errno));

                exit(EXIT_FAILURE);
        }

        /* Get file stats */
        if (fstat(fd, &file_stat) < 0)
        {
                fprintf(stderr, "Error fstat --> %s", strerror(errno));

                exit(EXIT_FAILURE);
        }

        fprintf(stdout, "File Size: \n%d bytes\n", file_stat.st_size);

        sock_len = sizeof(struct sockaddr_in);

        sprintf(file_size, "%d", file_stat.st_size);

        /* Sending file size */
        len = send(client_socket, file_size, sizeof(file_size), 0);
        if (len < 0)
        {
              fprintf(stderr, "Error on sending greetings --> %s", strerror(errno));

              exit(EXIT_FAILURE);
        }

        fprintf(stdout, "Client sent %d bytes for the size\n", len);

        offset = 0;
        remain_data = file_stat.st_size;
        /* Sending file data */
        while (((sent_bytes = sendfile(client_socket, fd, &offset, BUFSIZ)) > 0) && (remain_data > 0))
        {
                fprintf(stdout, "1. Client sent %d bytes from file's data, offset is now : %d and remaining data = %d\n", sent_bytes, offset, remain_data);
                remain_data -= sent_bytes;
                fprintf(stdout, "2. Client sent %d bytes from file's data, offset is now : %d and remaining data = %d\n", sent_bytes, offset, remain_data);
        }

        closeFd = close(fd);
        if(closeFd == -1)
        {
            fprintf(stderr, "Failed to close file --> %s\n", strerror(errno));

                exit(EXIT_FAILURE);
        }
        else
        {
            printf("\nFile closed\n");
        }

        closed = close(client_socket);
        if(closed == -1)
        {
            fprintf(stderr, "Failed to close client_socket --> %s\n", strerror(errno));

                exit(EXIT_FAILURE);
        }
        else
        {
            printf("\nClient Socket closed\n");
        }

        /*____________________________________________________*/


        //close(client_socket);
        printf("\nProcess ID is: %d\n", getpid());


        return 0;
}

最初，我认为这可能是因为某个端口处于打开状态。据我所知，我的代码关闭了它使用的所有文件和端口。我使用了 netstat 命令:

netstat -atn

查看打开的端口。发现客户端确实有一个开放的端口号5000。它的状态为“TIME_WAIT”。大约 2 分钟后，端口似乎自行关闭。我查了一下，这似乎是正在关闭的端口的正常行为。

有趣的是，即使在这个端口关闭后，程序仍然无法运行，只有在重启后才能运行。我发现的另一个奇怪的事情是，几天前，如果我在第一次成功执行程序后等待7-8分钟，然后再次尝试，它会正常执行并传输文件。但是，现在这不再有效，唯一可以解决问题的方法是重新启动系统。

我知道我的代码远非完美，但我的代码是否存在导致此问题的根本问题？

非常感谢您的帮助。

我正在编辑帖子以添加程序的输出。

首先，执行成功:

服务器输出:

Socket Created Successfully: 3
Binding to socket done
No error on listen

Accept has been performed
Accept peer --> 10.20.20.44

The message has been received and the length of it is 512

The size of file_size_A is 193
Receive 193 bytes and we hope :- 0 bytes
Len after skipping the while loop is 0

received_file closed

Peer Socket shutdown

Server Socket shutdown

Process ID is: 1397

客户端输出:

Socket created successfully 3

The ip address is converted back is 10.20.20.55

The re-converted port number is, 5000
File Size:
193 bytes
Client sent 512 bytes for the size
1. Client sent 193 bytes from file's data, offset is now : 193 and remaining data = 193
2. Client sent 193 bytes from file's data, offset is now : 193 and remaining data = 0

File closed

Client Socket shutdown

Process ID is: 1298

现在我将添加执行不成功时的输出。

服务器输出:

Socket Created Successfully: 3
Binding to socket done
No error on listen

Accept has been performed
Accept peer --> 10.20.20.44

The message has been received and the length of it is 705

The size of file_size_A is 193
Len after skipping the while loop is 0

received_file closed

Peer Socket shutdown

Server Socket shutdown

Process ID is: 1398

客户端输出:

Socket created successfully 3

The ip address is converted back is 10.20.20.55

The re-converted port number is, 5000
File Size:
193 bytes
Client sent 512 bytes for the size
1. Client sent 193 bytes from file's data, offset is now : 193 and remaining data = 193
2. Client sent 193 bytes from file's data, offset is now : 193 and remaining data = 0

File closed

Client Socket shutdown

Process ID is: 1299

我还没有解决这个问题。

谢谢

Answer 1

问题是:您希望对数据进行特定分块。这在 STREAM (TCP) 套接字上无效并且在您的情况下失败，当整个数据在第一个 recv()-call 中返回时:

在这里你阅读了整个数据包(包头+数据):

len = recv(peer_socket, buffer, BUFSIZ, 0);

这是你的 while 循环:

while (((len = recv(peer_socket, buffer, BUFSIZ, 0)) > 0) && (remain_data > 0))

在第一个(成功的)情况下，数据不是由第一个 recv() 读取，而是在 while() 条件内的调用中读取。

在第二种(不成功的)情况下，header+data 在第一个 recv() 调用中全部读取，因此对 recv() 的调用在while()-condition 返回 0 并且 while()-loop 根本不执行。

我不太清楚协议(protocol)是如何定义的。如果您的 header 始终为 512 字节(从您的输出看来就是这种情况)，则在第一次调用 recv 时仅读取 512 字节可能会有所帮助:

len = recv(peer_socket, buffer, 512, 0);

但您仍然必须确保，确实 512 个字节被读取(否则循环直到您获得其余字节)，否则它将不同步。

底线是:

永远不要期望从流式套接字接收数据时以某种方式分块。始终指定要读取的字节数，并检查是否实际 读取了该字节数(如果没有，则再次调用 read())。