c - 在 Linux 上使用 TCP 进行文件传输

Question

我正在 Linux 上尝试 TCP 文件传输。建立连接后，服务器要发送“send.txt”给客户端，客户端收到文件后保存为“receive.txt”。然后连接中断。

正确的输入输出应该是:

服务器端:

$./server &
[server] obtain socket descriptor successfully.
[server] bind tcp port 5000 in addr 0.0.0.0 successfully.
[server] listening the port 5000 successfully.
[server] server has got connect from 127.0.0.1.
[server] send send.txt to the client…ok!
[server] connection closed.

客户端:

$./client 
[client] connected to server at port 5000…ok!
[client] receive file sent by server to receive.txt…ok!
[client] connection lost.

并且服务端和客户端都应该在这个过程结束后退出。

但我现在得到的是

$ ./server &
[server] obtain socket descriptor successfully.
[server] bind tcp port 5000 in addr 0.0.0.0 sucessfully.
[server] listening the port 5000 sucessfully.
[server] server has got connect from 127.0.0.1.
[server] send send.txt to the client...ok!
[server] connection closed.

/*Here the server doesn't exit*/

$ ./client
[client] connected to server at port 5000...ok!

/*Here the client doesn't exit*/

此外，还会生成一个空的“receive.txt”。

我的代码最初是为传输简单字符串而编写的，并且运行正常。所以我猜问题出在文件传输部分。

我的代码如下:

服务器.c

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/wait.h>
#include <sys/socket.h>
#define PORT 5000 // The port which is communicate with server
#define BACKLOG 10
#define LENGTH 512 // Buffer length
int main ()
{
    int sockfd; // Socket file descriptor
    int nsockfd; // New Socket file descriptor
    int num;
    int sin_size; // to store struct size
    struct sockaddr_in addr_local;
    struct sockaddr_in addr_remote;
    /* Get the Socket file descriptor */
    if( (sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1 )
    {
        printf ("ERROR: Failed to obtain Socket Descriptor.\n");
        return (0);
    }
    else printf ("[server] obtain socket descriptor successfully.\n");
    /* Fill the local socket address struct */
    addr_local.sin_family = AF_INET; // Protocol Family
    addr_local.sin_port = htons(PORT); // Port number
    addr_local.sin_addr.s_addr = INADDR_ANY; // AutoFill local address
    bzero(&(addr_local.sin_zero), 8); // Flush the rest of struct
    /* Bind a special Port */
    if( bind(sockfd, (struct sockaddr*)&addr_local, sizeof(struct sockaddr)) == -1 )
    {
        printf ("ERROR: Failed to bind Port %d.\n",PORT);
        return (0);
    }
    else printf("[server] bind tcp port %d in addr 0.0.0.0 sucessfully.\n",PORT);
    /* Listen remote connect/calling */
    if(listen(sockfd,BACKLOG) == -1)
    {
        printf ("ERROR: Failed to listen Port %d.\n", PORT);
        return (0);
    }
    else printf ("[server] listening the port %d sucessfully.\n", PORT);
    int success = 0;
    while(success == 0)
    {
        sin_size = sizeof(struct sockaddr_in);
        /* Wait a connection, and obtain a new socket file despriptor for single connection */
        if ((nsockfd = accept(sockfd, (struct sockaddr *)&addr_remote, &sin_size)) == -1) 
            printf ("ERROR: Obtain new Socket Despcritor error.\n");
        else printf ("[server] server has got connect from %s.\n", inet_ntoa(addr_remote.sin_addr));
        /* Child process */
        if(!fork())
        {
            char* f_name = "send.txt";
            char sdbuf[LENGTH]; // Send buffer
            printf("[server] send %s to the client...", f_name);
            FILE *fp = fopen(f_name, "r");
            if(fp == NULL)
            {
                printf("ERROR: File %s not found.\n", f_name);
                exit(1);
            }
            bzero(sdbuf, LENGTH);
            int f_block_sz;
            while((f_block_sz = fread(sdbuf, sizeof(char), LENGTH, fp))>0)
            {
                if(send(nsockfd, sdbuf, f_block_sz, 0) < 0)
                {
                    printf("ERROR: Failed to send file %s.\n", f_name);
                    break;
                }
                bzero(sdbuf, LENGTH);
            }
            printf("ok!\n");
            success = 1;
            close(nsockfd);
            printf("[server] connection closed.\n");
            while(waitpid(-1, NULL, WNOHANG) > 0);
        }
    }
}

客户端.c

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>
#define PORT 5000
#define LENGTH 512 // Buffer length
int main(int argc, char *argv[])
{
    int sockfd; // Socket file descriptor
    char revbuf[LENGTH]; // Receiver buffer
    struct sockaddr_in remote_addr; 
    /* Get the Socket file descriptor */
    if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
    {
        printf("ERROR: Failed to obtain Socket Descriptor!\n");
        return (0);
    }
    /* Fill the socket address struct */
    remote_addr.sin_family = AF_INET; 
    remote_addr.sin_port = htons(PORT); 
    inet_pton(AF_INET, "127.0.0.1", &remote_addr.sin_addr); 
    bzero(&(remote_addr.sin_zero), 8);
    /* Try to connect the remote */
    if (connect(sockfd, (struct sockaddr *)&remote_addr, sizeof(struct sockaddr)) == -1)
    {
        printf ("ERROR: Failed to connect to the host!\n");
        return (0);
    }
    else printf("[client] connected to server at port %d...ok!\n", PORT);
    //printf ("OK: Have connected to %s\n",argv[1]);
    printf("[client] receive file sent by server to receive.txt...");
    char* f_name = "receive.txt";
    FILE *fp = fopen(f_name, "a");
    if(fp == NULL) printf("File %s cannot be opened.\n", f_name);
    else
    {
        bzero(revbuf, LENGTH);
        int f_block_sz = 0;
        int success = 0;
        while(success == 0)
        {
            while(f_block_sz = recv(sockfd, revbuf, LENGTH, 0))
            {
                if(f_block_sz < 0)
                {
                    printf("Receive file error.\n");
                    break;
                }
                int write_sz = fwrite(revbuf, sizeof(char), f_block_sz, fp);
                if(write_sz < f_block_sz)
                {
                    printf("File write failed.\n");
                    break;
                }
                bzero(revbuf, LENGTH);
            }
            printf("ok!\n");
            success = 1;
            fclose(fp);
        }
    }
    close (sockfd);
    printf("[client] connection lost.\n");
    return (0);
}

非常感谢!

Answer 1

您需要将代码添加到 f_block_sz代码。 recv()有一些可能的返回值:

• <0 -- 错误，错误编号在 errno 中( errno.h )
• 0 -- 连接关闭
• >0 -- 读取的数据，字节数

您需要处理第二种情况。添加此 else案例:

else if(f_block_sz)
{
  break;
}

这样当服务器关闭连接时循环就会被打破，你的代码会打印出你[client] connection lost并退出。