TCP套接字，发送文件，客户端服务器，服务器不保存整个文件，C语言

Question

我编写了一个简单的协议，使我能够在客户端和服务器之间交换文件/文本消息。如果客户端向服务器发送文本，服务器应该简单地回显它。另一方面，当客户端发送特殊命令（例如，SEND_TXT_FILE）时，服务器应该会收到客户端上传到服务器的文件。

我几乎成功了。但是，发送文件仍然存在问题。 Sever 不会保存整个文件，它只会创建它并断开连接。

这是协议：

CLIENT ---------- text1 ----------> SERVER
CLIENT <---------- text1 ---------- SERVER
CLIENT ---------- text2 ----------> SERVER
CLIENT <---------- text3 ---------- SERVER
CLIENT ---------- SENDTXTFILE ----------> SERVER
CLIENT <---------- OK ---------- SERVER
CLIENT ---------- FILENAME ----------> SERVER
CLIENT <---------- OK ---------- SERVER
CLIENT ---------- file content ----------> SERVER
CLIENT <--------- FILE_UPLOADED --------- SERVER
CLIENT ---------- text3 ----------> SERVER
CLIENT <---------- text3 ---------- SERVER

我该如何解决这个问题？

server.c

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <netdb.h>
#include <string.h>

#define BUFFER_SIZE 1024

int send_all(int sockfd, const char *buf, int len)
{
    ssize_t n;

    while (len > 0)
    {
        n = send(sockfd, buf, len, 0);
        if (n < 0)
            return -1;
        buf += n;
        len -= n;
    }

    return 0;
}

int recv_all(int sockfd, char *buf, int len)
{
    ssize_t n;

    while (len > 0)
    {
        n = recv(sockfd, buf, len, 0);
        if (n <= 0)
            return n;
        buf += n;
        len -= n;
    }

    return 1;
}

int recv_txt_file(int sockfd, int len, const char *filename)
{
    FILE *fp = fopen(filename, "wb");
    int total = 0, b = 0;
    char buffer[BUFFER_SIZE];
    memset(buffer, '\0', BUFFER_SIZE);

    if (fp != NULL)
    {
        while (recv_all(sockfd, buffer, len) != 1)
        {
            total += b;
            fwrite(buffer, 1, b, fp);
        }

        printf("Received byte: %d\n",total);
        if (b < 0)
            perror("Receiving");

        fclose(fp);
    }
    else
    {
        perror("File");
    }
    close(sockfd);
}

int main()
{
    int port = 6666;
    int server_fd, client_fd, read;
    struct sockaddr_in server, client;
    char buffer[BUFFER_SIZE], filename[BUFFER_SIZE];
    char remote_ip[16];
    int remote_port, res = 0;

    server_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (server_fd < 0)
    {
        perror("Could not create socket");
        return 1;
    }

    int optval = 1;
    setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR, (const void *)&optval, sizeof(int));

    memset(&server, '\0', sizeof(server));
    server.sin_family = AF_INET;
    server.sin_port = htons(port);
    server.sin_addr.s_addr = htonl(INADDR_ANY);

    if (bind(server_fd, (struct sockaddr *)&server, sizeof(server)) < 0)
    {
        perror("Could not bind socket");
        close(server_fd);
        return 1;
    }

    if (listen(server_fd, 1) < 0)
    {
        perror("Could not listen on socket");
        close(server_fd);
        return 1;
    }

    printf("Server TCP is listening on port %d ... \n", port);

    socklen_t client_len = sizeof(client);
    client_fd = accept(server_fd, (struct sockaddr *)&client, &client_len);

    if (client_fd < 0)
    {
        perror("Could not establish new connection");
        close(server_fd);
        return 1;
    }

    remote_port = ntohs(client.sin_port);
    inet_ntop(AF_INET, &client.sin_addr, remote_ip, sizeof(remote_ip));

    printf("Client IP address: %s, port %d\n", remote_ip, remote_port);

    while (1)
    {
        read = recv_all(client_fd, buffer, BUFFER_SIZE);
        if (read <= 0)
        {
            if (read < 0)
                perror("Client read failed");
            else
                printf("Client disconnected\n");
            break;
        }

        if ((res = strcmp(buffer, "SENDFILE_TXT\n") == 0))
        {

            printf("-------FROM CLIENT: %s-------\n", buffer);
            memset(buffer, BUFFER_SIZE, '\0');
            strcpy(buffer, "OK");
            if (send_all(client_fd, buffer, BUFFER_SIZE) < 0)
            {
                perror("Client write failed");
                break;
            }
            read = recv_all(client_fd, buffer, BUFFER_SIZE);
            if (read <= 0)
            {
                if (read < 0)
                    perror("Client read failed");
                else
                    printf("Client disconnected\n");
                break;
            }
            printf("-------FROM CLIENT: %s-------\n", buffer);
            memset(filename, '\0', BUFFER_SIZE);
            strcpy(filename, buffer);
            memset(buffer, BUFFER_SIZE, '\0');
            strcpy(buffer, "OK");
            if (send_all(client_fd, buffer, BUFFER_SIZE) < 0)
            {
                perror("Client write failed");
                break;
            }
            recv_txt_file(client_fd, BUFFER_SIZE, filename);
        }
        else
        {
            printf("FROM CLIENT: %.*s\n", BUFFER_SIZE, buffer);

            if (send_all(client_fd, buffer, BUFFER_SIZE) < 0)
            {
                perror("Client write failed");
                break;
            }
        }
    }

    close(client_fd);
    close(server_fd);

    return 0;
}

client.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <netdb.h>
#define BUFFER_SIZE 1024

socklen_t hostname_to_ip_port(char *hostname, int port, struct sockaddr_storage *addr)
{
    int sockfd;
    struct addrinfo hints, *servinfo;
    int rv;

    char service[20];
    sprintf(service, "%d", port);

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;

    if ((rv = getaddrinfo(hostname, service, &hints, &servinfo)) != 0)
    {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        return 0;
    }

    socklen_t addrlen = servinfo->ai_addrlen;
    memcpy(addr, servinfo->ai_addr, addrlen);

    freeaddrinfo(servinfo);

    return addrlen;
}

int send_all(int sockfd, const char *buf, size_t len)
{
    ssize_t n;

    while (len > 0)
    {
        n = send(sockfd, buf, len, 0);
        if (n < 0)
            return -1;
        buf += n;
        len -= n;
    }

    return 0;
}

int recv_all(int sockfd, char *buf, int len)
{
    ssize_t n;

    while (len > 0)
    {
        n = recv(sockfd, buf, len, 0);
        if (n <= 0)
            return n;
        buf += n;
        len -= n;
    }

    return 1;
}

long int get_file_size(char filename[])
{
    // opening the file in read mode
    FILE *fp = fopen(filename, "r");

    // checking if the file exist or not
    if (fp == NULL) {
        printf("File Not Found!\n");
        return -1;
    }

    fseek(fp, 0L, SEEK_END);

    // calculating the size of the file
    long int res = ftell(fp);

    // closing the file
    fclose(fp);

    return res;
}

int send_txt_file(int sockfd, int len, const char *filename)
{
    FILE *fp = fopen(filename, "r");
    int b;
    char buffer[BUFFER_SIZE];
    memset(buffer, '\0', BUFFER_SIZE);

    if (fp == NULL)
    {
        perror("Error while openning file");
        return 0;
    }

    while ((b = fread(buffer, 1, BUFFER_SIZE, fp)) > 0){
        send_all(sockfd, buffer, BUFFER_SIZE);
        memset(buffer, '\0', BUFFER_SIZE);
    }

    fclose(fp);
    return 1;
}

int main(int argc, char **argv)
{
    char *hostname = "127.0.0.1";
    int port = 6666;
    char buffer[BUFFER_SIZE], fname[BUFFER_SIZE];

    int sockfd, err, res;
    struct sockaddr_storage server_addr;
    socklen_t server_addr_len;

    server_addr_len = hostname_to_ip_port(hostname, port, &server_addr);
    if (server_addr_len == 0)
        return 1;

    sockfd = socket(server_addr.ss_family, SOCK_STREAM, IPPROTO_TCP);
    if (sockfd < 0)
    {
        perror("Could not create socket");
        return 1;
    }

    if (connect(sockfd, (struct sockaddr *)&server_addr, server_addr_len) < 0)
    {
        perror("Could not connect socket");
        return 1;
    }

    while (1)
    {
        memset(buffer, BUFFER_SIZE, '\0');
        memset(fname, BUFFER_SIZE, '\0');

        printf("> ");
        if (!fgets(buffer, BUFFER_SIZE, stdin))
            break;

        if (strstr(buffer, "SENDFILE_TXT") != NULL)
        {
            if (send_all(sockfd, buffer, BUFFER_SIZE) < 0)
            {
                perror("Could not send message");
                close(sockfd);
                return 1;
            }

            memset(buffer, BUFFER_SIZE, '\0');
            err = recv_all(sockfd, buffer, BUFFER_SIZE);
            if (err <= 0)
            {
                if (err < 0)
                    perror("Could not read message");
                else
                    printf("Server disconnected\n");
                break;
            }
            if ((res = strcmp(buffer, "OK") == 0))
            {
                printf("-------FROM SERVER: %s-------\n", buffer);

                printf("Give filename> ");
                memset(fname, BUFFER_SIZE, '\0');
                if (!fgets(fname, BUFFER_SIZE, stdin))
                    break;
                if (send_all(sockfd, fname, BUFFER_SIZE) < 0)
                {
                    perror("Could not send message");
                    close(sockfd);
                    return 1;
                }
            }

            err = recv_all(sockfd, buffer, BUFFER_SIZE);
            if (err <= 0)
            {
                if (err < 0)
                    perror("Could not read message");
                else
                    printf("Server disconnected\n");
                break;
            }

            fname[strlen(fname)-1] = 0;
            printf("----%s----\n", fname);
            send_txt_file(sockfd, BUFFER_SIZE, fname);

            printf("FROM SERVER: %.*s\n", BUFFER_SIZE, buffer);
        }
        else
        {
            if (send_all(sockfd, buffer, BUFFER_SIZE) < 0)
            {
                perror("Could not send message");
                close(sockfd);
                return 1;
            }

            err = recv_all(sockfd, buffer, BUFFER_SIZE);
            if (err <= 0)
            {
                if (err < 0)
                    perror("Could not read message");
                else
                    printf("Server disconnected\n");
                break;
            }
            printf("FROM SERVER: %.*s\n", BUFFER_SIZE, buffer);
        }
    }

    close(sockfd);

    return 0;
}

Answer 1

您使用send_txt_file() 和recv_txt_file() 传输文件的方式不正确。

---

在服务器端：

当客户端连接时，服务器等待客户端发送 确切 BUFFER_SIZE (1024) 字节为每个命令，不多也不少（这对于短命令来说是浪费带宽）。当接收到SENDFILE_TXT 命令时，服务器从客户端读取文件名（如果文件名恰好 BUFFER_SIZE字节长度会导致缓冲区溢出），然后调用recv_txt_file() .

recv_txt_file() 尝试循环读取客户端，准确读取BUFFER_SIZE块。但是，正在使用的while 循环编码不正确。它正在检查recv_all() 的返回值是否失败，而不是成功。 != 检查需要改为==。而且，用于递增total 并告诉fwrite() 要写入多少字节的b 变量永远不会设置为0 以外的任何值。它需要设置为 BUFFER_SIZE，因为如果 recv_all() 返回 1，这就是实际读取的字节数。

然而，即使while 循环编码正确，传输仍然无法正常运行，因为它要求文件以偶数倍发送BUFFER_SIZE。如果文件大小不是偶数倍，recv_all() 将最终等待客户端未发送的数据，直到套接字发生错误。

另外，recv_txt_file() 正在传输完成后关闭连接。它不应该这样做，因为这将阻止客户端在发送文件后能够发送进一步的命令。客户端在发送文件后没有关闭它的连接端，所以服务器在接收到文件后不应该关闭它的端。

---

在客户端：

当客户端发送SENDFILE_TXT 命令并得到确认返回时，它会调用send_txt_file()，它会循环读取文件，并将其以BUFFER_SIZE 大小的块发送到服务器。如果文件大小不是BUFFER_SIZE 的偶数倍，最后发送的块只会浪费带宽并向服务器发送随机垃圾。此外，如果套接字发生错误，您将忽略 send_all() 的返回值以中断循环。

---

在发送文件数据之前，客户端应该将实际的文件大小发送到服务器。然后，服务器可以先读取该大小值，以便知道何时停止读取。

话虽如此，请尝试以下类似的方法：

server.c

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <netdb.h>
#include <string.h>
#include <stdint.h>

#define BUFFER_SIZE 1024

int send_all(int sockfd, const void *buf, int len)
{
    ssize_t n;
    const char *pbuf = (const char*) buf;

    while (len > 0)
    {
        n = send(sockfd, pbuf, len, 0);
        if (n < 0)
        {
            perror("Client write failed");
            return n;
        }
        pbuf += n;
        len -= n;
    }

    return 0;
}

int send_uint32(int sockfd, uint32_t value)
{
    value = htonl(value);
    if (send_all(sockfd, &value, sizeof(value)) < 0)
        return -1;
    return 0;
}

int send_str(int sockfd, const char *s)
{
    uint32_t len = strlen(s);
    int res = send_uint32(sockfd, len);
    if (res == 0)
        res = send_all(sockfd, s, len);
    return res;
}

int recv_all(int sockfd, void * buf, int len)
{
    ssize_t n;
    char *pbuf = (char*) buf;

    while (len > 0)
    {
        n = recv(sockfd, pbuf, len, 0);
        if (n <= 0)
        {
            if (n < 0)
                perror("Client read failed");
            else
                printf("Client disconnected\n");
            return n;
        }
        pbuf += n;
        len -= n;
    }

    return 1;
}

int recv_uint32(int sockfd, uint32_t *value)
{
    int res = recv_all(sockfd, value, sizeof(*value));
    if (res > 0)
        *value = ntohl(*value);
    return res;
}

int recv_uint64(int sockfd, uint64_t *value)
{
    int res = recv_all(sockfd, value, sizeof(*value));
    if (res > 0)
        *value = ntohll(*value); // <-- use any implementation of your choosing...
    return res;
}

int recv_str(int sockfd, char **str)
{
    uint32_t len;

    int res = recv_uint32(sockfd, &len);
    if (res <= 0)
        return res;

    *str = (char*) malloc(len + 1);
    if (*str == NULL)
    {
        perror("Could not allocate memory");
        return -1;
    }

    res = recv_all(sockfd, *str, len);
    if (res <= 0)
        free(*str);
    else
        (*str)[len] = '\0';

    return res;
}

int recv_txt_file(int sockfd)
{
    char *filename;
    uint64_t filesize;

    if (recv_str(sockfd, &filename) <= 0)
        return -1;

    int res = recv_uint64(sockfd, &filesize);
    if (res <= 0)
    {
        free(filename);
        return -1;
    }

    printf("-------FROM CLIENT: %s-------\n", filename);

    FILE* fp = fopen(filename, "wb");
    if (fp == NULL)
    {
        perror("Could not create file");
        free(filename);
        send_str(sockfd, "NO");
        return 0;
    }

    free(filename);

    // optional: pre-size the new file to the specified filesize...

    if (send_str(sockfd, "OK") < 0)
        return -1;

    char buffer[BUFFER_SIZE];
    int b;
    uint64_t total = 0;

    while (filesize > 0)
    {
        b = (filesize > BUFFER_SIZE) ? BUFFER_SIZE : (int) filesize;

        res = recv_all(sockfd, buffer, b);
        if (res <= 0)
        {
            fclose(fp);
            return -1;
        }

        if (fwrite(buffer, b, 1, fp) < 1)
        {
            perror("Could not write to file");
            fclose(fp);
            send_str(sockfd, "ERROR");
            return -1;
        }

        total += b;
        filesize -= b;
    }

    fclose(fp);
    printf("Received bytes: %lu\n", total);

    if (send_str(sockfd, "OK") < 0)
        return -1;

    return 1;
}

int main()
{
    int port = 6666;
    int server_fd, client_fd, read;
    struct sockaddr_in server, client;
    char filename[BUFFER_SIZE], *cmd;
    char remote_ip[16];
    int remote_port, res = 0;

    server_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (server_fd < 0)
    {
        perror("Could not create socket");
        return 1;
    }

    int optval = 1;
    setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(int));

    memset(&server, '\0', sizeof(server));
    server.sin_family = AF_INET;
    server.sin_port = htons(port);
    server.sin_addr.s_addr = htonl(INADDR_ANY);

    if (bind(server_fd, (struct sockaddr *) &server, sizeof(server)) < 0)
    {
        perror("Could not bind socket");
        close(server_fd);
        return 1;
    }

    if (listen(server_fd, 1) < 0)
    {
        perror("Could not listen on socket");
        close(server_fd);
        return 1;
    }

    printf("Server TCP is listening on port %d ... \n", port);

    socklen_t client_len = sizeof(client);
    client_fd = accept(server_fd, (struct sockaddr *) &client, &client_len);

    if (client_fd < 0)
    {
        perror("Could not establish new connection");
        close(server_fd);
        return 1;
    }

    remote_port = ntohs(client.sin_port);
    inet_ntop(AF_INET, &client.sin_addr, remote_ip, sizeof(remote_ip));

    printf("Client IP address: %s, port %d\n", remote_ip, remote_port);

    while (recv_str(client_fd, &cmd) > 0)
    {
        printf("-------FROM CLIENT: %s-------\n", cmd);

        if (strcmp(cmd, "SENDFILE_TXT") == 0)
        {
            if (recv_txt_file(client_fd) < 0)
                break;
        }
        else
        {
            if (send_str(client_fd, cmd) < 0)
            {
                free(cmd);
                break;
            }
        }

        free(cmd);
    }

    close(client_fd);
    close(server_fd);

    return 0;
}

client.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <netdb.h>
#include <stdint.h>

#define BUFFER_SIZE 1024

socklen_t hostname_to_ip_port(char *hostname, int port, struct sockaddr_storage *addr)
{
    int sockfd;
    struct addrinfo hints, *servinfo;
    int rv;

    char service[20];
    sprintf(service, "%d", port);

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;

    if ((rv = getaddrinfo(hostname, service, &hints, &servinfo)) != 0)
    {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        return 0;
    }

    socklen_t addrlen = servinfo->ai_addrlen;
    memcpy(addr, servinfo->ai_addr, addrlen);

    freeaddrinfo(servinfo);

    return addrlen;
}

int send_all(int sockfd, const void *buf, size_t len)
{
    ssize_t n;
    const char *pbuf = (const char *) buf;

    while (len > 0)
    {
        n = send(sockfd, pbuf, len, 0);
        if (n < 0)
        {
            perror("Server write failed");
            return n;
        }
        pbuf += n;
        len -= n;
    }

    return 0;
}

int send_uint32(int sockfd, uint32_t value)
{
    value = htonl(value);
    return send_all(sockfd, &value, sizeof(value));
}

int send_uint64(int sockfd, uint64_t value)
{
    value = htonll(value); // <-- use any implementation of your choosing...
    return send_all(sockfd, &value, sizeof(value));
}

int send_str(int sockfd, const char *s)
{
    uint32_t len = strlen(s);
    int res = send_uint32(sockfd, len);
    if (res == 0)
        res = send_all(sockfd, s, len);
    return res;
}

int recv_all(int sockfd, void *buf, int len)
{
    ssize_t n;
    char *pbuf = (char*) buf;

    while (len > 0)
    {
        n = recv(sockfd, pbuf, len, 0);
        if (n <= 0)
        {
            if (n < 0)
                perror("Server read failed");
            else
                printf("Server disconnected\n");
            return n;
        }
        pbuf += n;
        len -= n;
    }

    return 1;
}

int recv_uint32(int sockfd, uint32_t *value)
{
    int res = recv_all(sockfd, value, sizeof(*value));
    if (res > 0)
        *value = ntohl(*value);
    return res;
}

int recv_str(int sockfd, char **str)
{
    uint32_t len;

    int res = recv_uint32(sockfd, &len);
    if (res <= 0)
        return res;

    *str = (char*) malloc(len + 1);
    if (*str == NULL)
    {
        perror("Could not allocate memory");
        return -1;
    }

    res = recv_all(sockfd, *str, len);
    if (res <= 0)
        free(*str);
    else
        (*str)[len] = '\0';

    return res;
}

int send_txt_file(int sockfd, const char *filename)
{
    char *resp;
    int res;
    long int filesize;
    char buffer[BUFFER_SIZE];
    int b;

    FILE* fp = fopen(filename, "rb");
    if (fp == NULL)
    {
        printf("Could not open file!\n");
        return 0;
    }

    fseek(fp, 0L, SEEK_END);
    filesize = ftell(fp);
    fseek(fp, 0L, SEEK_SET);

    if (filesize < 0)
    {
        fclose(fp);
        return 0;
    }

    if (send_str(sockfd, "SENDFILE_TXT") < 0)
    {
        fclose(fp);
        return -1;
    }

    if (send_str(sockfd, filename) < 0)
    {
        fclose(fp);
        return -1;
    }

    if (send_uint64(sockfd, filesize) < 0)
    {
        fclose(fp);
        return -1;
    }

    res = recv_str(sockfd, &resp);
    if (res <= 0)
        return -1;

    printf("-------FROM SERVER: %s-------\n", resp);

    if (strcmp(resp, "OK") != 0)
    {
        free(resp);
        fclose(fp);
        return 0;
    }

    free(resp);

    while (filesize > 0)
    {
        b = (filesize > BUFFER_SIZE) ? BUFFER_SIZE : (int) filesize;

        b = fread(buffer, 1, b, fp);
        if (b < 1)
        {
            fclose(fp);
            return -1;
        }

        if (send_all(sockfd, buffer, b) < 0)
        {
            fclose(fp);
            return -1;
        }

        filesize -= b;
    }

    fclose(fp);

    res = recv_str(sockfd, &resp);
    if (res <= 0)
        return -1;

    printf("-------FROM SERVER: %s-------\n", resp);

    free(resp);

    return 0;
}

int prompt(const char *text, char **input)
{
    *input = NULL;
    size_t size = 0;

    printf("%s> ", text);

    ssize_t len = getline(input, &size, stdin);
    if (len < 0)
        return len;

    if ((*input)[len-1] == '\n')
    {
        --len;
        (*input)[len] = '\0';
    }

    return len;
}

int main()
{
    char *hostname = "127.0.0.1";
    int port = 6666;
    char *cmd, *resp, *fname;
    size_t size;
    ssize_t len;

    int sockfd, res;
    struct sockaddr_storage server_addr;
    socklen_t server_addr_len;

    server_addr_len = hostname_to_ip_port(hostname, port, &server_addr);
    if (server_addr_len == 0)
        return 1;

    sockfd = socket(server_addr.ss_family, SOCK_STREAM, IPPROTO_TCP);
    if (sockfd < 0)
    {
        perror("Could not create socket");
        return 1;
    }

    if (connect(sockfd, (struct sockaddr *) &server_addr, server_addr_len) < 0)
    {
        perror("Could not connect socket");
        return 1;
    }

    while (prompt("", &cmd) >= 0)
    {
        if (strcmp(cmd, "SENDFILE_TXT") == 0)
        {
            if (prompt("Give filename", &fname) < 0)
                break;

            if (send_txt_file(sockfd, fname) < 0)
                break;

            free(fname);
        }
        else
        {
            if (send_str(sockfd, cmd) < 0)
            {
                free(cmd);
                close(sockfd);
                return 1;
            }

            if (recv_str(sockfd, &resp) <= 0)
                break;

            printf("FROM SERVER: %s\n", resp);
            free(resp);
        }

        free(cmd);
    }

    close(sockfd);

    return 0;
}

Answer 2

关于：

int total = 0, b = 0;

和

while(recv_all(sockfd, buffer, len) != 1)
{
    total += b;
    fwrite(buffer, 1, b, fp);

total 和 b 永远不会从它们的初始值 0 移动。所以 0 字节被写入输出文件。