c++ - Recv-Q+Send-Q>写入字节

Question

我写了一个 tcp 服务器和一个 tcp 客户端，客户端只向服务器发送数据并打印它写入了多少字节，服务器只接受连接，然后我使用 netstat 显示套接字的 Recv-Q 和 Send-问，我发现 Recv-Q+send-Q > write bytes。它是如何发生的？

client code:
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <netdb.h>
#include <stdarg.h>
#include <errno.h>

void error(int status, int err, const char *fmt, ...)
{
    va_list args;

    va_start(args, fmt);

    vfprintf(stderr, fmt, args);

    va_end(args);

    if (err != 0)
    {
        fprintf(stderr, "errno = %d, errmsg = %s\n", err, strerror(err));
    }

    if (status != 0)
    {
        exit(err);
    }
}

int set_address(const char *host, const char *serv, const char *protocol, sockaddr_in *addr)
{
    struct hostent *host_info;
    struct servent *serv_info;

    memset(addr, 0, sizeof(sockaddr_in));

    addr->sin_family = AF_INET;

    if (host != NULL)
    {
        if (inet_aton(host, &addr->sin_addr) != 0)
        {
            host_info = gethostbyname(host);

            if (host_info == NULL)
            {
                error(1, h_errno, "set_address failed, gethostbyname(%s) failed, errno = %d, errmsg = %s\n", host, h_errno, hstrerror(h_errno));
            }
            else
            {
                inet_aton(host_info->h_addr_list[0], &addr->sin_addr);
            }
        }
    }
    else
    {
        addr->sin_addr.s_addr = INADDR_ANY;
    }

    if (serv != NULL)
    {
        char * end_pos;

        addr->sin_port = htons(strtol(serv, &end_pos, 10));

        if (*end_pos != '\0')
        {
            serv_info = getservbyname(serv, protocol);

            if (serv_info == NULL)
            {
                error(1, h_errno, "set_address failed, getservbyname(%s, %s) faield\n", serv, protocol);
            }
            else
            {
                addr->sin_port = serv_info->s_port;
            }
        }
    }
}

int tcp_client(const char *host, const char *serv)
{
    struct sockaddr_in server;
    int fd;
    const int on = 1;

    set_address(host, serv, "tcp", &server);

    fd = socket(AF_INET, SOCK_STREAM, 0);

    if (fd < 0)
    {
        error(1, errno, "create socket failed\n");
    }

    if (connect(fd, (struct sockaddr *)&server, sizeof(server)) != 0)
    {
        error(1, errno, "connect to server failed\n");
    }

    return fd;
}

int main (int argc, char **argv)
{
    if (argc != 3)
    {
        printf("usage: %s <host> <port>\n", basename(argv[0]));
        exit(1);
    }

    char buf[1024];
    int fd;
    int total_write_bytes = 0, cur_write_bytes = 0;

    fd = tcp_client(argv[1], argv[2]);

    while (true)
    {
        cur_write_bytes = write(fd, buf, sizeof(buf));
        if (cur_write_bytes <= 0)
        {
            break;
        }
        total_write_bytes += cur_write_bytes;
        printf("total_write_bytes = %d, curr_write_bytes = %d\n", total_write_bytes, cur_write_bytes);
    }

    return 0;
}

server code:
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <netdb.h>
#include <stdarg.h>
#include <errno.h>

void error(int status, int err, const char *fmt, ...)
{
    va_list args;

    va_start(args, fmt);

    vfprintf(stderr, fmt, args);

    va_end(args);

    if (err != 0)
    {
        fprintf(stderr, "errno = %d, errmsg = %s\n", err, strerror(err));
    }

    if (status != 0)
    {
        exit(err);
    }
}

int set_address(const char *host, const char *serv, const char *protocol, sockaddr_in *addr)
{
    struct hostent *host_info;
    struct servent *serv_info;

    memset(addr, 0, sizeof(sockaddr_in));

    addr->sin_family = AF_INET;

    if (host != NULL)
    {
        if (inet_aton(host, &addr->sin_addr) != 0)
        {
            host_info = gethostbyname(host);

            if (host_info == NULL)
            {
                error(1, h_errno, "set_address failed, gethostbyname(%s) failed, errno = %d, errmsg = %s\n", host, h_errno, hstrerror(h_errno));
            }
            else
            {
                inet_aton(host_info->h_addr_list[0], &addr->sin_addr);
            }
        }
    }
    else
    {
        addr->sin_addr.s_addr = INADDR_ANY;
    }

    if (serv != NULL)
    {
        char * end_pos;

        addr->sin_port = htons(strtol(serv, &end_pos, 10));

        if (*end_pos != '\0')
        {
            serv_info = getservbyname(serv, protocol);

            if (serv_info == NULL)
            {
                error(1, h_errno, "set_address failed, getservbyname(%s, %s) faield\n", serv, protocol);
            }
            else
            {
                addr->sin_port = serv_info->s_port;
            }
        }
    }
}

int tcp_server(const char *host, const char *serv)
{
    struct sockaddr_in local;
    int fd;
    const int on = 1;

    set_address(host, serv, "tcp", &local);

    fd = socket(AF_INET, SOCK_STREAM, 0);

    if (fd < 0)
    {
        error(1, errno, "create socket failed");
    }

    if (bind(fd, (sockaddr *)&local, sizeof(local)) != 0)
    {
        error(1, errno, "bind to port %s:%s failed\n", host, serv);
    }

    if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on)) != 0)
    {
        error(1, errno, "set SO_REUSEADDR failed\n");
    }

    if (listen(fd, 100) != 0)
    {
        error(1, errno, "listen on %s:%s failed\n", host, serv);
    }

    return fd;
}

int main (int argc, char **argv)
{
    if (argc != 3)
    {
        printf("usage: %s <host> <port>\n", basename(argv[0]));
        exit(1);
    }

    int listen_fd, fd;
    struct sockaddr_in addr;
    socklen_t len;

    listen_fd = tcp_server(argv[1], argv[2]);

    fd = accept(listen_fd, (struct sockaddr *)&addr, &len);

    if (fd < 0)
    {
        error(1, errno, "accept an connection failed\n");
    }

    while (true)
    {
        sleep(5);
    }

    return 0;
}

这是结果:

标准输出结果: standard output

网络统计结果: netstat result

tcpdump 结果:

14:17:16.633105 IP localhost.54393 > localhost.personal-agent: S 4282375064:4282375064(0) win 32767 14:17:16.633106 IP localhost.personal-agent > localhost.54393: S 4268411460:4268411460(0) ack 4282375065 win 32767 14:17:16.633115 IP localhost.54393 > localhost.personal-agent: . ack 1 win 8192 14:17:16.633127 IP localhost.54393 > localhost.personal-agent: P 1:1025(1024) ack 1 win 8192 14:17:16.633130 IP localhost.personal-agent > localhost.54393: . ack 1025 win 8704 14:17:16.633156 IP localhost.54393 > localhost.personal-agent: P 1025:2049(1024) ack 1 win 8192 14:17:16.633159 IP localhost.personal-agent > localhost.54393: . ack 2049 win 8704 14:17:16.633167 IP localhost.54393 > localhost.personal-agent: P 2049:3073(1024) ack 1 win 8192 14:17:16.633169 IP localhost.personal-agent > localhost.54393: . ack 3073 win 8704 14:17:16.633176 IP localhost.54393 > localhost.personal-agent: P 3073:4097(1024) ack 1 win 8192 14:17:16.633179 IP localhost.personal-agent > localhost.54393: . ack 4097 win 8704 14:17:16.633185 IP localhost.54393 > localhost.personal-agent: P 4097:5121(1024) ack 1 win 8192 14:17:16.633188 IP localhost.personal-agent > localhost.54393: . ack 5121 win 8704 14:17:16.633195 IP localhost.54393 > localhost.personal-agent: P 5121:6145(1024) ack 1 win 8192 14:17:16.633198 IP localhost.personal-agent > localhost.54393: . ack 6145 win 8704 14:17:16.633204 IP localhost.54393 > localhost.personal-agent: P 6145:7169(1024) ack 1 win 8192 14:17:16.633206 IP localhost.personal-agent > localhost.54393: . ack 7169 win 8704 14:17:16.633213 IP localhost.54393 > localhost.personal-agent: P 7169:8193(1024) ack 1 win 8192 14:17:16.633215 IP localhost.personal-agent > localhost.54393: . ack 8193 win 8704 14:17:16.633222 IP localhost.54393 > localhost.personal-agent: P 8193:9217(1024) ack 1 win 8192 14:17:16.633224 IP localhost.personal-agent > localhost.54393: . ack 9217 win 8704 14:17:16.633230 IP localhost.54393 > localhost.personal-agent: P 9217:10241(1024) ack 1 win 8192 14:17:16.633233 IP localhost.personal-agent > localhost.54393: . ack 10241 win 8704 14:17:16.633239 IP localhost.54393 > localhost.personal-agent: P 10241:11265(1024) ack 1 win 8192 14:17:16.633242 IP localhost.personal-agent > localhost.54393: . ack 11265 win 8448 14:17:16.633249 IP localhost.54393 > localhost.personal-agent: P 11265:12289(1024) ack 1 win 8192 14:17:16.633251 IP localhost.personal-agent > localhost.54393: . ack 12289 win 8192 14:17:16.633258 IP localhost.54393 > localhost.personal-agent: P 12289:13313(1024) ack 1 win 8192 14:17:16.633261 IP localhost.personal-agent > localhost.54393: . ack 13313 win 7936 14:17:16.633269 IP localhost.personal-agent > localhost.54393: . ack 14337 win 7680 14:17:16.671777 IP localhost.personal-agent > localhost.54393: . ack 31757 win 3325 14:17:16.879921 IP localhost.54393 > localhost.personal-agent: P 31757:45057(13300) ack 1 win 8192 14:17:16.959771 IP localhost.personal-agent > localhost.54393: . ack 45057 win 0 14:17:17.175771 IP localhost.54393 > localhost.personal-agent: . ack 1 win 8192 14:17:17.175786 IP localhost.personal-agent > localhost.54393: . ack 45057 win 0 14:17:17.607770 IP localhost.54393 > localhost.personal-agent: . ack 1 win 8192 14:17:17.607782 IP localhost.personal-agent > localhost.54393: . ack 45057 win 0 14:17:18.471768 IP localhost.54393 > localhost.personal-agent: . ack 1 win 8192 14:17:18.471775 IP localhost.personal-agent > localhost.54393: . ack 45057 win 0

Answer 1

已接收但发送方尚未收到确认的字节同时在发送队列和接收队列中。它们保留在发送队列中，因为在它们被确认之前，发件人必须准备好重新发送它们。它们保留在接收队列中，直到应用程序调用接收函数并将它们从队列中取出。