recv返回值----任意长度的可读的数据_串口recv返回值是len吗-优快云博客

本文链接：https://blog.youkuaiyun.com/nodeathphoenix/article/details/31774885

本文通过一个客户端-服务端的例子展示了在UNIX套接字通信中，服务端一次性发送大量数据，客户端使用recv按4096字节接收时，recv返回值的情况。多数情况下返回4096字节，但有时也会返回小于4096的值，说明recv会返回当前可读取的数据长度，不一定是请求的全部长度。

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一、客户端代码

#include <sys/types.h>

#include <sys/socket.h>
#include <stdio.h>
#include <sys/un.h>
#include <unistd.h>
#include <stdlib.h>
#include <iostream>

using namespace std;

int main (int argc, char *argv[])
{
        struct sockaddr_un address;
        int sockfd;
        int len;
        int result;
        char buffer[5000] = {0};
        int rc = 0;

        if ((sockfd = socket(AF_UNIX, SOCK_STREAM, 0)) == -1) {
                perror ("socket");
                exit (EXIT_FAILURE);
        }

        address.sun_family = AF_UNIX;
        strcpy (address.sun_path, "server_socket");
        len = sizeof (address);

        result = connect (sockfd, (struct sockaddr *)&address, len);
        if (result == -1) {
                printf ("ensure the server is up\n");
                perror ("connect");
                exit (EXIT_FAILURE);
        }

        unsigned int sum = 0;
        while(1) {
                rc = recv(sockfd, buffer, 4096, 0);
                if (0 >= rc) {
                    cout << "rc is less than or equal to 0" << endl;
                    cout << "sum is : " << sum << endl;
                    return 0;
                }

                sum += rc;
                if (rc < 4096) {
                        cout << "rc is : " << rc << endl;
                }
        }

        return 0;
}

二、服务端代码

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <sys/un.h>
#include <unistd.h>
#include <stdlib.h>
#include <iostream>

using namespace std;

#define COUNT 5000000

int main (int argc, char *argv[])
{
        int server_sockfd, client_sockfd;
        int server_len, client_len;
        struct sockaddr_un server_address;
        struct sockaddr_un client_address;
        int i, bytes;
        char ch_send, ch_recv;
        unlink ("server_socket");

        server_sockfd = socket (AF_UNIX, SOCK_STREAM, 0);

        server_address.sun_family = AF_UNIX;

        strcpy (server_address.sun_path, "server_socket");

        server_len = sizeof (server_address);

        bind (server_sockfd, (struct sockaddr *)&server_address, server_len);

        listen (server_sockfd, 5);
        printf ("Server is waiting for client connect...\n");
        client_len = sizeof (client_address);

        client_sockfd = accept (server_sockfd, (struct sockaddr *)&server_address, (socklen_t *)&client_len);
        if (client_sockfd == -1) {
                perror ("accept");
                exit (EXIT_FAILURE);
        }

        char buffer[COUNT] = {0};
        memset(buffer, 'a', COUNT);

        if ((bytes = write (client_sockfd, buffer, COUNT)) == -1) {
                printf ("Failed to write data...\n");

        }
        cout << "written bytes is : " << bytes << endl;

        close (client_sockfd);
        unlink ("server socket");
}

三、执行测试

先执行server程序，然后再启动客户端程序，结果如下：
#./server
Server is waiting for client connect...
written bytes is : 5000000

# ./client
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 3776
rc is : 3776
rc is : 3776
rc is : 3776
rc is : 3776
rc is : 3776
rc is : 3776
rc is : 3776
rc is : 3776
rc is : 3776
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 4032
rc is : 3456
rc is less than or equal to 0
sum is : 5000000

可见，server端发送一次数据后，client在recv时指定期望长度4096，则循环接收数据的时候，绝大多数recv返回值是4096，但是可能间或收到的小于4096，即出现返回值为4096和小于4096的值交替的情况。故而验证了，recv确实是返回当前任意长度的可读的数据。

The receive calls normally return any data available,up to the requested amount, rather than waiting for receipt of the full amount requested.