阻塞

即为数据无法添加或者删除，一直开在此处，直到数据可以添加或者删除的时候；
如果是发送阻塞【send】，即为操作系统内的缓冲区满了，无法继续添加到操作系统内的缓冲区，会阻塞在当下，直到操作系统内的缓冲区有空闲为止；
如果是接收阻塞【recv】，即为操作系统内的缓冲区空了，无法继续获取到操作系统内的缓冲区的数据，会阻塞在当下，直到操作系统内的缓冲区有缓冲数据为止；

示例

服务端

#include <sys/types.h> 
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <iostream>
#include <string.h>

int main(int argc, char* argv[])
{
    //1.创建一个侦听socket
    int listenfd = socket(AF_INET, SOCK_STREAM, 0);
    if (listenfd == -1)
    {
        std::cout << "create listen socket error." << std::endl;
        return -1;
    }

    //2.初始化服务器地址
    struct sockaddr_in bindaddr;
    bindaddr.sin_family = AF_INET;
    bindaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    bindaddr.sin_port = htons(3000);
    if (bind(listenfd, (struct sockaddr *)&bindaddr, sizeof(bindaddr)) == -1)
    {
        std::cout << "bind listen socket error." << std::endl;
		close(listenfd);
        return -1;
    }

	//3.启动侦听
    if (listen(listenfd, SOMAXCONN) == -1)
    {
        std::cout << "listen error." << std::endl;
		close(listenfd);
        return -1;
    }

    while (true)
    {
        struct sockaddr_in clientaddr;
        socklen_t clientaddrlen = sizeof(clientaddr);
		//4. 接受客户端连接
        int clientfd = accept(listenfd, (struct sockaddr *)&clientaddr, &clientaddrlen);
        if (clientfd != -1)
        {         	
			//只接受连接，不调用recv收取任何数据
			std:: cout << "accept a client connection." << std::endl;
        }
    }
	
	//7.关闭侦听socket
	close(listenfd);

    return 0;
}

客户端

/**
 * 验证阻塞模式下send函数的行为，client端
 * zhangyl 2018.12.17
 */
#include <sys/types.h> 
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <iostream>
#include <string.h>

#define SERVER_ADDRESS "127.0.0.1"
#define SERVER_PORT     3000
#define SEND_DATA       "helloworld"

int main(int argc, char* argv[])
{
    //1.创建一个socket
    int clientfd = socket(AF_INET, SOCK_STREAM, 0);
    if (clientfd == -1)
    {
        std::cout << "create client socket error." << std::endl;
        return -1;
    }

    //2.连接服务器
    struct sockaddr_in serveraddr;
    serveraddr.sin_family = AF_INET;
    serveraddr.sin_addr.s_addr = inet_addr(SERVER_ADDRESS);
    serveraddr.sin_port = htons(SERVER_PORT);
    if (connect(clientfd, (struct sockaddr *)&serveraddr, sizeof(serveraddr)) == -1)
    {
        std::cout << "connect socket error." << std::endl;
        close(clientfd);
        return -1;
    }

    //3. 不断向服务器发送数据，或者出错退出
    int count = 0;
    while (true)
    {
        int ret = send(clientfd, SEND_DATA, strlen(SEND_DATA), 0);
        if (ret != strlen(SEND_DATA))
        {
            std::cout << "send data error." << std::endl;
            break;
        } 
        else
        {
            count ++;
            std::cout << "send data successfully, count = " << count << std::endl;
        }
    }

    //5. 关闭socket
    close(clientfd);

    return 0;
}

最后当客户端的操作系统缓冲区内的数据满的时候，进行阻塞了；
满了之后阻塞，直到数据被recv

参考：
https://codeantenna.com/a/SdTXxsiy6g