Blocking

That is, data cannot be added or deleted , Always drive here , Until the data can be added or deleted ;
If it is Send blocking 【send】, That is, the buffer in the operating system Full of , Unable to continue add to To the buffer in the operating system , Will block in the present , Until the buffer in the operating system is free ;
If it is Receive blocking 【recv】, That is, the buffer in the operating system empty , Unable to continue obtain Data to the buffer in the operating system , Will block in the present , Until the buffer in the operating system has buffered data ;

Example

Server side

#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. Create a listener socket
    int listenfd = socket(AF_INET, SOCK_STREAM, 0);
    if (listenfd == -1)
    {
        std::cout << "create listen socket error." << std::endl;
        return -1;
    }

    //2. Initialize server address 
    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. Start listening 
    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.  Accept client connections 
        int clientfd = accept(listenfd, (struct sockaddr *)&clientaddr, &clientaddrlen);
        if (clientfd != -1)
        {         	
			// Accept connections only , Do not call recv Receive any data 
			std:: cout << "accept a client connection." << std::endl;
        }
    }
	
	//7. Turn off listening socket
	close(listenfd);

    return 0;
}

client

/**
 *  Verify that in blocking mode send Behavior of functions ,client End 
 * 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. Create a socket
    int clientfd = socket(AF_INET, SOCK_STREAM, 0);
    if (clientfd == -1)
    {
        std::cout << "create client socket error." << std::endl;
        return -1;
    }

    //2. Connect to server 
    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.  Constantly send data to the server , Or exit with an error 
    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.  close socket
    close(clientfd);

    return 0;
}

Finally, when the data in the operating system buffer of the client is full , It's blocking ;
Blocked when full , Until the data is recv

Reference resources ：
https://codeantenna.com/a/SdTXxsiy6g