Why nothing is send to the socket after few minutes?

Client send data to socket every 10 sec. Like this:

rv Write: 1
1
rv Write: 1
2
rv Write: 1
3
rv Write: 1
4

Server stops receiving anything from this client after few minutes. Server log:

14:56:07  32
14:56:17  33
14:56:27  34
14:56:37  35
14:56:47  36
14:56:57  37

37 is last data from client. But the program continues send data to the server:

rv Write: 1
38
rv Write: 1
39
rv Write: 1
40
rv Write: 1
41
rv Write: 1
42
rv Write: 1

while(1){
    fd_set set;
    int rv;
    struct timeval timeout;
    int n;

    FD_ZERO(&set);
    FD_SET(sockfd, &set);

    timeout.tv_sec = 10;
    timeout.tv_usec = 0;


    rv = select(sockfd + 1, NULL, &set, NULL, &timeout);
    std::cout << "rv Write: " << rv << std::endl;
    if(rv == -1){
        printf("ERROE select\n");

    }
    else if(rv == 0){
        printf("WRITE timeout\n");

    }
    else{

        //char result[100];


        std::string s = std::to_string(h);
        char const *result = s.c_str();




        n = write(sockfd,result,strlen(result));

        if (n < 0){
            printf("Write error\n");

        }
        else if (n == 0){
            printf("Socket disconnected (write)\n");

        }
        else{
            std::cout << h << '\n';
            //break;
        }


    }


    h++;

    sleep(10);

}

Help to understand what is the problem?