Select in loop – work all the time – linux

I got next question about select: How to make select in loop ?

I try to do like that:

struct timeval timeout;
        int sel;
        size_t rozmiar = sizeof(pid_t);
        char buf[rozmiar];
        int i;

        FD_ZERO(&set);
        for(i = 0; i< val; i++)
        {       FD_SET(fd[i][0], &set); // val -> N pipe2
        }
        timeout.tv_sec = 2;
        timeout.tv_usec = 0;

        while(1)
        {
                sel = select(val+1,&set,NULL,NULL,&timeout);

                if(sel < 0)
                        perror("select");
                else if(sel == 0)
                        printf("No communicate n");
                else{
                        for(i = 0; i < val; i++)
                        {
                                if(FD_ISSET(fd[i][0],&set))
                                {
                                   while(read(fd[i][0],&buf,rozmiar) > 0)
                                   write(1,&buf,rozmiar);
                                } // check if exist and write to stdout
                        }
                } // end SELECT

                timeout.tv_sec = 2;
                timeout.tv_usec = 0;
        }

But there all the time show: ,, no communicate”. Is it the correct way to create select which work all the time? I am not sure so I prefer to ask. I try to find information in books but with no lucky.

Answer

The set is changed by select, you need to refill it each time