How can I clear select() state after reaching timeout in c++ on Raspberry PI 3

Question

I currently work on Raspberry PI 3. In order to loop serial port communication I connected TXD with RXD in GPIO port. So I receive the bytes that I've just sent. In my program I need to use the timeout limitation for reading the port. To do so I used following command select(fd + 1, &read_fds, NULL, NULL, &timeout). If the timeout is met with first byte (i.e. I've sent nothing), I can't read next byte - the state of reading is still "Timeout".

My question is: how can I clear tthis state or what do I have to set to port parameters to recive second byte which was send after reaching timeout during previous port reading. My code bellow:

#include <stdio.h>
#include <errno.h>
#include <termios.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/select.h>

int main ()
{
    struct termios RSopt;
    char byte = 0;
    char str[2] = {0,0};
    int fd;
    fd = open( "/dev/ttyS0", O_RDWR| O_NOCTTY );

    // Parameters setting for serial communication
    tcgetattr(fd, &RSopt);
    cfsetospeed (&RSopt, (speed_t)B300);
    cfsetispeed (&RSopt, (speed_t)B300);
    RSopt.c_cflag &= ~CSIZE;  // Mask out size
    RSopt.c_cflag |= CS8;     // Or in 7-bits
    RSopt.c_cflag |= PARODD;  // Enable Parity - even by default
    tcsetattr (fd, TCSANOW, &RSopt);   // Set new options

    // Timeout setting
    struct timeval timeout;
    timeout.tv_sec = 2;
    timeout.tv_usec = 0;
    // File descriptor initialization
    fd_set read_fds;

    //-----------------------------------------------------------------
    FD_ZERO(&read_fds);
    FD_SET(fd, &read_fds);
    str[0] = 0xFA;
    //write(fd, str, 1);
    //printf("Send.\n");
    //printf("Wait...\n");

    if (select(fd + 1, &read_fds, NULL, NULL, &timeout) == 1) {
        read(fd, &byte, 1);
        printf("Recive: %x\n", byte);
    } else {
        printf("Timeout.\n");
    }

    //-----------------------------------------------------------------
    FD_ZERO(&read_fds);
    FD_SET(fd, &read_fds);
    str[0] = 0xFB;
    write(fd, str, 1);
    printf("Send.\n");
    printf("Wait...\n");

    if (select(fd + 1, &read_fds, NULL, NULL, &timeout) == 1) {
        read(fd, &byte, 1);
        printf("Recive: %x\n", byte);
    } else {
        printf("Timeout.\n");
    }
}

Answer 1

You need to set

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

before each select since some kernels will reduce your timeout with the time spent in the select, leaving you with a zero timeout value if you've already timed out once.

Edit: You can confirm that this is what is happening to you by printing the timeout values before the select that times out directly.