select/poll/epoll on serial port

In this article, I will use three asynchronous conferencing--select, poll and epoll on serial port to transmit data between PC and Raspberry pi.

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Outline

1. Character device file of serial port
   
2. Naive serial communication
   
3. Asynchronous conferencing
   
4. Select
5. Poll
6. Epoll

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Character device of serial port

My device is Raspberry pi with debian system and PC with ubuntu12.04 system.

And I have used a USB-TTL to link the these device.

The character device files on the two device is :

/dev/ttyUSB0 #Ubuntu

/dev/ttyAMA0 #Debian Raspberry pi

These two files are what we must use to achieve the lab.

But there is a little trap of /dev/ttyAMA0.

By default, Raspberry pi uses /dev/ttyAMA0 as a output of serial. Therefor we could use minicom or putty to control our device. However, we have to modify the default function of serial, since we will use our own method to use serial port.

So we should modify two files:

/boot/comdline.txt

dwc_otg.lpm_enable=0 console=ttyAMA0,115200 rootfstype=ext4 elevator=deadline rootwait console=tty1 root=/dev/mmcblk0p2

Delete console=ttyAMA0,115200

/etc/inittab

T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100

Comment the line above.

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Now, start to code:

Naive version

The naive serial port communication version

Open the device, set the baud rate, and set parity

#include     <stdio.h>      /*標准輸入輸出定義*/
#include     <string.h>
#include     <stdlib.h>     /*標准函數庫定義*/
#include     <unistd.h>     /*Unix標准函數定義*/
#include     <sys/types.h>  /**/
#include     <sys/stat.h>   /**/
#include     <fcntl.h>      /*文件控制定義*/
#include     <termios.h>    /*PPSIX終端控制定義*/
#include     <errno.h>      /*錯誤號定義*/
#define FALSE 0
#define TRUE 1

void set_speed(int fd) {
    struct  termios Opt;
    tcgetattr(fd, &Opt);
    cfsetispeed(&Opt,B115200);
    cfsetospeed(&Opt,B115200);
    tcsetattr(fd,TCSANOW,&Opt);
    return;
}

void set_Parity(int fd) {
    struct termios options;
    tcgetattr(fd, &options);
    options.c_lflag  &= ~(ICANON | ECHO | ECHOE | ISIG);  /*Input*/
    options.c_oflag  &= ~OPOST;   /*Output*/
    tcsetattr(fd,TCSANOW,&options);
    return;
}

int OpenSerial(char *Dev) {
    int fd = open( Dev, O_RDWR );         //| O_NOCTTY | O_NDELAY
    if (-1 == fd) { /*設置數據位數*/
        perror("Can't Open Serial Port");
        return -1;
    }
    else {
        set_speed(fd);
        set_Parity(fd);
        return fd;
    }

}

int main(){
    int fd; 
    ssize_t length;
    char buff[512];
    char *dev ="/dev/ttyAMA0";
    fd = OpenSerial(dev);
    for(;;){
        length = read(fd,buff,sizeof(buff));
        if(length > 0) {
            buff[length] = 0;
            printf("plain:%s\n",buff);
        }
    }
    close(fd);
    exit(0);
}

---

Select version

#include <sys/time.h>
#include <sys/types.h>
#include "serial/serial.h"

int main() {
    int fd;
    fd_set rfds;
    struct timeval tv;
    char buff[512];
    ssize_t length;
    fd = OpenSerial("/dev/ttyAMA0");
   
    for(;;) {
        FD_ZERO(&rfds);
        FD_SET(fd, &rfds);

        //timeout = 5s
        tv.tv_sec = 5;
        tv.tv_usec = 0;
        //Wait for 5 seconds, then go
        int n;
        n = select(fd + 1, &rfds, NULL, NULL, &tv);
        //choose the target from set
        if(n > 0) {
            if (FD_ISSET(fd, &rfds)) {
                length = read(fd, &buff, sizeof(buff));
                buff[length] = 0;
                printf("select:%s\n", buff);
            }
        } else {
            printf("No data within 5 seconds.\n");
        }
    }
    return 0;
}

Poll version

#include <sys/poll.h>
#include "serial/serial.h"
int main(void) {
    struct pollfd fds[1];
    ssize_t length;
    char buff[512];
    fds[0].fd = OpenSerial("/dev/ttyAMA0");
    fds[0].events = POLLIN ;
    for(;;) {
        int n;
        n = poll( fds, 1, 5000);
        //got data, and look up which fd has data, but we just have 1
        if(n > 0) {
        //if( fds[0].revents & POLLIN ) {
            length = read(fds[0].fd, buff, sizeof(buff) );
            buff[length] = 0;
            printf("poll:%s\n",buff);

        } else {
            printf("No data within 5 seconds.\n");
        }
    }
} 

Epoll version

#include <sys/epoll.h>
#include "serial/serial.h"

#define MAXEVENTS 64

int main(void){
    int fd;
    int efd;
    struct epoll_event event;
    struct epoll_event *events;
    int length;
    char buff[512];
    fd = OpenSerial("/dev/ttyAMA0");
    efd = epoll_create1 (0);//initial is 0

    event.data.fd = fd;
    event.events = EPOLLIN | EPOLLET;

    epoll_ctl (efd, EPOLL_CTL_ADD, fd, &event);
    /* Buffer where events are returned */
    events = calloc (MAXEVENTS, sizeof event);

    /* The event loop */
    for(;;) {
        int n;
        n = epoll_wait (efd, events, MAXEVENTS, 5000);
        if(n > 0) {
            length = read(events[0].data.fd, buff, sizeof(buff));
        
            if(length > 0) {
                buff[length] = 0;
                printf("epoll:%s\n", buff);
            }
        } else {
            printf("No data whthin 5 seconds.\n");
        }
    }
    free (events);
    close (fd);
    return 0;
}