Linux Serial Port: missing data on input

Question

    #include <fcntl.h>   
    #include <termios.h> 
    #include <unistd.h>  
    #include <errno.h>   
    #include <cerrno>
    #include <iostream>
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <fstream>
    #include <sys/types.h>
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <cstdlib>
    #include <ctime>

    using namespace std;
    typedef struct termios ComSet;

    int main()
    {
    int acm=-1;                                   
    acm=open("/dev/ttyACM0",O_RDWR | O_NOCTTY);
    if(acm == -1)
    {
    cout<<"Error Opening ttyACM0"<<endl;
    exit(1);
    }
    else
    {
    cout<<"Preparing ttyACM0..."<<endl;
    ComSet SerialSettings;
    tcgetattr(acm, &SerialSettings);
    cfsetispeed(&SerialSettings,B9600);
    cfsetospeed(&SerialSettings,B9600);

 /* 8N1 Mode */
    SerialSettings.c_cflag &= ~PARENB;   
    SerialSettings.c_cflag &= ~CSTOPB;   
    SerialSettings.c_cflag &= ~CSIZE;   
    SerialSettings.c_cflag |=  CS8;     

    SerialSettings.c_cflag &= ~CRTSCTS;       
    SerialSettings.c_cflag |= CREAD | CLOCAL; 


    SerialSettings.c_iflag &= ~(IXON | IXOFF | IXANY);          
    SerialSettings.c_iflag &= ~(ICANON | ECHO | ECHOE | ISIG);  
    SerialSettings.c_oflag &= ~OPOST;                           

 /* Setting Time outs */
    SerialSettings.c_cc[VMIN]  = 10;     /* Read at least 10 characters */
    SerialSettings.c_cc[VTIME] = 0;     /* Wait indefinetly   */

    if((tcsetattr(acm,TCSANOW,&SerialSettings)) != 0)   
    {
        cout<< " ERROR ! in Setting attributes"<<endl;

    }
    else
    {
        cout<< "=======ttyACM0 Setting====="<<endl;
        cout<<"BaudRate = 9600 StopBits = 1 Parity = none"<<endl;
        cout<<"Reading ttyACM0... "<<endl;
        char read_buffer[1024];                                
        bzero(read_buffer,1024);
        int read_bytes=0;                                      
        while(1)
         {
                tcflush(acm, TCIFLUSH);                        
                read_bytes=read(acm,&read_buffer,1024);        
                if(read_bytes>0)
                {
                    cout<<read_buffer;
                    bzero(read_buffer,1024);
                }

         }
    }



  close(acm); /* Close the serial port */

}

return 0;
}

for some reason code its not working properly im trying to read from an arduino as it was working properly earlier its giving me grabled output such as

lo World ! Hello World ! ello World ! llo World ! llo World !

one thing that changed was

there is one extra new line in terminal output this is using cat /dev/ttyAMC0

Hello World !

Hello World !

Hello World !

Hello World !

Hello World !

i want to read line by line and as it was sent by the arduino to terminal

Answer 1

When you get garbled output like lo World ! Hello World ! ello World ! lo World ! Hello World ! ello World ! then that usually indicates that your clock speed (baud rate) isn't matched. Make sure your Arduino is writing at a speed of 9600 baud.

Additionally, when the Arduino outputs characters, it puts them in a hardware buffer. The read() function takes the number of bytes you specify out of that buffer- in this case 1024. But you probably don't want to read 1024 bytes every time. Instead, I think you should read until you reach a newline character. See if there is a readline() function in the library you are using.

The Linux cat command is reading correctly, but the command itself adds an extra newline at the end.