Thread: read() is not reading the whole data

Hi all,

I am doing serial port programming very first time.
what i am trying to do is some machine is there which is communicating to the computer(LINUX) through serial port. whatever data machine writes comes on a device file "/dev/ttyS0".
now after setting all the configuration settings, i am trying to read the file.
suppose machine writes 240 bytes on the file. but when i read that file in a buffer it gives only first 8 bytes. rest of data doesnt comes.

one thing i would also like to say that , the behavior of this program is very confusing to me.
like sometime after doing some manipulation it goes working in right way. but as i close the terminal and restart the program again it shows only 8 bytes of data. machine is inserting some carriage return ,linefeed also.

with read even EOF is not working.

Code:

int main(){
        struct termios new_options;
        int fd;         /* File descriptor for the port         */
        int i,t=0,b_read=0;
        char c[240];
        char *outbuf=NULL,cr;
        cr='\xd';
        outbuf=(char*)malloc(10);
        bzero(c,240);
        //Open the first serial port, read/write, no controlling terminal,
        // not care about the Data Carrier Detect signal
                                                                                                 
        fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY |O_ASYNC);
        if (fd == -1){
        // Could not open the port.
         perror("Unable to open /dev/ttyS0\n");
 }
        else {
        // Set all flags to 0 for the read call to be blocking!???
              fcntl(fd, F_SETFL, 0);
        // Get the current options for the port...
              tcgetattr(fd, &new_options);
        // Set the baud rates to 2400...
              cfsetispeed(&new_options, B2400);
              cfsetospeed(&new_options, B2400);
        // Enable the receiver and set local mode...
              new_options.c_cflag |= (CLOCAL | CREAD);
        // Set the option for 8N1
              new_options.c_iflag |= IGNPAR;
              new_options.c_cflag &= ~PARENB;
              new_options.c_cflag &= ~CSTOPB;
              new_options.c_cflag &= ~CSIZE;
              new_options.c_cc[VTIME]=20;
              new_options.c_cflag |= CS8;
        //Set the option for raw input
              new_options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
        // Set the option for raw output
              new_options.c_oflag &=~OPOST;
                                                                                                 
        // Set the new options for the port...
              tcsetattr(fd, TCSANOW, &new_options);
      //we r sending password to the machine to active for furthe communication.
              bzero(outbuf,10);
              sprintf(outbuf,"$U876",5);
              write(fd,outbuf,5);
              write(fd,&cr,1);
        }
                                                                                                 
                do{
                t=read(fd,c,sizeof(c));
                printf("%d\n",t);
                }while(t==240);
                                                                                                                                                                                                
        for(i=3; i<=240; i++){
                printf("%c",c[i]);
                }

 close(fd);
                                                                                                     
        return 0;
}

thanks in advance.

Thanks a lot dear, but my problem isn't solved.

1. It is an assumption on your part that anything if anything outside the machine which sends a block of data, that some part of your machine will magically reassemble those bytes into the SAME sized block. It doesn't work for serial ports, and it doesn't work for networking.

2. 8 bytes is a common buffer size for serial ports. The first 8 bytes are received, then the others are either lost, or are held up because the hardware negated the "CTS" line to cause data transmission to stop, or the low level driver sent "xoff" to pause transmission at the other end.

I tried it in lot of ways but neither a single method is going to solve my problem.
MY problem is under many constraints.
like "/dev/ttyS0" conatins some data

READY SIGNAL

HI PROGRAM. 18:30:12

AD 18:30:10

now first i have to check whether ready comes in file or not (usually it comes when program send password to machine on serial port after which only the machine can send data to "ttyS0").
then whatever data comes i need to parse that in different variables.

please tell me how can i do that.

i have removed this timeout option from my program.

and as a read() case, its behaving anonymously. after giving it that u have to read 240 bytes [read(fd,buff,240);] its reading only 8 bytes. and one amazing behaviour of it is after mutliple run of the program it gives the bytes after those 8 bytes and then after run it give the desired result.

and as again i close the terminal and start the same program it gives again the same first 8 bytes.

i am very confused. about wahts going wrong with it.

Why is this confusing? You're saying, "Hey, read this amount please." Read replies, "Well, when I tried, I got 8 bytes." ... So why are you confused? You're sending a request for up to 240 bytes. Read tells you how many it received. Where's the problem? And why would it be surprising that you get different, or even the same, results from multiple runs? You're communicating with another program. Why does it have to give you what you ask?

I connect to and play various games over the "interweb". Some times it tells me my connection speed is fast. Some times it says it is slow. It's the same game, connecting to the same server. I'm surprised! ... No, wait, I'm not surprised at all. So why are you?


Quzah.