Re: result=read(fd,buffer,200);

[Gerald Emig <gme@emig-software.de>]

HI,

According to the documentation provided in info:libc, read 
returns the number of character it actually has read.
Especially if you are using serial ports, the amount read is often fewer
as you expected.

Additionally, checking errno is stronly recommended.

Use something like the following to read exactly 200 chars:


int read_amount (int iwantread)
{
	int readpointer, fetch;
readpointer=0;
while (iwantread>0) {
	fetch=read (fd,&buffer[readpointer],iwantread);
	if (fetch >= 0) {
		readpointer += fetch;
		iwantread -= fetch;
	} else 	if ((errno!=EINTR) && (errno!=EAGAIN)) return -1;
	}
	return iwantread; // should be 0 on success
}

		
Best regards,

Gerald Emig





On Tue, 20 Jul 2004 15:12:24 +0200
"Sayang Oin" <sayangoin@hotmail.com> wrote:

> Hello,
> 
> I would like to read for example 200 bytes from my UART device
> throught the serial port.
> 
> I use
> 
> result=read(fd,buffer,200);
> 
> I don't know why the result always lesser then 200 bytes.
> 
> my open_port  is like this:
> 
> int open_port ()
> {
> 
>   struct termios options, oldtio;
> 
>   int fd;
> 
>   fd = open (COM,O_RDWR | O_NOCTTY );
> 
> 
> if (fd < 0)
> {
>   fprintf(stderr, "Open_port: unable to open %s\n", COM);
>   exit (1);
> }
> else
> {
>   fcntl (fd, F_SETFL, FNDELAY);
>   printf("\nopen_port: 
> %s,%s,%d,%d,%s\n",COM,BAUD,stopbits,datenbits,PARITY);
> }
> 
> bzero(&options, sizeof(options));
> 
> 
> if(!strcmp(BAUD,"B9600"))
>    {
> 	cfsetispeed (&options, B9600);
> 	cfsetospeed (&options, B9600);
>    }
> if(!strcmp(BAUD,"B19200"))
>    {
> 	cfsetispeed (&options, B19200);
> 	cfsetospeed (&options, B19200);
>    }
> if(!strcmp(BAUD,"B38400"))
>    {
> 	cfsetispeed (&options, B38400);
> 	cfsetospeed (&options, B38400);
>    }
> if(!strcmp(BAUD,"B57600"))
>    {
> 	cfsetispeed (&options, B57600);
> 	cfsetospeed (&options, B57600);
>    }
> if(!strcmp(BAUD,"B115200"))
>    {
> 	cfsetispeed (&options, B115200);
> 	cfsetospeed (&options, B115200);
>    }
> 
> 
> 
> switch(stopbits)
> {
>    case 1:
>     options.c_cflag &= ~CSTOPB;
>     break;
>    case 2:
>     options.c_cflag |= CSTOPB;
>     break;
> }
> 
> options.c_cflag &= ~CSIZE;
> switch(datenbits)
> {
>   case 5:
>     options.c_cflag |= CS5;
>     break;
>   case 6:
>     options.c_cflag |= CS6;
>     break;
>   case 7:
>     options.c_cflag |= CS7;
>     break;
>   case 8:
>     options.c_cflag |= CS8;
>     break;
> }
> 
> if(!strcmp(PARITY,"even"))
>              {
> 	      options.c_cflag |= PARENB;
> 	      options.c_cflag &= ~PARODD;
>              }
> else if(!strcmp(PARITY,"odd"))
>              {
> 	      options.c_cflag |= PARENB;
> 	      options.c_cflag |= PARODD;
>              }
> else if(!strcmp(PARITY,"no"))
>              {
> 	      options.c_cflag &= ~PARENB;
>              }
> 
> options.c_cflag &= ~CRTSCTS;
> 
> 
> options.c_cflag |= (CLOCAL | CREAD);
> 
> options.c_lflag &= ~(ICANON | ECHO | ECHOE |ISIG);
> 
> cfmakeraw (&options);
> 
> tcflush(fd, TCIFLUSH);
> tcsetattr (fd, TCSAFLUSH, &options);
> 
> return fd;
> }
> 
> 
> Thanks for any advice..
> 
> Sayangoin
> 
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