Trouble reading a character device.

Trouble reading a character device.

[saravanaraj v]

Hi,
   I am facing few issues in reading from a character device.
   I have created few character devices using 'mknod' and I am writing
continuously to those devices using 'printk'. After some time I start
another process to read from the device. The open/read is done using
normal sys calls for file open/reading. I have pasted the code for
your ref. When I run this program the read happens for some time and
then there is no progress. I am suspecting that  everything written to
the device before this program was started is read. The flags I tried
for opening the file were RD, RDWR with/without NONBLOCK. The output
was same in all the cases. I tried "cu -l /dev/virt_uart8". It was
able to read continuosuly without any problem. Can someone tell where
the problem is ?


int main(){

int fd,counter=0;
fd = open("/dev/virt_uart8",O_RDONLY|O_NONBLOCK);
if(fd<0) printf("\nError in opeining file");
while(1){

                                        int nread = 0;
                                        nread =
read(fd,buffer,MAX_BUFFER_SIZE-1);
                                        if(nread>0) {
                                                buffer[nread]='\0';
                                                printf("%s",buffer);
                                        }

}
}



Thanks,
Saravanaraj

Re: Trouble reading a character device.

[Glynn Clements]

saravanaraj v wrote:

>    I am facing few issues in reading from a character device.
>    I have created few character devices using 'mknod' and I am writing
> continuously to those devices using 'printk'. After some time I start
> another process to read from the device. The open/read is done using
> normal sys calls for file open/reading. I have pasted the code for
> your ref. When I run this program the read happens for some time and
> then there is no progress. I am suspecting that  everything written to
> the device before this program was started is read. The flags I tried
> for opening the file were RD, RDWR with/without NONBLOCK. The output
> was same in all the cases. I tried "cu -l /dev/virt_uart8". It was
> able to read continuosuly without any problem. Can someone tell where
> the problem is ?
> 
> int main(){
> 
> int fd,counter=0;
> fd = open("/dev/virt_uart8",O_RDONLY|O_NONBLOCK);
> if(fd<0) printf("\nError in opeining file");
> while(1){
> 
>     int nread = 0;
>     nread = read(fd,buffer,MAX_BUFFER_SIZE-1);
>     if(nread>0) {
>         buffer[nread]='\0';
>         printf("%s",buffer);
>     }

If you want to print data which doesn't necessarily have a trailing
newline, you need fflush(stdout) or setvbuf(stdout, NULL, _IONBF, 0).

If you make that change and still have problems, it's likely to be an
issue with the device. E.g. you may need to disable XON/XOFF
processing with tcsetattr(), or put the device into raw mode.

-- 
Glynn Clements <glynn@gclements.plus.com>

Re: Trouble reading a character device.

[saravanaraj v]

Thanks Glynn

On Tue, Dec 16, 2008 at 8:21 PM, Glynn Clements
<glynn@gclements.plus.com> wrote:
>
> saravanaraj v wrote:
>
> >    I am facing few issues in reading from a character device.
> >    I have created few character devices using 'mknod' and I am writing
> > continuously to those devices using 'printk'. After some time I start
> > another process to read from the device. The open/read is done using
> > normal sys calls for file open/reading. I have pasted the code for
> > your ref. When I run this program the read happens for some time and
> > then there is no progress. I am suspecting that  everything written to
> > the device before this program was started is read. The flags I tried
> > for opening the file were RD, RDWR with/without NONBLOCK. The output
> > was same in all the cases. I tried "cu -l /dev/virt_uart8". It was
> > able to read continuosuly without any problem. Can someone tell where
> > the problem is ?
> >
> > int main(){
> >
> > int fd,counter=0;
> > fd = open("/dev/virt_uart8",O_RDONLY|O_NONBLOCK);
> > if(fd<0) printf("\nError in opeining file");
> > while(1){
> >
> >     int nread = 0;
> >     nread = read(fd,buffer,MAX_BUFFER_SIZE-1);
> >     if(nread>0) {
> >         buffer[nread]='\0';
> >         printf("%s",buffer);
> >     }
>
> If you want to print data which doesn't necessarily have a trailing
> newline, you need fflush(stdout) or setvbuf(stdout, NULL, _IONBF, 0).
>
> If you make that change and still have problems, it's likely to be an
> issue with the device. E.g. you may need to disable XON/XOFF
> processing with tcsetattr(), or put the device into raw mode.
>
> --
> Glynn Clements <glynn@gclements.plus.com>