Hi

I have opened a serial port and then blocked on a select system call .
but select returns even if no data is comming from other end. So when
I read the buffer after select returns, read gives the number of 0s in
buffer and this continues very fast.

void main()
{
int fd_uart =-1 ;
int retval,cErr;
fd_set read_fds;
int   max_fd = 0;

/* intialize the data set */
if( fd_uart = open("/dev/ttyS0", O_RDWR | O_NOCTTY ) == -1)
{
exit(0);
}

FD_ZERO( &read_fds );
FD_SET(fd_uart, &read_fds );
max_fd = fd_uart;

while(1)
{  /* This select returns very fast*/
select( max_fd + 1, &read_fds, 0, 0, NULL);
printf("\nData\n");
}


}

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On Jun 19, 12:30 pm, alok <alok....@gmail.com> wrote:
> Hi
>
> I have opened a serial port and then blocked on a select system call .
> but select returns even if no data is comming from other end. So when
> I read the buffer after select returns, read gives the number of 0s in
> buffer and this continues very fast.
>
> void main()
> {
>   int fd_uart =-1 ;
>     int retval,cErr;
>     fd_set read_fds;
>     int   max_fd = 0;
>
>     /* intialize the data set */
>       if( fd_uart = open("/dev/ttyS0", O_RDWR | O_NOCTTY ) == -1)
>    {
>      exit(0);
>    }
>
>     FD_ZERO( &read_fds );
>     FD_SET(fd_uart, &read_fds );
>     max_fd = fd_uart;
>
>    while(1)
>    {  /* This select returns very fast*/
>       select( max_fd + 1, &read_fds, 0, 0, NULL);
>       printf("\nData\n");
>    }
>
> }

You shouldn't coding like this, you should look at the return value to
check why select returns.

man select

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In article <1182227444.270607.280580@i38g2000prf.googlegroups.com>,
alok  <alok.net@gmail.com> wrote:
>Hi
>
>I have opened a serial port and then blocked on a select system call .
>but select returns even if no data is comming from other end. So when
>I read the buffer after select returns, read gives the number of 0s in
>buffer and this continues very fast.
>
>void main()

In a C program, main returns int.

>{
>  int fd_uart =-1 ;
>    int retval,cErr;
>    fd_set read_fds;
>    int   max_fd = 0;
>
>    /* intialize the data set */
>      if( fd_uart = open("/dev/ttyS0", O_RDWR | O_NOCTTY ) == -1)

The relative precedence of = and == is not what you think. You've set fd_uar
t
to either 0 or 1, depending on whether the open failed.

gcc -Wall says:
warning: suggest parentheses around assignment used as truth value

>   {
>     exit(0);
>   }

If the open failed, fd_uart was set to 1 and then the if() condition was tru
e
so you've exited. If you got this far, the open must have succeeded, and
fd_uart has been set to the result of the == operator, which is 0.

>
>    FD_ZERO( &read_fds );
>    FD_SET(fd_uart, &read_fds );
>    max_fd = fd_uart;
>
>   while(1)
>   {  /* This select returns very fast*/
>      select( max_fd + 1, &read_fds, 0, 0, NULL);

Now you're selecting on file descriptor 0 (stdin) instead of the one you
opened.

--
Alan Curry
pacman@world.std.com

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