problem with select() - 2.4.5

[Thomas Speck]

Hi !
I have a problem with reading from a serial port using select() under
2.4.5. What I am doing is basically the following: 

fd_set readfds;
struct timeval timeout;
int s;

serialfd = open("/dev/ttyS0", O_RDWR );

init_serial(B9600);

timeout.tv_sec = 2; /* ! */
timeout.tv_usec = 0;
FD_ZERO(&readfds);
FD_SET(serialfd,&readfds);

s=select(serialfd+1, &readfds, NULL, NULL, &timeout);
...

But s is always equal to 0 even when I am sure there are data to read.
If I use 

s=select(serialfd+1, NULL, &writefds, NULL,  &timeout);

(with the corresponding initialisation of writefds) it returns s=1 and I
can write to the serial port. I can see that since the lights of the modem
are flashing. 
I noticed that behavior since I tried to send some "ATZ" with the
write-function but I never got the "OK" back.

However, the same programme works under 2.2.19.

Any help, please ?

--
Thomas