Programming :: Open /dev/ttyS0 Hangs

I have a problem with programming serial port lines. my issue is that i must toggle and read single lines like rts and cts. but my program stops at the line when i open the port. here my sampe:

Code:
int port;
int i;
port = open("/dev/ttyS0", O_RDWR);

[Code].....

when I start the program it hangs on the open-function. I have tried ps with grep for ttyS0 but there is nothig found. Why hangs this function and gives me no error?

If I use ttyS1, it works. where is the problem and why I can't get an error?

target is an embedded system with busybox.