Comms port code locks if no device return

knobby67 · 05-01-2009, 11:35 AM

Hi all,
A while ago I did some code to talk to comms ports. It appeared to work fine, except I discovered that my receive functions locks up if no data is being sent from an rs232 device?

Basically I use open,read and write function. The problem is the read function which hangs if no data is being sent. So I can't even error.

I set up the port with an open (and all the rest of the baud rate stuff)

Code:

(*comlist)->id = open((*comlist)->portname, O_RDWR | O_NOCTTY | O_NDELAY);

I've also tried adding a non block but this has no effect

Code:

(*comlist)->id = open((*comlist)->portname, O_RDWR | O_NOCTTY | O_NDELAY | O_NONBLOCK);

I read with

Code:

read(port->id, readletter, 1)

This just hangs on read unless I connect a device which is sending data.
Is there a way to make read return if nothing is there? I thought I would get a -1 returned but this does not appear to be the case. Thanks

wje_lq · 05-01-2009, 09:39 PM

Try doing the read() only if select() tells you that data is available.

knobby67 · 05-02-2009, 07:23 AM

Thanks found it you need to set F_SETFT with fcntl function