FreeBSD programming question

Michael Conlen meconlen at
Wed Aug 6 10:43:18 PDT 2003

select() should work for you, similar to trigering an interrupt. Instead 
of triggering an ISR select() will sleep until there's an event on the 
file descriptors. So you open() the device for the serial port and 
select() on it. When you return from select() the return value will tell 
you why you returned and you handle the situation similar to programing 
for the 8250 (read from the port to see which event).

In any case, you can select() on the file descriptors for the standard 
input and the serial port, though remember that STDIN uses buffered IO 
and open() will return an unbuffered file descriptor, which is what 
select() uses, so you need to find the unbuffered file descriptor for 
the stadard IO, which is either 0, 1 or 2, but I forget which on FreeBSD 
(I've been doing network daemons to much lately).

In any case, you create an FD_SET

fd_set mySet;
FD_SET(fd, &mySet);

where fd is the file descriptor returned from open, or the file 
descriptor for the standard input.

Use the set as a read set with select along iwth a timeout. struct 
timeval is

struct timeval {
        long    tv_sec;         /* seconds */
        long    tv_usec;        /* and microseconds */

if the pointer to the struct timeval is NULL then it waits forever. (or 
until a signal causes an exit).

(Note, usleep() is often implemented using select on no file descriptors 
and a timeval).

int rc;
struct timeval myTimeout;
rc = select(2, &mySet, NULL, NULL, &myTimeout);

This call will return when either timeval is up or there's data to read 
on your file descriptors. Be sure to check errno if select returns -1. 
When select returns the fd_set will be set to the descriptors that are 
actionable. Use FD_ISSET(fd, &mySet) to see if that file descriptor is 
waiting to be actioned on (read, write, or other) until you've found all 
the ones that are ready (the number returned by select()) and do your thing.

There's a really great book called "Advanced Programing in the UNIX 
environment" and it will show you all the system calls you ever needed 
to know to work with UNIX, though it's light on the concurrency issues, 
but it doesn't sound like your writing multithreaded memory shared 
programs so it's no worry.

I haven't really looked at the sio driver, but I doubt it, it still 
works with the 8250, which only had one IO address (tell it what you 
want to do, read the result, tell it what you want to do, send it info, 
tell it what you want to know, read the info it has... ...programing was 
much more fun back then).

J. Seth Henry wrote:

>It appears that my experience on microcontrollers is throwing me off.
>I'm used to having a touch more control at the hardware level.
>It sounds like I would be best served by setting up a loop that sleeps
>for a certain number of milliseconds, and then looks for new data in the
>serial port buffers. Knowing the amount of time per loop, I could handle
>the periodic data polling as well. My largest concern was in creating a
>CPU hog. I don't want to slow the system down by constantly accessing
>the serial port.
>It occurred to me that I may be able to deal with this another way. I
>can poll the thermostat for MOST things, only the user interface
>requires fairly speedy interactions. I can simply listen for the "ENTER"
>button, and then increase the polling rate until the UI exits.
>As it were, I'm poking around in the ports to see how other programs
>have dealt with this.
>Just out of curiousity, since I can check the driver source, does the
>sio driver add any additional buffering, or does it simply read the
>16byte FIFO on the serial port? Most of the messages I am expecting
>should fit in that FIFO anyway.
>Seth Henry
>On Wed, 2003-08-06 at 09:58, Malcolm Kay wrote:
>>On Wed, 6 Aug 2003 07:00, J. Seth Henry wrote:
>>>Not sure if this is the right list or not, but I could really use some
>>>How can I code trap serial port interrupts in my C program?
>>For any modern hosted system interrupt trapping and servicing is in the 
>>province of the system -- it should not be a userland activity.
>>>For example, I want to read values from a serial device every
>>>user-specified number of seconds, calculate some stuff and then sit for
>>>a while. Should the serial device decide it wants to send some data
>>>unsolicited, I would like to enter an interrupt service routine, handle
>>>the communication, and then return to the previous loop.
>>There are a number of techniques which may or may not suit your needs;
>>it is not too clear just what you are trying to do.
>>Generally the system will provide some buffering of input so it is not usually
>>important that your code processes each character immediately on arrival.
>>In many cases using placing the select(2) system call in a loop will meet the 
>>In more difficult cases you may need to look at threading pthread(3) or 
>>forking fork(2) or vfork(2)
>>>I can get the loop going by using sleep(n), but I don't know how to
>>>write the ISR in C, and (additionally) make it such that it will run on
>>>any *nix like platform.
>>You might be able to do something at system level by adding your driver to the 
>>kernel possibly as a kernel module. This is not generally the way to go if 
>>userland alternatives work and it certainly will be very operating system and 
>>platform specific possibly even requiring significant editing from one OS 
>>version to the next.
>>>Any pointers, HOWTO's, or examples would be greatly appreciated!
>>Malcolm Kay
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