select call on a UDP socket with BSD library?

Chris Johns chrisj at
Tue Sep 1 02:48:25 UTC 2020

On 1/9/20 6:14 am, Cudmore, Alan P. (GSFC-5820) wrote:
> I am having problems using the “select” call with an RTEMS + LibBSD network socket.
> Here is what I am using:
> RTEMS 5.1 RSB, RTEMS source, and RTEMS LibBSD release files.
> ARM/Beagleboneblack + LibBSD
> I have verified that the network is initialized (ethernet device on the
> beaglebone) and I can receive UDP packets from the network.

Great. I use the shell and the `ifconfig` and `netstat` commands. You also have
`tcpdump` which is rather useful.

> I’m trying to use “select” to check the socket (using a timeout of zero), but
> when I call select I get a -1 return code with EBADF errno.

I suspect you have hit the FD set size limit.

> The fd in this instance is 77 and I have RTEMS setup for 200 file descriptors.
> Are there any other LibBSD configuration parameters I should be aware of?

I am sorry but 77 is to high because newlib by default limits the fd set size to

#  ifndef       FD_SETSIZE
#       define  FD_SETSIZE      64
#  endif

I suggest you define `FD_SETSIZE` on the command line to the compiler to 256 and
see what happens. It should be fine to do this.

> When I build a SPARC/LEON3 BSP with the RTEMS legacy network stack, the select
> call seems to work. Admittedly I am just doing this on the loopback interface on
> QEMU since I do not have a real or simulated ethernet device to use. But the
> select call seems to work as I would expect in this instance.

What is the fd for that stack? I suspect the fd is less than 64.

> Is there any setup I need to do, or perhaps the select call operates differently
> with LibBSD? 

The only difference I know of is signals are not supported in libbsd so a signal
will not wake a select call.

For code you write we suggest you look at the kqueue set of calls. These provide
a better and more efficient interface for blocking on sockets.

> Does it work on a UDP socket?


> If I wanted to continue debugging, is the select call in the libbsd source tree?

Yes select is in libbsd and called kern_select ...

Notice the calls to check the fds .. select_check_badfd(). The code is a little
lower down in the file.


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