RTEMS5 and file descriptors

[Miroslaw Dach]

Thank you Joel for all of the hints.
I find the maximum number - 64 - of file descriptors somehow low when
compared with Linux and RTEMS 4
(RTEMS 4 did not have the restriction with FD_SETSIZE).

Anyhow, my application is EPICS base and most of the open file
descriptors refer to the clients which connect using sockets.
I will place the EPICS channel access gateway "in front" of my RTEMS epics
server to narrow down the number of clients to just one.
This is just a work around but should satisfy the problem.

Best Regards
Mirek



pon., 17 paź 2022 o 16:20 Chris Johns <chrisj at rtems.org> napisał(a):

> On 18/10/2022 9:19 am, Miroslaw Dach wrote:
> >>AFAIK you'd have to patch the header in the C Library when building the
> tools
> > using the RSB to have a possible clean solution. Editing the installed
> header
> > would be uncool.
> > I see , I thought that it is somehow simpler thing.
> >>How many descriptors do you need? And will you be using select()?
> > I need maximum 128 file descriptors to be used.
>
> The issue is a little more complicated because of the way the fd number are
> allocated. RTEMS implements not reusing fd number once free (closed)
> preferring
> to move to an unused fd number until all numbers have been used. The idea
> is to
> try and catch the accidental continued use of an fd after close when an
> open
> follows.
>
> If you allocate 200 descriptors the following select code will work:
>
> /* RTEMS entry point */
> void Init(void) {
>   /* start networking */
>   int fd = socket(...);
>   FD_ZERO(&set);
>   FD_SET(fd, &set);
>   nfds = sd + 1;
>   r = select(nfds, &set, NULL, NULL, NULL);
>   /* blah blah */
> }
>
> while this will not:
>
> /* RTEMS entry point */
> void Init(void) {
>   /* use up all the fd spots select has by default */
>   for (i = 0; i < 65; i++) {
>     fd = open("afile", ...);
>     close(fd);
>   }
>   /* start networking */
>   fd = socket(...);
>   FD_ZERO(&set);
>   /* out of range access */
>   FD_SET(fd, &set);
>   nfds = sd + 1;
>   rv = select(nfds, &set, NULL, NULL, NULL);
>   /* blah blah */
> }
>
> If you control the code in quesiton and can make changes the following are
> some
> suggestions:
>
> 1. Consider a thread per descriptor. Select is slow
>
> 2. Look at kqueue, it is a better interface for this type of blocking
>
> 3. Look at the following select work around ..
> https://git.rtems.org/rtems/tree/cpukit/mghttpd/mongoose.c#n1522
>
> Chris
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.rtems.org/pipermail/users/attachments/20221020/c5117323/attachment.htm>