Re: Open master pty (/dev/ptmx) non blocking

[christos%astron.com@localhost (Christos Zoulas)]

In article <25389.33419.131713.821547%gargle.gargle.HOWL@localhost>,
Anthony Mallet  <anthony.mallet%laas.fr@localhost> wrote:
>-=-=-=-=-=-
>
>Hi,
>
>I have a piece of software that opens a master pty non-blocking:
>fd = open("/dev/ptmx", O_RDWR | O_NOCTTY | O_NONBLOCK);
>
>The intent is to make further read(2) on the master non blocking. But
>the O_NONBLOCK flag seems to be ignored. Attached is a minimal sample
>C program showing the issue.
>
>Several remarks:
>* open(2) manual does not mention a master pty as special regarding the
>  O_NONBLOCK flag, and even says "this flag also has the effect of making
>  all subsequent I/O on the open file non-blocking",
>* explicitly setting the file descriptor as non-blocking with fcntl(2)
>  works fine,
>* a slave pty has no such issue (O_NONBLOCK in open(2) is honoured),
>* FWIW, linux has no surprise here, the flag behaves as one would expect,
>* POSIX does not mention the flag as supported in posix_openpt(3) (it
>  does not says it's not supported either :).
>
>So, I'm not sure if something should be changed here, and if someone
>is willing to check that? I tried to track down where in the kernel
>this happens, maybe in pty_alloc_master in kern/tty_ptm.c? I really
>don't master those kernel aspects so I'm not confident in providing a
>patch.
>
>I guess the principle of least surprise would say that the flag should
>be supported (e.g. for maximum portability). OTOH, if the current
>behaviour is deemed correct or sufficient, maybe at least some manual
>(posix_openpt, open, ...) should mention this is not supported?
>
>Best,
>Anthony

Fixed in HEAD.

christos