Cannot open() /dev/tty0

Sometimes I cannot open() /dev/ttyN (N=0,1,…)
for read and write.
/ttyN is my current tty;

I found out that I cannot open it,
because its attributes are “write only”.
/normaly (in 99% of cases) it has rw attributes;

What and why change /dev/ttyN atributes to wr only ?

Try: open 1 pterm>mqc>see /dev/ttyp0 w-only ttyp1-rw,
open 2 pterm>mqc>see /dev/ttyp1 w-only… and so on.
So if /dev/ttyN is in used it wr.

so everytime when I logon ttyN a have to change its attributes to a+rw ?

I think, that you may open ttyNX for read only from one process…