filedesc struct in filedesc.h

[Yan Yu]

Hello, all,
  I have a Q on filedesc.h:
  i am wondering whether the order of the field in struct
_filedesc_ (in sys/filedesc.h) matters?
  I added a field to _filedesc_ struct in filedesc.h. It is a dynamically
allocated list, just as fd_ofiles, or fd_ofilefalgs.
I put my new added list right after fd_fileflags, and allocate/initialize
it just as fd_ofiles or fd_ofileflags.
Even if I put little of my
extra code in  except this data field, and codes to allocate and
initialize it ( i put
it in fdgrowtable(), just as the existing kernel initialize fd_ofiles, or
fd_ofileflgs. (i believe i did this allcaotion/initialization  part
right:)
the problem i had is page fault and then crashes the kernel.
If I put my extra data fields at the end of _filedesc_ or _filedesc0_
struct, it then works..

My Q is:
is there places in the kernel that not directly use the field name to
access a member
of "filedesc" struct, but implicitly assuming
the ordering of the fields in "strcut filedesc", e.g., they may use
offset to access a field?
so then my new added field may mess up the original code?

I found one such place in fdgrowtable() in kern_descrip.c
there is one such line:
------------------------------------------
nfileflags = (char *)&ntable[nnfiles];
------------------------------------------
so it allocates and assumes that fd_ofileflags is right after fd_ofiles in
_filedesc_ struct.
I am curious  is there other places in the kernel that implicitly use the info
on the order that those fields are defined?
i hope my Q makes sense..

many thanks,
yan