From:	MERC::"uunet!CRVAX.SRI.COM!RELAY-INFO-VAX" 22-APR-1993 09:44:45.68
To:	Info-VAX@KL.SRI.COM
CC:	
Subj:	Re: VMS LINK PROBLEM

In article <1993Apr20.043053.16235@dbased.nuo.dec.com>, 
  winalski@adserv.enet.dec.com (Paul S. Winalski) writes:
> In article <1564@se.alcbel.be>, adc1@se.alcbel.be (Alexander De Clercq) writes:
> |>
> |>I have a question concerning the VMS linker.
> |>We have written some allocation routines (XtAlloc,XtRealloc,...)
> |>which we want to use in our program. However we also need
> |>the sys$share:decw$dwtlibshr.exe shareable image where the
> |>DEC allocation routines are residing.
> |>
> |>Is there any way (via the linker) to use our own routines instead 
> |>of the DECWindows routines ?
> 
> The only way to do this is to not link against DECW$DWTLIBSHR.EXE directly.
> Instead, use calls to LIB$FIND_IMAGE_SYMBOL to activate the DWTLIBSHR image,
> and to look up the addresses of the entry points in it that you wish to call.
> Even then, of course, the DWTLIBSHR image itself will not be using your
> XtAlloc and XtRealloc--it will be using its own and there's no way you can
> change that.
Maybe - maybe not.  Without actually looking at the source [and perhaps you
have], I couldn't say for sure, but I've found the following to generally
work quite well where other things have failed [excepting protected images]:
  . Use LIB$FIND_IMAGE_SYMBOL to activate the target image you want to
    alter & find the entrypoint addresses for the target routines.
  . Drop into the debugger & disassemble the first instruction or so...
    the objective here is to locate the first 'real' executable instruction
    in the target routine... i.e. the first thing you see will usually be a
    jmp l^xxxxx instruction [for which the target can easily be calculated]
    that jumps to the first 'real' instruction.
  . Modify the page protection on the page containing the first 'real'
    instruction of the target routine using the following method:
    : Make a copy of the page
    : Do a $DELTVA on the page
    : Do a $CRETVA on the page
    : Restore the original contents of the page
  . Write a JMP @#xxxxxxxx instruction over the first instruction[s] of the
    modified target routine's original code... where 'xxxxxxxxx' is the
    virtual address of your replacement code.
Comments?  I realize it's kind of nasty, but it can be a neat trick to pull
if you're trying to do an image-global replacement of a faulty RTL routine
like C's getenv(). :(
> --PSW

In reference to the original post, the 'best' solution would be to rename
the site-local routines & not require DWTLIBSHR routines to be replaced
with the site-local ones.
-- 
David G. North, CCP
D_North@onDEC.LoneStar.Org {...!letni!tachyon!ondec!d_north}
onDEC systems                                 "Everyone's crazy but you & me...
P.O.Box 795848, Dallas TX  75379-5848          and come to think about it, I'm
(214)985-1316 ... (214)902-3957                not so sure about you either!"