Re: Fun trick

On 1/9/2025 8:01 AM, hb0815 wrote:

On 1/9/25 02:00, Arne Vajhøj wrote:

Hmmm.
>
It works in C, but it does not seem to work in
any other language (tested with Pascal and Fortran).
...
I assume the difference relates to user code main not being
the real program entry.

 So, make it a main entry.
 This is on Alpha. It should work on IA64 and x86. I don't have access to any of the latter systems. If this does not work on these systems, I know how to make it work, anyway.
 $ gdiff -ub lib.pas-orig lib.pas

$ gdiff -ub lib.for-orig lib.for

Also, I should have mentioned ... Your shareable with printing a message from init code can not be used as a "normal" shareable image. The message will always be printed. That's probably not what you want.

 >
 > Init code of an image is always run. For a shareable image it is run at
 > activation time, for a main image it is run at image startup time. For
 > some reasons I distinguish these two phases although others prefer to
 > say that "startup" is part of activation.
 >
 > My shareable image only prints its message if a user (accidently) runs
 > it as main image.
My Pascal code a few revisions back looked like:
    lib$getjpi(item_code := jpi$_imagname, resultant_string := imgnam.body, resultant_length := imgnam.length);
    if index(imgnam, ']libshr.EXE') > 0 then begin
       writeln('This is a shareable image to link against not run');
       $exit(ss$_normal);
    end;
Maybe not elegant, but it did check.

All this works because VMS defines a weak transfer (or entry) address. The C compiler in absence of a "main" assigns this weak transfer to the first function seen in a source module. (I admit, I initially didn't test this with other compilers; obviously FORTRAN and PASCAL do not define this). The linker keeps track of the first weak transfer it encounters and uses it as image transfer, if there is no "strong" transfer, in C no "main". This makes the shareable image "runnable".

There are probably a lot of shareable images written in C out
there where the developer has no idea that they are runnable or
what code will run.
Arne