Re: Fun trick

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
--- lib.pas-orig        2025-01-09 07:34:25 -0500
+++ lib.pas     2025-01-09 07:37:14 -0500
@@ -1,19 +1,11 @@
-[inherit('sys$library:starlet')]
-module lib(input, output);
-
-[global]
-procedure ooops;
-
-begin
-   writeln('This is a shareable image to link against not run');
-   $exit(ss$_normal);
-end;
+program lib(input, output);
  [global]
  procedure say;
-
  begin
     writeln('Hi');
  end;
+begin
+   writeln('This is a shareable image to link against not run');
  end.
$
...
$ def/user libshr sys$disk:[]libshr.exe;
$ r prg
Hi
$ r libshr
This is a shareable image to link against not run
$
and
$ gdiff -ub lib.for-orig lib.for
--- lib.for-orig        2025-01-09 07:43:03 -0500
+++ lib.for     2025-01-09 07:43:25 -0500
@@ -1,7 +1,5 @@
-      subroutine ooops
-      include '($ssdef)'
+      program ooops
        write(*,*) 'This is a shareable image to link against not run'
-      call sys$exit(ss$_normal)
        end
  c
        subroutine say
$
...
$ def/user libshr sys$disk:[]libshr.exe;
$ r prg
Hi
$ r libshr
This is a shareable image to link against not run
$
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.
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". If This often ends up in an unexpected error, if a user (accidently) runs it as main image: the first (static) function may expect arguments and may therefore miserably fail. I bet that there are only a few shareable images that are prepared for being run as a main image.
For C this works at least on Alpha, IA64 and x86. On the latter two this is more visible, because there is a VMS weak symbol in the symbol table - of the object module. On Alpha the information is in the EOBJ$C_EEOM record (and on VAX I assume it is the OBJ$C_EOM record).