[lug] q: export/import of variables

[Chan Kar Heng]

thanks..
but i'm not sure if it's what i'm looking for...

perhaps we have differing levels of the term 'general'.. :)

"RTLD_GLOBAL - The object's global symbols are made
available for the relocation processing of any other object.
In addition, symbol lookup using dlopen(0, mode) and an
asso- ciated dlsym(), allows objects loaded with
RTLD_GLOBAL to be searched. "

it really doesn't give enough insight...

under windows global identifiers are often defaulted
to dllimport or dllexport types depending on the
build target (executable or dll) ....

when an executable is linked with an import library
of a dll, what gets linked in are things somewhat like
proxy functions.
these "proxy functions" will call the actual functions
in respective dlls after dlls are loaded and symbols
are resolved.

the case might be simpler with exported functions,
function symbols from import libraries may actually
be function pointers, or are actual proxy functions
ultimately calling function pointers.
in this case, symbol resolution might only involve
redirecting pointer to functions.

it is however different with exported variables...
i get the idea that things like:

extern int myint;

... where myint is an int variable from another dll.
it would eventually be converted into some sort of
"proxy function" (?) during either compilation or
linking. this "proxy function" then allows access
to the actual variable from the dll.
the way it could be done on some other platform
is to have:

#define int int *
#define extern
extern int myint;
#undef extern
#undef int
#define myint *myint

.. and then resolve myint to point to the actual myint
from the dll after the dll is loaded.

otherwise, of none of those are done, when an
import library is linked in, there would be 2 instances
of a variable (one in the dll and one in the executable).
each (executable and dll) would only be accessing
and changing its own copy of the variable...

so i'm just curious as to how this is avoided under linux
and most other *nix systems considering they only
require an extern tag minus things like dllimport and
dllexport.

(i only know of these through other mailing lists and
don't know of any actual references to specs detailing
these yet, otherwise, i could have incorrectly said
many things there).  :)



At 2002/12/05 03:35, you wrote:
>Chan Kar Heng wrote:
>>hi
>>would generally like to know how linux/elf binaries
>>export variables in such a way that doesn't require
>>dllimport/dllexport attributes.
>>i'll probably dig up the elf binary specification next
>>on this...
>>thanks in advance...
>>rgds,
>>kh
>
>Check out the RTLD_GLOBAL option (bitmask) in dlopen.
>
>D. Stimits