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Subject:

Re: Dynamic loading of shared object files in Fortran (how?)

From:

"Russell, Richard" <[log in to unmask]>

Reply-To:

Fortran 90 List <[log in to unmask]>

Date:

Thu, 10 Nov 2005 14:44:32 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (64 lines)

On Nov 9, 2005, at 5:20 PM, Neil Carlson wrote:



> Some software apps are made user-extensible by providing a mechanism 

> for dynamically loading a user-written shared object file into the 

> executable code.  I'm know the details are platform-specific, and 

> almost certainly involve some special C code, but can anyone point me 

> to some how-tos, examples, etc., especially when it is done in the





This is reminiscent of something I used back in the days of VM/370 on

IBM mainframes. In building the original executable, you could tell the

link editor to keep the symbol table with the executable, and at run

time the table would be loaded into high memory, with code in low

memory, replicating how the link editor built the thing in the first

place. A SVC assembler call could invoke the LOAD command of CMS to load

in an object file created by the compiler, as though the linking process

were still underway. The new object would be loaded above existing code

and relocated, and external references specified within the new object

file would be resolved using the existing symbol table. The address of

where the LOAD processing put the loaded object file was returned to the

caller, which would save the address. The main group of subprograms, by

design, provided for a number of stubs for "user-written routines," with

standardized names like "USER01." These were used by passing the name to

the assembler routine that managed the loading:

      CALL LINKER('USER01', arg1, arg2, ....)

LINKER would cause the LOAD to occur the first time and save the address

of what got loaded. On subsequent calls, the already loaded routine

would simply be called directly and given the rest of the argument list.

Running things this way allowed the routine loaded on the fly to

reference various utility/service routines previously linked with the

executable, through the in-memory symbol table. Thus the main package

could be designed to handle nearly all of what most users needed done,

while providing the ability for power users to write their own routines

for special calculations. By providing documented interfaces to standard

facilities within the main package, not only could the main package call

user-written routines to do extra things, those routines could call back

to routines already in the main package to do part of the calculations.



Since this sort of thing is technically feasible, it really would be

nice if the vendors of linkers for Windows and other operating systems

provided the equivalent facility for developers. Have they?



Dick Russell



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