[petsc-dev] PetscDLSym called on an empty handle

[Dmitry Karpeev]

On Thu, Aug 5, 2010 at 11:42 AM, Matthew Knepley <knepley at gmail.com> wrote:

> On Thu, Aug 5, 2010 at 11:40 AM, Dmitry Karpeev <karpeev at mcs.anl.gov>wrote:
>
>> What is the intended behavior of PetscDLSym when called on an empty
>> handle: PetscDLSym(handle=PETSC_NULL, symbol, &value)
>> My understanding was that this should look for symbol in the symbol table
>> of the main executable.
>> This is based on what happens in the Windows case (#ifdef
>> PETSC_HAVE_WINDOWS_H),
>> but when using dlopen (#ifdef PETSC_HAVE_DLFCN_H), this probably won't
>> behave correctly,as currently implemented.
>> This is because the above call will boil down to
>>    value = dlsym((dlhandle_t)0, symbol),
>> which on many systems returns NULL (e.g., on Ubuntu 9.10).
>> There is a relatively easy fix, which is to obtain the handle of the main
>> executable with, for example,
>>   dlhandle = dlopen(0,RTLD_LOCAL|RTLD_LAZY)
>> followed by
>>   value = dlsym(dlhandle,symbol)
>>
>> I have played around with it on my box, and it appears to work.
>> I can push that fix, if that's what we decide we want, and if it doesn't
>> alter the currently expected behavior.
>> There is one caveat: symbols can be dlsym'ed from the executable only if
>> they are exported as dynamically-loadable,
>> which is not the default behavior for linking executable (so as not to
>> pollute the dynamic symbol table).
>> On linux the linker needs to get -Wl,--export-dynamic, but it would be
>> nice to enable some sort of portable option at compile time.
>>
>
> This is sharedLinkerFlags.
>

Sorry, I meant configure-time option.
Does sharedLinkerFlags alter the behavior of CLINKER?
Unless there is another way to link an executable.

Dmitry.


>
>   Matt
>
>
>> Incidentally, currently there appears to be no way to supply additional
>> linker flags, as used to be the case with LDFLAGS.
>>
>> Dmitry.
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
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