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HP-UX : shl_load & related functions

If anybody could explain the advantages of using explicit
run time dynamic linking in HP-UX programs(using shl_load or
dlopen functions) over implicit linking using linker options
e.g in GCC compiler i am using following options.
gcc -c -fPIC -shared -export-dynamic -o <lib.so>
then linking it with any other program using the functions &
data defined in <lib.so>
Please tell me any code sample if possible to show proper use of shl_load or related functions.

With Best Regards,
Flower
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flower020397
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flower020397
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1 Solution
 
seedyCommented:
The same advantages that a 'LoadLibrary' and 'GetProcAddress' provide under Windows.
Lets assume we are building a product under windows that interfaces to one ore more mail services, like MAPI, VIM or SMTP.
Lets also assume that you do this depending on user's choice.  Normally, these service APIs are in seperate DLLs (MAPI32.DLL or WINSOCK.DLL, etc).

If you code does explicit LoadLibrary, and then GetProcAddress to get function pointer and call the fucniton via the pointer.  In this case, when the product is installed on a users' machine, it will interface with only the mail interface that machine provides.  For example, if the user does not have MAPI32.DLL or VIM.DLL, but has only WINSOCK, you program will still work fine.

On the other hand, if you have "implicitly linked" the DLLs than, the product will not run, if any of these DLLs are not found.  (Lets' say, VIM.DLL is not available).

The same will hold good for HP-UX.  Replace "shl_load" for 'LoadLibray' and 'shl_findsym' for 'GetProcAddress'.
But the dynamic/shared libraries (or the linker) on most UNIX have few additional feature than on the Windows.  For example, you can use -B option of HP-UX linker to specify run-time binding behavior of a program using shared libraries.  For more info man "ld".
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ahoffmannCommented:
> On the other hand, if you have "implicitly linked" the DLLs than, the product will not run, if any of  these DLLs are not found.
NO, this depends on the linker options: -B defered  or  -B immediate  or  -B nonfatal

> But the dynamic/shared libraries (or the linker) on most UNIX have few additional feature than  on the Windows.
I would state the other way around, see comment about -B option  ;-))

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The advantage using implicit linking is that there is only one precondition at runtime: the linked libs must exist (but see -B option also).
You don't need anything special in your code for this behavior.

While the advantage of using shl_load, dlopen etc. has the advantage that you can
load any kind of shared object dynamically from within your code. You don't need to have the libs (and its functions) at compile- and runtime.
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seedyCommented:
ahoffmann:
>On the other hand, if you have "implicitly linked" the DLLs than, the product will not run, if any of these DLLs are not found.

I was still talking about Windows.
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ahoffmannCommented:
aha, so it's time to tell us the "Windows features" ;-)
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seedyCommented:
ahoffmann:
>On the other hand, if you have "implicitly linked" the DLLs than, the product will not run, if any of these DLLs are not found.

I was still talking about Windows.
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