dynamic variable with static storage behaviour with when libs loaded using dlopen

Question

I have a global variable defined in a c-library [ex: int globalcount = 0] and in library initialization function [say library_init] globalcount is incremented by 1.

The issue here is : when library open with dlopen and appln call library_init function and closed the libs using dlclose then again dlopen call is reseting the globalcount to 0.

I want to know how to prevent reset of global variable [I want globalcount must be 1 not 0] when next dlopen is called because process is not exit so global variable also must not reset.

If library linked to proceess using -l at link time , I am not facing the abv issue. the issue only when libs open using dlopen/dlclose [multiple times]

OS is Linux Prog Lan : C Compiler : gcc

How to do, memory in the data segment must not freed when dlclose called ?

This is an OS issue rather than a language issue. It may help (though I doubt it) if you tell us the OS. What about persisting the state of the variable to a file when dlclose() is called. — Loki Astari
– Loki Astari, Commented Nov 5, 2011 at 6:50
OS is Linux, We don't want to save any think into a file, I think its not a good way . Is any way using static storage class can I solve abv issue — Syedsma
– Syedsma, Commented Nov 5, 2011 at 7:05
I think you should just ask yourself the question why you ever do dlclose on that library. I can't figure out much use for that. — Jens Gustedt
– Jens Gustedt, Commented Nov 5, 2011 at 8:43

David Feurle · Accepted Answer · 2011-11-05 07:13:08Z

1

When you call dlclose your dll is unloaded that means any memory in the data segment of your library is freed. This is the reason why the value is reset to 0. You could use shared memory to have a variable that is persistent even when your library gets unload.

answered Nov 5, 2011 at 7:13

David Feurle

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7 Comments

Syedsma Over a year ago

Is any simplest method , such that I can make a verialbe permenant for entire process life ?

David Feurle Over a year ago

@syedsma as I said: when the .so is unload all memory ocupied by it is freed. There is no way around it. It is how shared objects are designed.

Syedsma Over a year ago

How to do memory in the data segment must not freed when dlclose called ?

David Feurle Over a year ago

@Syedsma One last time: There is no way. It is freed by the operating system. You can use shared memory.

Syedsma Over a year ago

Thanks for u r Response:). But why dynamic memeory allocated using malloc inside the library will not automaticaly freed when dlclose called , valgrind/purify report this memory as leaked

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Yogesh · Accepted Answer · 2014-03-06 13:51:03Z

0

Use RTLD_NODELETE flag while loading library using dlopen. the global and global static variables persist in memory when we use this flag. Even if the reference count of the library becomes zero.

answered Mar 6, 2014 at 13:51

Yogesh

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