c/c++ Linux equivalent of "bool DllMain()" - but I need to return failure to dlopen()

Question

I am porting a DLL from Windows to Linux (OS X actually). I used this StackOverflow article to do that change.

i.e. I've ported the Windows "bool DllMain()" to the Linux way:

__attribute__((constructor)) void dllLoad();

__attribute__((destructor)) void dllUnload();

... but both are void return types. I need to be able to do the same as Windows and return FALSE if a condition isn't met in the constructor so that the dlopen() fails and the .so doesn't load.

How do I get the calling dlopen() to fail?

Try throwing an exception. I don't know if this will work or not, but if dlopen is sane (and it probably is) it should catch it and indicate a failure. — Wug, Mar 22 '13 at 18:15
you want dlopen to fail if some runtime criteria within the shared library isn't met? — Sam Miller, Mar 22 '13 at 18:17
What does this have to do with C? C doesn't have formal constructors. — beatgammit, Mar 22 '13 at 18:19
@tjameson This isn't a constructor in the C++ sense. It's a special function called when the dynamic library is loaded. — James M, Mar 22 '13 at 18:25
@JamesMcLaughlin - Huh, didn't know that. That's actually pretty cool. — beatgammit, Mar 22 '13 at 18:26
@SamMiller Yes. Based on hardware capabilities I need to abort loading so that the parent program doesn't use this .so — Edward, Mar 22 '13 at 22:08
Are you just interested in the architecture as a capability ? If so you can hack your way through it. — user1952500, Mar 22 '13 at 23:48

score 3 · Answer 1 · answered Mar 25 '13 at 15:50

3

The answer is that it isn't possible. As stated above you can't error out in the constructor - be it an exception or exit()

answered Mar 25 '13 at 15:50

Edward

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score 1 · Answer 2 · answered Feb 20 '15 at 21:22

You need to approach this differently.

If you are dynamically loading a library, than you must also be using GetProcAddress() and dlsym() to actually do anything with it. dlsym() is your path forward here.

You clearly control the plugin's code, since otherwise you couldn't even add these APIs to it. So, all your 'dllmain' on either platform needs to do is set some global 'is valid' state information. You then simply call a known API like "ModuleIsValid()" whose entire job is to simply read that state info & return true/false. If it returns false, you close the library and report failure.

c/c++ Linux equivalent of "bool DllMain()" - but I need to return failure to dlopen()

2 Answers2