BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Importing function pointers from a DLL          BorlandTalk.com Forum Index -> C++ Builder (Language C++) View previous topic :: View next topic   Author Message Andrue Cope [TeamB] Guest Posted: Tue Dec 12, 2006 8:11 pm    Post subject: Importing function pointers from a DLL Hiya, This was a new one on me. I've used GetProcAddress() many times but this is the first time I've run across someone exporting a function pointer instead of a function. My fix for the moment is the slightly scary: typedef int WINAPI (*TFn)( void * ); .... TFn Fn=TFn( GetProcAddress( HInstance, "FN" ) ); memcpy( &Fn, reinterpret_cast<void *>( Fn ), sizeof( Fn ) ); I had a quick play with the typedef to see if I could work out how to import the function pointer properly but couldn't get my head around. Since I have a solution that works for me I'm moving on to other problems but can some kind soul suggest a way to get rid of the memcpy() operation? -- Andrue Cope [TeamB] [Bicester, Uk] http://info.borland.com/newsgroups/guide.html Back to top Roddy Pratt Guest Posted: Tue Dec 12, 2006 8:11 pm    Post subject: Re: Importing function pointers from a DLL Andrue Cope [TeamB] wrote: Quote: typedef int WINAPI (*TFn)( void * ); ... TFn Fn=TFn( GetProcAddress( HInstance, "FN" ) ); Maybe I've misunderstood, but how about this? typedef int WINAPI (*TFn)( void * ); typedef TFn* PTFn; int test(void *param) { TFn Fn=*PTFn( GetProcAddress( HInstance, "test" ) ); return Fn(param); } My motto: Never do in one typedef what you can do in two! - Roddy Back to top Alan Bellingham Guest Posted: Tue Dec 12, 2006 9:00 pm    Post subject: Re: Importing function pointers from a DLL "Andrue Cope [TeamB]" <no.spam (AT) not (DOT) a.valid.address> wrote: Quote: This was a new one on me. I've used GetProcAddress() many times but this is the first time I've run across someone exporting a function pointer instead of a function. My fix for the moment is the slightly scary: typedef int WINAPI (*TFn)( void * ); Defining a function pointer type. So far, so good. Quote: ... TFn Fn=TFn( GetProcAddress( HInstance, "FN" ) ); OK, you've successfully assigned to a function pointer. Now onto stage 2 .... Quote: memcpy( &Fn, reinterpret_cast<void *>( Fn ), sizeof( Fn ) ); You're trying to cast a function pointer to a normal pointer. You're not allowed to do that. Having attempted to cast your function pointer to a normal pointer, you're then attempting to copy the result back to the function pointer. Do you see something wrong with this scenario? Umm, what does the export itself look like? I'm confused as to how it could look any different from the normal export, but I suspect that what you are missing is probably an extra level of indirection. So, maybe something like typedef int WINAPI (*TFn)( void * ); typedef TFn* TfnIndirect; TfnIndirect FnInd = TfnIndirect(GetProcAddress(0, "FN")); TFn Fn = *FnInd; (Compiles, but definitely not tried!) Alan Bellingham -- ACCU Conference: 11-14 April 2007 - Paramount Oxford Hotel Back to top Andrue Cope [TeamB] Guest Posted: Tue Dec 12, 2006 10:21 pm    Post subject: Re: Importing function pointers from a DLL Alan Bellingham wrote: Quote: memcpy( &Fn, reinterpret_cast<void *>( Fn ), sizeof( Fn ) ); You're trying to cast a function pointer to a normal pointer. You're not allowed to do that. I am - the compiler is happy (probably only because I used reinterpret_cast) but it's legal C++ :) Quote: Having attempted to cast your function pointer to a normal pointer, you're then attempting to copy the result back to the function pointer. Which works fine. The problem I'm trying to resolve is that the address returned by GetProcAddress() is not the address of the function being exported. Instead it's the address of the address. What my hack is doing (and yes it's a hack, legal, but a hack) is to take the four bytes pointed to by GetProcAddress() and copy them into the function pointer so that the function pointer is useable. In effect I'm doing a one-time-only dereference of what GetProcAddress() returns. In pseudo code: Function_Pointer=*( GetProcAddress() ); And this works fine. My only problem is that I don't like having to use memcpy() and I'd like to know how to import it properly. Quote: Umm, what does the export itself look like? _declspec(dllexport) int (*Fn)(void); Quote: I'm confused as to how it could look any different from the normal export, but I suspect that what you are missing is probably an extra level of indirection. So, maybe something like typedef int WINAPI (*TFn)( void * ); typedef TFn* TfnIndirect; TfnIndirect FnInd = TfnIndirect(GetProcAddress(0, "FN")); TFn Fn = *FnInd; (Compiles, but definitely not tried!) I'll give that a try tomorrow. As is usually the case with these things this was supposed to be a minor background task to fit into a quiet moment away from my primary C# project. I had a couple of goes at fixing the declaration then gave up. -- Andrue Cope [TeamB] [Bicester, Uk] http://info.borland.com/newsgroups/guide.html Back to top Andrue Cope [TeamB] Guest Posted: Tue Dec 12, 2006 10:23 pm    Post subject: Re: Importing function pointers from a DLL Yah, that's what Alan has suggested. I'll let you know how I get on tomorrow. My motto:Nothing is ever as simple as it appears. The code that uses the DLL (the stuff that does all the work) turned out to be simple, elegant and worked like a charm. The bit that I've done dozens of times before and is hardly rocket science (loading a DLL) had me tearing my hair out. Ho hum. I'm hoping to find out /why/ it was done this way but atm I can only assume it's deliberate obfustication. -- Andrue Cope [TeamB] [Bicester, Uk] http://info.borland.com/newsgroups/guide.html Back to top Alan Bellingham Guest Posted: Tue Dec 12, 2006 10:57 pm    Post subject: Re: Importing function pointers from a DLL "Andrue Cope [TeamB]" <no.spam (AT) not (DOT) a.valid.address> wrote: Quote: Yah, that's what Alan has suggested. I'll let you know how I get on tomorrow. But Roddy has his clock set so it looks like he got in before me. In fact, it even looks like he answered before you asked! Alan Bellingham -- ACCU Conference: 11-14 April 2007 - Paramount Oxford Hotel Back to top Alan Bellingham Guest Posted: Tue Dec 12, 2006 11:00 pm    Post subject: Re: Importing function pointers from a DLL "Andrue Cope [TeamB]" <no.spam (AT) not (DOT) a.valid.address> wrote: Quote: I am - the compiler is happy (probably only because I used reinterpret_cast) but it's legal C++ Ah, OK. I had thought that was going across a boundary it wasn't allowed across. Maybe I was thinking normal pointer <=> member pointer. I can only think that the reason the writer exports the pointer is so that the same export can provide different functions behind it, depending on circumstances. I can sort of image a scenario in which it makes perfect sense, but it is definitely unusual. Alan Bellingham -- ACCU Conference: 11-14 April 2007 - Paramount Oxford Hotel Back to top Roddy Pratt Guest Posted: Wed Dec 13, 2006 1:26 am    Post subject: Re: Importing function pointers from a DLL Alan Bellingham wrote: Quote: But Roddy has his clock set so it looks like he got in before me. In fact, it even looks like he answered before you asked! I had Xananews "Generate Date Headers" disabled, so the time (with offset) seems to actually come from the newsgroup server! My PC *is* correct. I will accept you actually beat me by a whole 56 seconds, although the 15:00:00 time looks definitely dodgy ;-) - Roddy Back to top Remy Lebeau (TeamB) Guest Posted: Wed Dec 13, 2006 3:23 am    Post subject: Re: Importing function pointers from a DLL "Andrue Cope [TeamB]" <no.spam (AT) not (DOT) a.valid.address> wrote in message news:457ed73b (AT) newsgroups (DOT) borland.com... Quote: I'm hoping to find out /why/ it was done this way but atm I can only assume it's deliberate obfustication. Maybe the DLL wants to export a single entry point that maps to different internal functions under different conditions? Gambit Back to top Alisdair Meredith[TeamB] Guest Posted: Wed Dec 13, 2006 3:29 am    Post subject: Re: Importing function pointers from a DLL Andrue Cope [TeamB] wrote: Quote: I am - the compiler is happy (probably only because I used reinterpret_cast) but it's legal C++ Technically, it is undefined behaviour that has the desired effect on the platform you use <g> Data pointers and function pointers are not interchangable, and indeed there are platforms where they are different sizes. In 32 bit Windows world, which is all we are usually worried about in these groups, then you will probably get away with this for some considerable time to come. I might be happier with some static assertions that your assumptions hold though (sizeof(void*) == sizeof(TFn)) in case 64 bit windows or some future platform holds an interesting surprise... -- AlisdairM(TeamB) Back to top Andrue Cope [TeamB] Guest Posted: Wed Dec 13, 2006 3:59 pm    Post subject: Re: Importing function pointers from a DLL Remy Lebeau (TeamB) wrote: Quote: I'm hoping to find out why it was done this way but atm I can only assume it's deliberate obfustication. Maybe the DLL wants to export a single entry point that maps to different internal functions under different conditions? Could be I suppose. OTOH for this DLL it may be an attempt at obfustication as well. It sure confused me. -- Andrue Cope [TeamB] [Bicester, Uk] http://info.borland.com/newsgroups/guide.html Back to top Andrue Cope [TeamB] Guest Posted: Wed Dec 13, 2006 4:01 pm    Post subject: Re: Importing function pointers from a DLL Alisdair Meredith[TeamB] wrote: Quote: Technically, it is undefined behaviour that has the desired effect on the platform you use <g Yeah, which is why I wanted a better solution :) Thankfully Roddy's and Alan's solution works so I have removed the memcpy. -- Andrue Cope [TeamB] [Bicester, Uk] http://info.borland.com/newsgroups/guide.html Back to top Alan Bellingham Guest Posted: Wed Dec 13, 2006 5:38 pm    Post subject: Re: Importing function pointers from a DLL "Andrue Cope [TeamB]" <no.spam (AT) not (DOT) a.valid.address> wrote: Quote: Yeah, which is why I wanted a better solution :) Thankfully Roddy's and Alan's solution works so I have removed the memcpy. Thanks. It did appear closer to the description of what was going on. 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