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i have a c file which contains some global variables.
and i have one cpp file which needs to access these global variables.
it did the following but failed:
Code:/* file.c */
int value;
i got a linker error saying _value is unresolved symbol.Code:// file.cpp
extern "C" {
extern int value;
}
i am using MSVC .Net 2003, but i need to be able to compile this on gcc as well. so a portable solution is needed here :(
How do you compile these? With one C and one C++ file I'm sure you don't compile into one single binary?
Hi Magos!
The .c file ends up in a .dll file, while .cpp file is used for building .exe file which then depends on .dll file as well.
I tried DUMPBIN on the .dll as follows:
DUMPBIN /SYMBOLS cfile.dll
but nothing was output!
Code:File Type: DLL
Summary
4A000 .data
2000 .idata
30000 .rdata
F000 .reloc
154000 .text
you have to declare value as dllexport...
But I personally prefer not to export globals from dll - instead I provide some reasonable API
Thanks Magos! Thanks vart!
The above solution would be MS only, i need to do the same in gcc also.
I tried to use 2 functions instead of globals, one function will set the values, other will get the values.
But again same linker error of unresolved symbols :(
vart what do you suggest as a reasonable API here?
DLL's are windows-specific anyways :)
Yeah Magos, but as you asked:
"How do you compile these? With one C and one C++ file I'm sure you don't compile into one single binary?"
So i told you what the files end up as. It may not be in .dll or .so file on Linux, maybe different situation on Mac :(
But the c file and cpp file, both are needed for final .exe file. it's just that c file happens to be compiled in a .dll on windows, on linux it will be different.
but why i am unable to access global variables or functions from my c file? :(
I do not know what you want to do with this global var...Quote:
vart what do you suggest as a reasonable API here?
One of the ways...
Provide Init that will dynamically allocate all neded data and return handle to this memory
Every other funtion in Dll will receive the handle - cast it to the real data type and perform required actions
End function will release the allocated memory making handle invalide
In such approach DLL could be used in MT environment without big problems, and also work on several instancies of the data simulteneously...
vart, the c file contains some low level audio stuff, cpp file is using Qt to make gui over the low level stuff in c file. so using gui controls i want to set some values in the c file. my first thought was making the values global in c file and using them in cpp file. but then i made a set/get pair of functions to do the same. both approaches give me unresolved symbol linker error.
have you used __declspec(dllexport) declarations as suggested by the links provided?
No vart, that will work with MSVC only. I need to compile on Windows, MacOS & Linux.
Isn't there a portable approach?
I thought that at least in case of functions I will be able to access them from my c file, regardless of whether the c file ends up as a .dll or .so or anything.
you cannot compile dll on other OSes... so use #ifdef
Or - do not use dll at all, compile it as static library
No. You need OS-specific code.Quote:
Isn't there a portable approach?
So what is the reason you can't pass the variable from one side to the other?
Global variables are a nuisance, and it gets worse when you have to pass them across shared library/DLL boundaries.
--
Mats
CornedBee, you are probably right about OS specific code :(
Mats, I also tried to avoid globals and tried using functions that would set/get the variables.
Now as provided in the links I can do this on Windows but don't know how to do this on Linux & Mac.
Well on Linux i did not have to use any kind of trickery to access the functions from a .so file.
I just made a .c file with two functions in it, into a Linux .so file.
Then in .cpp i used those functions with this guard around prototypes:
Code:
#ifdef __cplusplus
extern "C" {
#endif
char* toUpper(char* str);
#ifdef __cplusplus
}
#endif
Now, the good news is for my small test, my .so files and functions within are easily used by the .cpp program i made.
But the bad news is, that it does not work with my project, which also have a similar situation (although already mentioned in my posts above) which i summraize here again:
The .c files ends up in a .so/.dll or anything OS specific (similiar for a dynamic library). And the program contains all .cpp files. Now the program uses this .dll/.so file (links with it) and needs some functions from it. On Windows there is this dllexport trickery involved. But on Linux nothing needed to be done and i was able to create a .so and access the functions just fine. Is there something more to it (involving .so files) on Linux, which i may be missing?
From my understanding, .so files exports everything by default, which means you have to do nothing to access everything in your file.
Windows dlls are different. They export nothing by default. You have to use either __declspec(export) or a definition file (.def) to export your variables/functions to use them elsewhere.