OS/2, eCS & ArcaOS - Technical > Programming

GCC/G++ - how to export global variable without underscore?

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Dave Yeo:

--- Quote from: Andi B. on July 15, 2024, 09:04:07 am ---I tried various combinations of dllexport (which seems to be mandatory to export the symbol) and stdcall cdecl... But I ever end up with exporting _NdpTypes instead NdpTypes. I was under the impression that c++ don't add the _ but obviously I misinterpreted some stuff.

Another idea, is there any emx tool which can modify the exported symbols? Or add a new symbol to a dll which references to the orignal?

Btw. this all started cause I didn't manage to convince c++/linker to work with a .def file as it was used with icc or watcom. You know I'm not experienced with all this stuff so there is much guessing on my side.

--- End quote ---

You only need dllexport if you don't have the symbols in the def file. Traditionally, before we got dllexport, you hand wrote a DEF file including the exports, either by name or ordinal or you ran emxexp on the
object files and piped the results to the def file. Run emxexp with no parameters to see usage..
I've only done it the other way, pulling in symbols from a GCC DLL to a OW binary.
Note that with OW, you have to be careful to use the right calling convention to begin with, it defaults to its own calling convention which is incompatible with GCC DLLs, it uses registers instead of the stack.
See "Open Watcom 2.0 C/C++ Programmers Guide under Commonly Asked Questions, Converting to OpenWatcom C/C++, Conversion from IBM-compatible PC compilers.
In the case of porting from icc or OW, I simply edit the DEF, adding the underscores

Edit: You might also need the extern keyword

KO Myung-Hun:
Hi/2.

You can append `asm("variable_name")' to a variable declaration like:


--- Quote ---__attribute__ ((dllexport)) extern const char *NdpTypes[] asm("NdpTypes");

--- End quote ---

You can use `__asm__' instead of `asm'.

For details, see http://trac.netlabs.org/libc/wiki/Faq#Howtodeclareavariablesoitwontgetanunderscore and https://gcc.gnu.org/onlinedocs/gcc/Asm-Labels.html.

Lars:
OS/2 32-bit calling convention for public interfaces should always be syscall in order to avoid all these name decoration problems. Depending on compiler, this is _System or syscall or _syscall and proper header files for the compiler will map that to APIENTRY and also EXPENTRY.
Note: OS/2 16-bit calling convention is Pascal. There are technical reason for using Pascal in 16-bit code (you need a calling convention where the called function cleans up the stack in order to support call gates. And Pascal is a calling convention that expects the called function to clean up the stack). Pascal uppercases all symbols ...

As has been stated, the calling convention also applies to data variables (only for the naming, of course. Nothing is called from a data variable). And before you start fuzzing with tools,aybe the easiest way is to use:
EXPENTRY char * myGlobalVar;
or (I cannot remember)
char * EXPENTRY myGlobalVar;

and to then use the def file to export a symbol named myGlobalVar because the syscall calling convention neither decorates names (as C callin convention does) nor uppercases names (as the Pascal calling convention does).
It will leave the symbol name untouched.

It should be obvious that on the accessing side, you will need to also properly use the calling convention:
extern char EXPENTRY *theGlobalVar;

Andi B.:

--- Quote from: KO Myung-Hun on July 16, 2024, 02:06:56 am ---Hi/2.

You can append `asm("variable_name")' to a variable declaration like:


--- Quote ---__attribute__ ((dllexport)) extern const char *NdpTypes[] asm("NdpTypes");

--- End quote ---

You can use `__asm__' instead of `asm'.

For details, see http://trac.netlabs.org/libc/wiki/Faq#Howtodeclareavariablesoitwontgetanunderscore and https://gcc.gnu.org/onlinedocs/gcc/Asm-Labels.html.

--- End quote ---

Thank you so much. This works, though I've to test even more.


--- Quote ---_System
--- End quote ---
I get -

--- Code: ---./h/ndextpl2.h:354:102: error: '__system__' was not declared in this scope
  354 | extern const char __attribute__((visibility("default"))) __attribute__ ((dllexport)) __attribute__ ((_System))*NdpTypes[];

--- End code ---
with that. As with a lot of other combinations I tried.

.def file - the whole thing started as I could not find out how to convince

--- Code: ---g++.exe (GCC) 9.2.0 20190812 (OS/2 RPM build 9.2.0-5.oc00)
--- End code ---
to use a .def file. So I thought it's more clear to put such thing straight into the code.

Thanks for all inputs.

Dave Yeo:

--- Quote from: Andi B. on July 17, 2024, 07:41:19 pm ---
.def file - the whole thing started as I could not find out how to convince

--- Code: ---g++.exe (GCC) 9.2.0 20190812 (OS/2 RPM build 9.2.0-5.oc00)
--- End code ---
to use a .def file. So I thought it's more clear to put such thing straight into the code.

--- End quote ---

Just add the def file to the linker line. Should also use -Zomf for linking to use a native linker and be able to debug.

--- Code: ---g++ -Zdll -Zomf foo.def foo.o -o foo.dll

--- End code ---
or for an executable,

--- Code: ---g++ -Zomf bar.def bar.o -o bar.exe

--- End code ---
Add -Zmap to produce a map file, -Zhigh-mem to use high memory, -g for debug symbols etc.

BTW, libc seems to use _system, not __system__

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