Hello,

I'm trying to use a c library where I can define a logging plugin.

The c prototype for the callback is

     void (*log)(void *logContext, UA_LogLevel level, UA_LogCategory
category, const char *msg, va_list args);

however I couldn't find a way to define a suitable callback in fpc since the "array of const" construct is not allowed with cdecl without external, i.e.

procedure LogLog(logContext: Pointer; level: UA_LogLevel; category:
UA_LogCategory; msg: PAnsiChar; const args:array of const);cdecl;

gives an error "Error: VarArgs directive (or '...' in MacPas) without CDecl/CPPDecl/MWPascal/StdCall and External".

I cannot add "external" (I don't want to call an external function, it's the library calling my function) or remove the cdecl (parameter passing wouldn't work then).

Is there some trick to define a function with varargs callable from the c library?

Bye

It is indeed not possible to implement a function with C varargs in FPC.

Jonas

El 19/12/20 a les 23:15, Jonas Maebe via fpc-pascal ha escrit:

I was afraid that's the answer :-(

as an ugly workaround, I wrote a c library that vsnprintf msg and args to a buffer and then calls a pascal function with no varargs.

It works but, as I said, it's ugly and I'd need to provide a .a for every platform I plan to use.

Is there a better/simpler way (apart from asking the library developers to use a callback with no varargs)?

Bye

No.

Jonas

Not necessarily better than platform specific .a files mentioned by Luca, but the required playing with stack and registers should be still doable with a function written in assembler (inside the Pascal sources), shouldn't it?

Tomas

Yes, but you'd have to write it separately for each ABI.

Jonas

El 20/12/20 a les 14:00, Jonas Maebe via fpc-pascal ha escrit:

:-(

Oh, well, that's life.

Now I'm struggling to integrate my stub function under win32 (no problem with linux/64).

The linker complains about

Error: Undefined symbol: _vsnprintf
Error: Undefined symbol: _ua_pascallog

the first it's probably because it's missing some library (which?) but the second is defined in my program (and, as I said, it builds and works fine under linux/64) as:

procedure ua_pascallog(logContext: Pointer; level: UA_LogLevel;
   category: UA_LogCategory; msg: PAnsiChar);cdecl;export;

Any hint?

Bye