Re: Extern "C" when defining aSub records

[Michael Davidsaver via Tech-talk <tech-talk at aps.anl.gov>]

On 2/2/21 5:04 AM, Simon Rose via Tech-talk wrote:
> Hello -
> 
>  
> 
> I am trying to understand how to register a cpp function for an EPICS aSub record (or something similar). In particular, I am trying to understand where and how to use the extern “C” directive.

While in theory I think that extern "C" could effect calling convention, in
practice it does not.  So the only place extern "C" is needed is around
the epicsExport*() macros.

The actual global symbol name is determined by the epicsExport*,
which is working from a pointer to your function, so c++ name mangling
isn't relevant.

> From what I understand, the extern “C” directive is there to tell the c++ compiler to use C-style linking and name mangling. This seems reasonable for any function that is going to be exposed as a function that could be used outside of my specific library, e.g. by an aSub record.
> 
>  
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> The function that is actually calling the aSub function is do_sub from aSubRecord.c. For reference:
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>  
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> static long do_sub(aSubRecord *prec)
> 
> {
> 
>     GENFUNCPTR pfunc = prec->sadr;
> 
>     long status;
> 
>  
> 
>     if (prec->snam[0] == 0)
> 
>         return 0;
> 
>  
> 
>     if (pfunc == NULL) {
> 
>         recGblSetSevr(prec, BAD_SUB_ALARM, INVALID_ALARM);
> 
>         return S_db_BadSub;
> 
>     }
> 
>     status = pfunc(prec);
> 
>     if (status < 0)
> 
>         recGblSetSevr(prec, SOFT_ALARM, prec->brsv);
> 
>     else
> 
>         prec->udf = FALSE;
> 
>  
> 
>     return status;
> 
> }
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>  
> 
> And the function is called in the line
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>  
> 
>     status = pfunc(prec);
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>  
> 
> Given that this is from a .c file and not a .cpp file, does pfunc need to be defined with extern “C” linkage? The closest analog that I can find is from asyn, specifically asynPortDriver.cpp, which defines a number of functions such as
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>  
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> extern "C" {static asynStatus disconnect(void *drvPvt, asynUser *pasynUser)
> 
> {
> 
> …
> 
> }}
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>  
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> And these functions are later added into interfaces that, as I understand, are called from other parts of the code, hence their C-style linkages.
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>  
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> Unfortunately, the example from the documentation seems to be implicitly in C, so that doesn’t help.
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>  
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> In summary, my question is roughly as follows:
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>  
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> 1. For a function defined in a .cpp file which is to be used as either the INAM or SNAM parameter in an aSub record, does that function need to be declared extern “C”?
> 
> 2. What about the epicsExportFunction() line? What is the relationship between these two, and do either/both/none of them need to be extern “C”d?
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>  
> 
> Thanks for your help,
> 
>  
> 
> Cheers,
> 
>  
> 
> -- 
> 
> Simon Rose
> 
> Software Engineer
> 
> European Spallation Source
>