Experimental Physics and
Industrial Control System

Till Straumann via Tech-talk <tech-talk at aps.anl.gov> · Tue, 2 Feb 2021 18:12:21 +0100

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.

The function that is actually calling the aSub function is do_sub from aSubRecord.c. For reference:

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;

}

And the function is called in the line

    status = pfunc(prec);

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

extern "C" {static asynStatus disconnect(void *drvPvt, asynUser *pasynUser)

{

…

}}

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.

Unfortunately, the example from the documentation seems to be implicitly in C, so that doesn’t help.

In summary, my question is roughly as follows:

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?

Thanks for your help,

Cheers,

-- 

Simon Rose

Software Engineer

European Spallation Source

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System