The aSub record $(prefix):ARRAY:PARSE processes input waveform $(prefix):ARRAY:1 that has 10 elements, generates an array of 20 elements, and writes it to output waveform $(prefix):ARRAY:2.

1. In xxxApp/Db/

1.1 array.db

record(waveform, "$(prefix):ARRAY:1")
{
    field(DESC, "Input array")
    field(DTYP, "stream")
    field(INP,  "@array.proto read_int_array(read_array, $(prefix)) vs")  # read array using protocol read_int_array
    field(NELM, "10")
    field(FTVL, "USHORT")
    field(FLNK, "$(prefix):ARRAY:PARSE")
}

record(aSub, "$(prefix):ARRAY:PARSE")
{
    field(SNAM, "arrayProc")                   # name of the processing function
    field(INPA, "$(prefix):ARRAY:1 NPP NMS")   # input record
    field(FTA,  "USHORT")                      # data type of elements in the input record
    field(NOA,  "10")                          # number of elements in the input record
    field(OUTA, "$(prefix):ARRAY:2")           # output record
    field(FTVA, "USHORT")                      # data type of elements in the output record
    field(NOVA, "20")                          # number of elements in the output record
}

record(waveform, "$(prefix):ARRAY:2")
{
    field(NELM, "20")
    field(FTVL, "USHORT")
}

1.2 Makefile

DB += array.db

2. In xxxApp/src/

2.1 arrayProc.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <registryFunction.h>
#include <aSubRecord.h>
#include <epicsExport.h>
#include <dbDefs.h>
#include <waveformRecord.h>
#include <dbAccess.h>
#include <dbFldTypes.h>
#include <epicsTime.h>

static long arrayProc(aSubRecord *prec)
{
    unsigned short * data_raw = (unsigned short*)prec->a;  // point to input record
    unsigned short *data = (unsigned short*)prec->vala;    // point to output record
    
    DBLINK * plink;
    DBADDR * paddr;
    waveformRecord * pwf;

    plink = &prec->outa;

    if(DB_LINK != plink->type)
        return -1;

    paddr     = (DBADDR*)plink->value.pv_link.pvt;
    pwf       = (waveformRecord *)paddr->precord;
    pwf->nelm = 20;

    for (int i=0; i<10; i++)
    {
        data[i]    = 2 * data_raw[i];
        data[i+10] = 3 * data_raw[i];
    }

    return 0;
}

epicsRegisterFunction(arrayProc);

2.2 arrayProc.h

extern long arrayProc(aSubRecord *prec);

2.3 array.dbd

function(arrayProc)

2.4 Makefile

...
maia_DBD += array.dbd
...
maia_SRCS += arrayProc.c
...