sample.c

/*
$ gcc -shared -fpack-struct=4 -Isrc/include/ -o sample.so sample.c
$ ./r3core -qs
Loading boot extensions...
>> import %./sample.so
>> add-mul 10 20 30
== 900
*/

#include "reb-c.h"
#include "reb-ext.h"

const char *init_block =
    "REBOL [\n"
        "Title: {Example extension}\n"
        "Name: example\n"
        "Type: extension\n"
        "Exports: [add-mul]\n"
    "]\n"
    "add-mul: command [{Add and multiply integers.} a b c]\n"
;

RXIEXT const char *RX_Init(int opts, RXILIB *lib) {
    RXI = lib;
    if (lib->version == RXI_VERSION) return init_block;
    return 0;
}

RXIEXT int RX_Call(int cmd, RXIFRM *frm, void *data) {
    RXA_INT64(frm, 1) =
        (RXA_INT64(frm, 1) + RXA_INT64(frm, 2)) *
        RXA_INT64(frm, 3);
    return RXR_VALUE;
}

sample2.c

/*
$ gcc -shared -fpack-struct=4 -Isrc/include/ -o sample2.so sample2.c
$ ./r3core -qs
Loading boot extensions...
>> import %./sample2.so
>> sum-chars "testing"
== 766
*/

#include "reb-c.h"
#include "reb-ext.h"

const char *init_block =
    "REBOL [\n"
        "Title: {Another example extension}\n"
        "Name: example2\n"
        "Type: extension\n"
        "Exports: [\n"
            "sum-chars\n"
        "]\n"
    "]\n"
    "sum-chars: command [{Checksum a string.} str [string!]]\n"
;

RXIEXT const char *RX_Init(int opts, RXILIB *lib) {
    RXI = lib;
    if (lib->version == RXI_VERSION) return init_block;
    return 0;
}

RXIEXT int RX_Call(int cmd, RXIFRM *frm, void *data) {
    i32 idx, tail;
    i64 sum = 0;
    REBSER *ser;

    ser = RXA_SERIES(frm, 1);
    idx = RXA_INDEX(frm, 1);
    tail = RXI_SERIES_INFO(ser, RXI_SER_TAIL);

    for (; idx < tail; ++idx) {
        sum += RXI_GET_CHAR(ser, idx);
    }

    RXA_INT64(frm, 1) = sum;
    RXA_TYPE(frm, 1) = RXT_INTEGER;
    return RXR_VALUE;
}