Demonstrates a minimal R3 extension that returns a block containing a string

block.c

#include "reb-host.h"

const char *init_block =
    "REBOL [\n"
        "Name: block\n"
        "Type: extension\n"
    "]\n"
    "export get-block: command []\n"
;

RL_LIB *RL;

const char *RX_Init(int opts, RL_LIB *lib) {
    RL = lib;
    return init_block;
}

int RX_Call(int cmd, RXIFRM *frm, void *data) {
    REBSER *s = RL_MAKE_STRING(3, FALSE);
    RL_SET_CHAR(s, 0, 'f');
    RL_SET_CHAR(s, 1, 'o');
    RL_SET_CHAR(s, 2, 'o');

    RXIARG a;
    a.series = s;
    a.index = 0;

    REBSER *b = RL_MAKE_BLOCK(1);
    RL_SET_VALUE(b, 0, a, RXT_STRING);

    RXA_SERIES(frm, 1) = b;
    RXA_INDEX(frm, 1) = 0;
    RXA_TYPE(frm, 1) = RXT_BLOCK;

    return RXR_VALUE;
}