fail.c

#include "libucl/include/ucl.h"
#include <sys/types.h>
#include <unistd.h>

bool handler(const unsigned char *data, size_t len,
                unsigned char **replace, size_t *replace_len, bool *need_free, void* ud) {


    write(1, data, len);
    if (memcmp(data, "WAT", len)) {
        const char *val = "xxx";
        *replace = malloc(3);
        *replace_len = 3;
        memcpy (*replace, val, 3);
        *need_free=true;
        return true;
    }
    return false;
}

int
main (int argc, char **argv)
{
        struct ucl_parser *parser = NULL;
        ucl_object_t *obj;
    int flags;

    const char *fname_in = NULL;

    fname_in = argv[0];

    flags = UCL_PARSER_DEFAULT;

        parser = ucl_parser_new (flags);

    ucl_parser_set_variables_handler (parser, handler, NULL);
    ucl_parser_add_file(parser, "test.ucl");

        if (ucl_parser_get_error (parser) != NULL) {
                fprintf (stdout, "Error occurred (phase 1): %s\n",
                                                ucl_parser_get_error(parser));
        exit(1);
    }


        obj = ucl_parser_get_object (parser);


        ucl_parser_free (parser);
        ucl_object_unref (obj);
}

gistfile1.txt

key = "${VAR}"