Create the lld_copy_subset function

/* LLD SAPI interface example */

#include <mi.h>
#include <lldsapi.h>

/* append a (small) subset of a large object to another large object */

MI_ROW*
lld_copy_subset (MI_ROW* src,           /* source LLD_Locator */
                 MI_ROW* dest,          /* destination LLD_Locator */
                 mi_int8* offset,       /* offset to begin copy at */
                 mi_integer nbytes,     /* number of bytes to copy */
                 MI_FPARAM* fp)
{
    MI_ROW*        new_dest;       /* return value */
    MI_CONNECTION* conn;           /* database server connection */
    mi_string*     buffer;         /* I/O buffer */
    LLD_IO*        io;             /* open large object descriptor */
    mi_int8        new_offset;     /* offset after seek */
    mi_integer     bytes_read;     /* actual number of bytes copied */
    mi_integer     error;          /* error argument */
    mi_integer     _error;         /* extra error argument */
    mi_boolean     created_dest;   /* did we create the dest large object? */

    /* initialize variables */
    new_dest = NULL;
    conn = NULL;
    buffer = NULL;
    io = NULL;
    error = LLD_E_OK;
    created_dest = MI_FALSE;

    /* open a connection to the database server */
    conn = mi_open (NULL, NULL, NULL);
    if (conn == NULL)
        goto bad;

    /* allocate memory for I/O */
    buffer = mi_alloc (nbytes);
    if (buffer == NULL)
        goto bad;

    /* read from the source large object */
    io = lld_open (conn, src, LLD_RDONLY, &error);
    if (error != LLD_E_OK)
        goto bad;

    lld_seek (conn, io, offset, LLD_SEEK_SET, &new_offset, &error);
    if (error != LLD_E_OK)
        goto bad;

    bytes_read = lld_read (conn, io, buffer, nbytes, &error);
    if (error != LLD_E_OK)
        goto bad;

    lld_close (conn, io, &error);
    if (error != LLD_E_OK)
        goto bad;

    /* write to the destination large object */
    new_dest = lld_create (conn, dest, &error);
    if (error == LLD_E_OK)
        created_dest = MI_TRUE;
    else if (error != LLD_E_EXISTS)
        goto bad;

    io = lld_open (conn, new_dest, LLD_WRONLY | LLD_APPEND | LLD_SEQ, &error);
    if (error != LLD_E_OK)
        goto bad;

    lld_write (conn, io, buffer, bytes_read, &error);
    if (error != LLD_E_OK)
        goto bad;

    lld_close (conn, io, &error);
    if (error != LLD_E_OK)
        goto bad;

    /* free memory */
    mi_free (buffer);

    /* close the database server connection */
    mi_close (conn);

    return new_dest;

    /* error clean up */
bad:
    if (io != NULL)
        lld_close (conn, io, &_error);
    if (created_dest)
        lld_delete (conn, new_dest, &_error);
    if (buffer != NULL)
        mi_free (buffer);
    if (conn != NULL)
        mi_close (conn);
    lld_error_raise (conn, error);
    mi_fp_setreturnisnull (fp, 0, MI_TRUE);
    return NULL;
}