/** maximum number of Xconnections **/
#define MAXXCONNS 128

typedef enum {
    XMODE_NULL,
    XMODE_R,
    XMODE_W,
    XMODE_RW,
    XMODE_A,
    XMODE_RA
} Xmode;

typedef struct Xconn  *Xconnection;
struct Xconn {
    int      (*open)(Xconnection, Xmode, const char *);
    void     (*close)(Xconnection);
    void     (*destroy)(Xconnection);
    int      (*seek)(Xconnection, long, int, int);
    int      (*flush)(Xconnection);
    void     (*truncate)(Xconnection);
    size_t   (*read)(void *, size_t, size_t, Xconnection);
    size_t   (*write)(const void *, size_t, size_t, Xconnection);
    SEXP     (*control)(Xconnection, SEXP);
    void     *data;
};