struct bio_st
{
        BIO_METHOD *method;
 /* bio, mode, argp, argi, argl, ret */
      long (*callback)(struct bio_st *,int,const char *,int, long,long);
  char *cb_arg; /* first argument for the callback */
 
    int init;
   int shutdown;
       int flags;      /* extra storage */
 
    int retry_reason;
 
      int num;
    void *ptr;
  struct bio_st *next_bio;        /* used by filter BIOs */
   struct bio_st *prev_bio;        /* used by filter BIOs */
   int references;
     unsigned long num_read;
     unsigned long num_write;
 
       CRYPTO_EX_DATA ex_data;
};