Ceph » rbd

  union {
    uint8_t flags;
    uint8_t flags = 0;
    struct {
      uint8_t entry_valid :1; /* if 0, this entry is free */
      uint8_t sync_point :1;  /* No data. No write sequence number. Marks sync
                                 point for this sync gen number */
      uint8_t sequenced :1;   /* write sequence number is valid */
      uint8_t has_data :1;    /* write_data field is valid (else ignore) */
      uint8_t discard :1;     /* has_data will be 0 if this is a discard */
      uint8_t writesame :1;   /* ws_datalen indicates length of data at write_bytes */
    };
  };

  DENC(WriteLogCacheEntry, v, p) {
    DENC_START(1, 1, p);
    ... 
    denc(v.flags, p);
    ...
    DENC_FINISH(p);
  }

Bit fields are not suit for anything where layout actually matters. Here, depending on the platform and the compiler, entry_valid can refer to either least significant bit of flags or the most significant bit:

An implementation may allocate any addressable storage unit large enough to hold a bit-field. If enough space remains, a bit-field that immediately follows another bit-field in a structure shall be packed into adjacent bits of the same unit. If insufficient space remains, whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is implementation-defined. The order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is implementation-defined. The alignment of the addressable storage unit is unspecified.