What is zero-width bit field [duplicate]

Answer 1

As stated here, zero-length bitfields add alignment between bitfields. If we have several bitfields in a row, their layout is compact, but if we want to align one of them to byte/word/dword boundary, we need to put a zero-length bitfield between this and the previous one.

Example from the link above:

struct on_off {
                  unsigned light : 1;
                  unsigned toaster : 1;
                  int count;            /* 4 bytes */
                  unsigned ac : 4;     // this and
                  unsigned : 4;        // this and
                  unsigned clock : 1;  // this bitfields are next to each other
                  unsigned : 0;
                  unsigned flag : 1;   // this bitfield is at a 4 bytes boundary.
                 } kitchen ;