A bit field declares a structure or union member with a specified bit width after a colon. Successive fields may share larger storage units, reducing padding while remaining accessible through ordinary member expressions and assignments. section 6.3.4
Compact calendar fields
The declarations in struct tib assign widths such as 6 bits for seconds, 5 for hours, and 1 for the daylight-saving flag. This traditional version is marked erroneous: an int bit-field may be signed or unsigned depending on the implementation. A safer alternative gives fields explicit types, as in struct tbi; its members remain usable in expressions such as x.tbi_min < 60.
struct tbi {
unsigned _BitInt(6) tbi_sec :6; // Seconds after the minute [0, 60]
unsigned _BitInt(6) tbi_min :6; // Minutes after the hour [0, 59]
unsigned _BitInt(5) tbi_hour :5; // Hours since midnight [0, 23]
unsigned _BitInt(5) tbi_mday :5; // Day of the month [1, 31]
unsigned _BitInt(4) tbi_mon :4; // Months since January [0, 11]
signed tbi_year; // Years since 1900
unsigned _BitInt(3) tbi_wday :3; // Days since Sunday [0, 6]
unsigned _BitInt(9) tbi_yday :9; // Days since January [0, 365]
bool tbi_isdst:1; // Daylight Saving Time flag
};
Compare explicitly typed widths for calendar values, including _BitInt fields and a bool flag. section 6.3.4
