Q:I came across some structure declarations with colons and numbers next to certain fields, like this:
struct record {
char *name;
int refcount : 4;
unsigned dirty : 1;
};
What gives?
A:Those are bit-fields; the number gives the exact size of the field, in bits. (See any complete book on C for the details.) Bit-fields can be used to save space in structures having several binary flags or other small fields, and they can also be used in an attempt to conform to externally-imposed storage layouts. (Their success at the latter task is mitigated by the fact that bit-fields are assigned left-to-right on some machines and right-to-left on others).
Note that the colon notation for specifying the size of a field in bits is
only valid in structures (and in unions); you cannot use this mechanism to specify the size of arbitrary variables. (See questions
1.2 and
1.3.)
