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comp.lang.c FAQ list · Question 2.25

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.)

References: K&R1 Sec. 6.7 pp. 136-8
K&R2 Sec. 6.9 pp. 149-50
ISO Sec.
H&S Sec. 5.6.5 pp. 136-8

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