Document: WG14 N1204
Date: 2006/12/15
Submitter: Fred Tydeman (USA)
Submission Date: 2006-12-12
Source: WG 14
Reference Document: ISO/IEC
WG14 N1204
ISO/IEC
WG14 DR 262
ISO/IEC
WG14 DR 315
Version: 1.0
Date: 2006-12-12
Subject:_Bool bit-fields
Summary
What are the constraints on and semantics of _Bool bit-fields?
#include <stdbool.h>
struct bits {
_Bool bbf1 : 1; /* unsigned 1-bit _Bool bit-field */
_Bool bbf3 : 3; /* unsigned 3-bit _Bool bit-field */
} bits;
int main(void){
bits.bbf1 = true; /* the value 1u */
bits.bbf1 = ~ bits.bbf1; /* undefined? 0u? 1u? */
bits.bbf3 = true; /* the value 1u */
bits.bbf3 = ~ bits.bbf3; /* undefined? 0u? 1u? 6u? */
return 0;
}
What is the maximum width of a _Bool bit-field allowed that does not cause a constraint violation? 1? CHAR_BIT? Something else? Is bbf3 a constraint violation?
DR 262 changed 6.7.2.1#3 to require a constraint violation if a bit-field width is too large.
6.2.5#2 says _Bool can hold the values 0 and 1.
6.2.6.2#6 discusses width.
I see nothing that says the width of a _Bool is 1.
6.7.2.1#9 has: If the value 0 or 1 is stored into a nonzero-width bit-field of type _Bool, the value of the bit-field shall compare equal to the value stored.
So, if a value other than 0 or 1 is stored into a nonzero-width bit-field of type _Bool, is that undefined?
The first assignment gives the bit-field the value 1u. The ~ of that valud yields ...1111110u. When that value is then stored into the _Bool bit-field, what value is stored?
6.3.1.2 Boolean type has: When any scalar value is converted to _Bool, the result is 0 if the value compares equal to 0; otherwise, the result is 1.
6.7.2.1#9 has: A bit-field is interpreted as a signed or unsigned integer type consisting of the specified number of bits.
So, does a _Bool bit-field have the semantics of a _Bool (as per 6.3.1.2) or of an unsigned integer type (as per 6.7.2.1)? DR 315 might be relavent.
Suggested Technical Corrigendum
Proposed Technical Corrigendum
Committee Discussion (for history only)
Committee Response