Microchip Technology SW006023-3N 数据表
Implementation-Defined Behavior
2012 Microchip Technology Inc.
DS51686E-page 197
18.12 STRUCTURES, UNIONS, ENUMERATIONS, AND BIT FIELDS
18.13 QUALIFIERS
ISO Standard:
“A member of a union object is accessed using a member of a different
type (C90 6.3.2.3).”
type (C90 6.3.2.3).”
Implementation:
The corresponding bytes of the union object are interpreted as an
object of the type of the member being accessed without regard for
alignment or other possible invalid conditions.
object of the type of the member being accessed without regard for
alignment or other possible invalid conditions.
ISO Standard:
“Whether a “plain” int bit field is treated as a signed int bit field or
as an unsigned int bit field (C90 6.5.2, C90 6.5.2.1, C99 6.7.2, C99
6.7.2.1).”
as an unsigned int bit field (C90 6.5.2, C90 6.5.2.1, C99 6.7.2, C99
6.7.2.1).”
Implementation:
By default, a plain
int
bit field is treated as a signed integer. This
behavior can be altered by use of the
-funsigned-bitfields
command line option.
ISO Standard:
“Allowable bit field types other than
_Bool
, signed int, and
unsigned int
(C99 6.7.2.1).”
Implementation:
No other types are supported.
ISO Standard:
“Whether a bit field can straddle a storage unit boundary (C90 6.5.2.1,
C99 6.7.2.1).”
C99 6.7.2.1).”
Implementation:
No.
ISO Standard:
“The order of allocation of bit fields within a unit (C90 6.5.2.1, C99
6.7.2.1).”
6.7.2.1).”
Implementation:
Bit fields are allocated left to right.
ISO Standard:
“The alignment of non-bit field members of structures (C90 6.5.2.1, C99
6.7.2.1).”
6.7.2.1).”
Implementation:
Each member is located to the lowest available offset allowable accord-
ing to the alignment restrictions of the member type.
ing to the alignment restrictions of the member type.
ISO Standard:
“The integer type compatible with each enumerated type (C90 6.5.2.2,
C99 6.7.2.2).”
C99 6.7.2.2).”
Implementation:
If the enumeration values are all non-negative, the type is
unsigned
int
, else it is
int
. The
-fshort-enums
command line option can
change this.
ISO Standard:
“What constitutes an access to an object that has volatile-qualified type
(C90 6.5.3, C99 6.7.3).”
(C90 6.5.3, C99 6.7.3).”
Implementation:
Any expression which uses the value of or stores a value to a volatile
object is considered an access to that object. There is no guarantee
that such an access is atomic.
If an expression contains a reference to a volatile object but neither
uses the value nor stores to the object, the expression is considered an
access to the volatile object or not depending on the type of the object.
If the object is of scalar type, an aggregate type with a single member
of scalar type, or a union with members of (only) scalar type, the
expression is considered an access to the volatile object. Otherwise,
the expression is evaluated for its side effects but is not considered an
access to the volatile object.
For example:
object is considered an access to that object. There is no guarantee
that such an access is atomic.
If an expression contains a reference to a volatile object but neither
uses the value nor stores to the object, the expression is considered an
access to the volatile object or not depending on the type of the object.
If the object is of scalar type, an aggregate type with a single member
of scalar type, or a union with members of (only) scalar type, the
expression is considered an access to the volatile object. Otherwise,
the expression is evaluated for its side effects but is not considered an
access to the volatile object.
For example:
volatile int a;
a; /* access to ‘a’ since ‘a’ is scalar */