Can offsetof be used to determine the offset of a member within an
embedded struct member?
For example, let 'struct foo' be a structure with an embedded structure
'struct bar' which has a member 'mem'. Can one do:
offsetof(struct foo, bar.mem)
Thanks,
Mike
3  2834 
Michael B Allen wrote: Can offsetof be used to determine the offset of a member within an
embedded struct member?
For example, let 'struct foo' be a structure with an embedded structure
'struct bar' which has a member 'mem'. Can one do:
offsetof(struct foo, bar.mem)
7.17p3...
The macros are ... ; and
offsetof(type, member-designator)
which expands to an integer constant expression that has type
size_t, the value of which is the offset in bytes, to the
structure member (designated by member-designator), from the
beginning of its structure (designated by type). The type and
member designator shall be such that given static type t;
then the expression &(t.member-designator) evaluates to an address
constant. (If the specified member is a bit-field, the behavior
is undefined.)
Technically, I'd say no, but there is a sure fire way...
offsetof(struct foo, bar_member) + offsetof(struct bah, mem)
--
Peter
On Thu, 01 Sep 2005 21:18:54 -0700, Peter Nilsson wrote: Michael B Allen wrote: Can offsetof be used to determine the offset of a member within an
embedded struct member?
For example, let 'struct foo' be a structure with an embedded structure
'struct bar' which has a member 'mem'. Can one do:
offsetof(struct foo, bar.mem)
7.17p3...
The macros are ... ; and
offsetof(type, member-designator)
which expands to an integer constant expression that has type
size_t, the value of which is the offset in bytes, to the
structure member (designated by member-designator), from the
beginning of its structure (designated by type). The type and
member designator shall be such that given static type t;
then the expression &(t.member-designator) evaluates to an address
constant. (If the specified member is a bit-field, the behavior
is undefined.)
Technically, I'd say no, but there is a sure fire way...
offsetof(struct foo, bar_member) + offsetof(struct bah, mem)
Actually from the wording "expression &(t.member-designator) evaluates
to an address constant" I would say it IS ok as &(t.bar.mem) is a valid
expression.
Mike
Michael B Allen wrote: On Thu, 01 Sep 2005 21:18:54 -0700, Peter Nilsson wrote: Michael B Allen wrote: Can offsetof be used to determine the offset of a member within an
embedded struct member?
For example, let 'struct foo' be a structure with an embedded structure
'struct bar' which has a member 'mem'. Can one do:
offsetof(struct foo, bar.mem)
7.17p3...
The macros are ... ; and
offsetof(type, member-designator)
which expands to an integer constant expression that has type
size_t, the value of which is the offset in bytes, to the
structure member (designated by member-designator), from the
beginning of its structure (designated by type). The type and
member designator shall be such that given static type t;
then the expression &(t.member-designator) evaluates to an address
constant. (If the specified member is a bit-field, the behavior
is undefined.)
Technically, I'd say no, but there is a sure fire way...
offsetof(struct foo, bar_member) + offsetof(struct bah, mem)
Actually from the wording "expression &(t.member-designator) evaluates
to an address constant" I would say it IS ok as &(t.bar.mem) is a valid
expression.
But bar.mem is not a member of struct foo, bah is; mem is a member of
whatever struct bar is.
You're probably better off asking csc for the committee's intent, but
I think that in the rare circumstances where I might need such an
offset,
I'd have an object handy, and I can use that...
#define obj_offsetof(obj, mem) \
((size_t) ((char *) &(obj) - (char *) &(obj).mem))
--
Peter This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
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