On 8 Aug 2008, at 01:59, Johannes Fortmann
<[EMAIL PROTECTED]> wrote:
The problem here is that UTCDateTime is defined with #pragma pack 2 in
effect. That means the compiler packs with an alignment of 2, so the
whole structure has 8 bytes. The proper alignment (4) results in 12
bytes. Since there's nothing in the @encode'd information specifying
the non-standard alignment, NSGetSizeAndAlignment (which NSValue
probably uses internally) will return 12 bytes as the struct's size.
As an example,
#include <Foundation/Foundation.h>
void main()
{
NSUInteger size, align;
NSGetSizeAndAlignment (@encode(UTCDateTime),
&size,
&align);
printf("%i, %i, %s\n", sizeof(UTCDateTime), size,
@encode(UTCDateTime));
}
prints "8, 12, {UTCDateTime=SIS}".
Thank you. This explains the problems I had.
So:
some_type a;
NSValue *data = [ NSValue value: &a withObjCType: @encode
(some_type) ];
followed by:
some_type b;
[ data getValue: &b ];
is unsafe, dangerous and strictly to be avoided - especially if the
definiton of "some_type" is buried in some frameworks.
Instead one must use:
some_type *bPointer = [ data bytes ];
The only problem: NSValue has no method "bytes".
So we have to do:
const char *objCType = [ data objCType ];
unsigned int size;
NSGetSizeAndAlignment (objCType, &size, NULL);
// The alloca() function is machine and compiler dependent; its use
is discouraged. Why?
some_type *bPointer = alloca(size);
[ data getValue: bPointer ];
Or does anyone have a better idea?
Another question:
sizeof(UTCDateTime) < NSValue.
Now this begs the question: Can there be some bloated_type with:
sizeof(bloated_type) > NSValue ?
That is: is there some alignment pragma taking more bytes than the
alignment used by NSValue?
FInal question: May the dangerous and (at least in Tiger)
undocumented behaviour of getValue: be labeled as a severe design error?
Kind regards,
Gerriet.
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