Problem:
I have a structure which needs to store its data in contiguous memory
by there is a dynamic element which can't be defined at compile time.
It needs to be aligned along a 4 byte boundary. This is what my
structure looks like:
START
struct tsBob
{
unsigned int fieldA;
unsigned int fieldB;
unsigned short varLen[0];
}
tsBob* myBob;
myBob = (tsBob*)malloc( sizeof(tsBob) + sizeof(unsigned short) * 1);
END
myBob needs an additional 3 bytes alloc'd to it to be aligned. I
don't want to use %
Finally the question,
Can I use a bitwise something to determine how many extra bytes I need
and malloc that with the
rest of the function call?
Oct 3 '08
14  1845 
On Oct 4, 3:57 pm, Hrvoje Prge?a <hrvoje.prg...@ gmail.hrwrote:
Maxim Yegorushkin wrote:
Was it not the original intent of C++ to be compatible with C?
Up to a certain point. The original C++ required function
prototypes, for example, which didn't even exist in C at the
time.
It still is, but the c++ standard is always catching up with
the c standard. Unspecified/flexible length arrays are from
C99, the latest c++ standard is C++03 (which is an updated
version of c++98). It'll probably get in the new standard,
sometime in the future.
I don't think so. It would require at least a minimum of work
(a formal proposal, etc.), and no one thought it worth the
effort.
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
On Oct 4, 3:20 pm, Hrvoje Prge?a <hrvoje.prg...@ gmail.hrwrote:
James Kanze wrote:
No. All that malloc (or operator new()) guarantee is that the
returned pointer is sufficiently aligned for any type.
I knew about malloc, but are you sure this is true for
operator new? It should know the requirement of the needed
type in advance so the largest aligment seems like a
overspecificati on?
The operator new function doesn't know what type it is being
called for, and must return a pointer suitably aligned for any
function. (See §3.7.4.1.)
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
On Oct 4, 10:14 pm, Ian Collins <ian-n...@hotmail.co mwrote:
Hrvoje Prge¹a wrote:
James Kanze wrote:
No. All that malloc (or operator new()) guarantee is that
the returned pointer is sufficiently aligned for any type.
I knew about malloc, but are you sure this is true for
operator new? It should know the requirement of the needed
type in advance so the largest aligment seems like a
overspecificati on?
No, operator new does not know which type is being allocated,
it is only passed the size.
There's a possibility of ambiguity with the expression "operator
new": it can be interpreted to mean a "new expression" or the
"new operator", or to mean the operator new function. In the
first case, it does know the type.
I specifically wrote "operator new()", with the parenthesis, to
make it clear that I was talking about the function.
Apparently, it wasn't clear enough.
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
In article
<a4************ *************** *******@f63g200 0hsf.googlegrou ps.com>, James
Kanze <ja*********@gm ail.comwrote:
On Oct 4, 3:20 pm, Hrvoje Prge?a <hrvoje.prg...@ gmail.hrwrote:
James Kanze wrote:
No. All that malloc (or operator new()) guarantee is that the
returned pointer is sufficiently aligned for any type.
I knew about malloc, but are you sure this is true for
operator new? It should know the requirement of the needed
type in advance so the largest aligment seems like a
overspecificati on?
The operator new function doesn't know what type it is being
called for, and must return a pointer suitably aligned for any
function. (See =A73.7.4.1.)
Couldn't it do the same that new char [] and new unsigned char []
(operator, not function) does (C++03 section 5.3.4 paragraph 10)? There,
the memory is suitably aligned for any object of that size or less, since
one can't (legally) store an object of greater size.
On Oct 7, 6:37 pm, blargg....@gish puppy.com (blargg) wrote:
In article
<a479a519-6609-4a9b-976b-7ccd23ad8...@f6 3g2000hsf.googl egroups.com>, James
Kanze <james.ka...@gm ail.comwrote:
On Oct 4, 3:20 pm, Hrvoje Prge?a <hrvoje.prg...@ gmail.hrwrote:
James Kanze wrote:
No. All that malloc (or operator new()) guarantee is
that the returned pointer is sufficiently aligned for
any type.
I knew about malloc, but are you sure this is true for
operator new? It should know the requirement of the needed
type in advance so the largest aligment seems like a
overspecificati on?
The operator new function doesn't know what type it is being
called for, and must return a pointer suitably aligned for
any function. (See =A73.7.4.1.)
Couldn't it do the same that new char [] and new unsigned char
[] (operator, not function) does (C++03 section 5.3.4
paragraph 10)? There, the memory is suitably aligned for any
object of that size or less, since one can't (legally) store
an object of greater size.
The standard says (concerning the return value of an "allocation
functionr", §3.7.3.1/2): "The pointer returned shall be suitably
aligned so that it can be converted to a pointer of any complete
object type and then used to access the object or array in the
storage allocated (until the storage is explicitly deallocated
by a call to a corresponding deallocation function)." The first
part of the sentence is clear: it must be suitably aligned for
any type. The second part makes it a little less clear; it's
obvious that you can't use the pointer to access an object
larger than the ammount of memory allocated.
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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