struct and union alignment

[Richard Bos]

Re: struct and union alignment

pete <pfiland@mindsp ring.com> wrote:
[color=blue]
> S.Tobias wrote:
>[color=green]
> > And yes, I think char type can have alignment issues. Consider this
> > setup: Type char is composed of 4 octets.[/color]
>
> What are you talking about?
> sizeof(char) is one, always.[/color]

So? Byte != octet.

Richard

[pete]

Re: struct and union alignment

S.Tobias wrote:[color=blue]
>
> pete <pfiland@mindsp ring.com> wrote:[color=green]
> > S.Tobias wrote:[color=darkred]
> > > pete <pfiland@mindsp ring.com> wrote:
> > > > S.Tobias wrote:
> > > >
> > > > > Since void* does not point to any object, is a "free pointer",
> > > > > the Standard must specify that the void* values which are valid
> > > > > for char*, are also valid for void*, otherwise undefined. And
> > > > > this is what a programmer must be concerned about.
> > >
> > > > void* can hold the address of any byte of any object,
> > > > just like char*.
> > >
> > > And which address void* in my example implementation cannot hold?[/color][/color]
>[color=green]
> > I don't understand your question.[/color]
>
> I asked because I didn't understand where to apply your statement to.
> Perhaps, never mind.
>[color=green][color=darkred]
> > > > N869
> > > > 6.3.2.3 Pointers
> > > > [#1] A pointer to void may be converted to or from a pointer
> > > > to any incomplete or object type. A pointer to any
> > > > incomplete or object type may be converted to a pointer to
> > > > void and back again; the result shall compare equal to the
> > > > original pointer.[/color][/color]
>[color=green][color=darkred]
> > > Similarly, when you convert, say, an int* to void*, how do you know
> > > that the result fulfills alignment requirements?[/color][/color]
>[color=green]
> > The standard says so.[/color]
>
> Where?
>[color=green]
> > You must be reading 6.3.2.3 differently from the way that I read it.[/color]
>
> I think that the difference between our reading of 6.3.2.3 may be
> that you might assume that #1 demands that conversions to and from
> void* must always succede. IMO if that were the case, then all
> conversions from void* to unaligned int*
> would have to complete
> without a cough. I think this is not what the Standard requires.[/color]

If you have an unaligned int*, then you are invoking
undefined behavior and you code is no longer C code.
The rules don't apply after that.
[color=blue]
> (In fact, I don't see why #1 is even neccessary.[/color]

That doesn't prove that you don't understand it, but it's a clue.

--
pete

[S.Tobias]

Re: struct and union alignment

pete <pfiland@mindsp ring.com> wrote:[color=blue]
> S.Tobias wrote:[/color]
[color=blue][color=green]
> > I think that the difference between our reading of 6.3.2.3 may be
> > that you might assume that #1 demands that conversions to and from
> > void* must always succede. IMO if that were the case, then all
> > conversions from void* to unaligned int*
> > would have to complete
> > without a cough. I think this is not what the Standard requires.[/color][/color]
[color=blue]
> If you have an unaligned int*, then you are invoking
> undefined behavior and you code is no longer C code.
> The rules don't apply after that.[/color]

int *ip;
char *cp;
void *vp;

cp = malloc(2 * sizeof(int));
cp++; //defined
vp = cp; //defined
ip = vp; //???
[color=blue][color=green]
> > (In fact, I don't see why #1 is even neccessary.[/color][/color]
[color=blue]
> That doesn't prove that you don't understand it, but it's a clue.[/color]

Maybe I don't. Would you, please, care to explain it to me.

--
Stan Tobias
sed 's/[A-Z]//g' to email

[pete]

Re: struct and union alignment

S.Tobias wrote:[color=blue]
>
> pete <pfiland@mindsp ring.com> wrote:[color=green]
> > S.Tobias wrote:[/color]
>[color=green][color=darkred]
> > > I think that the difference between our reading of 6.3.2.3 may be
> > > that you might assume that #1 demands that conversions to and from
> > > void* must always succede. IMO if that were the case, then all
> > > conversions from void* to unaligned int*
> > > would have to complete
> > > without a cough. I think this is not what the Standard requires.[/color][/color]
>[color=green]
> > If you have an unaligned int*, then you are invoking
> > undefined behavior and you code is no longer C code.
> > The rules don't apply after that.[/color]
>
> int *ip;
> char *cp;
> void *vp;
>
> cp = malloc(2 * sizeof(int));
> cp++; //defined
> vp = cp; //defined
> ip = vp; //???[/color]

Assuming that sizeof(int) is greater than one,
ip = vp;
is not defined.

--
pete

[S.Tobias]

Re: struct and union alignment

pete <pfiland@mindsp ring.com> wrote:[color=blue]
> S.Tobias wrote:[color=green]
> >
> > int *ip;
> > char *cp;
> > void *vp;
> >
> > cp = malloc(2 * sizeof(int));
> > cp++; //defined
> > vp = cp; //defined
> > ip = vp; //???[/color][/color]
[color=blue]
> Assuming that sizeof(int) is greater than one,
> ip = vp;
> is not defined.[/color]

Why?

--
Stan Tobias
sed 's/[A-Z]//g' to email

[pete]

Re: struct and union alignment

S.Tobias wrote:[color=blue]
>
> pete <pfiland@mindsp ring.com> wrote:[color=green]
> > S.Tobias wrote:[color=darkred]
> > >
> > > int *ip;
> > > char *cp;
> > > void *vp;
> > >
> > > cp = malloc(2 * sizeof(int));
> > > cp++; //defined
> > > vp = cp; //defined
> > > ip = vp; //???[/color][/color]
>[color=green]
> > Assuming that sizeof(int) is greater than one,
> > ip = vp;
> > is not defined.[/color]
>
> Why?[/color]

Because vp holds an address which is not aligned for type int.

--
pete

[S.Tobias]

Re: struct and union alignment

pete <pfiland@mindsp ring.com> wrote:[color=blue]
> S.Tobias wrote:[color=green]
> >
> > pete <pfiland@mindsp ring.com> wrote:[color=darkred]
> > > S.Tobias wrote:
> > > >
> > > > int *ip;
> > > > char *cp;
> > > > void *vp;
> > > >
> > > > cp = malloc(2 * sizeof(int));
> > > > cp++; //defined
> > > > vp = cp; //defined
> > > > ip = vp; //???[/color]
> >[color=darkred]
> > > Assuming that sizeof(int) is greater than one,
> > > ip = vp;
> > > is not defined.[/color]
> >
> > Why?[/color][/color]
[color=blue]
> Because vp holds an address which is not aligned for type int.[/color]

Which point in the Std did you apply to explain this?

--
Stan Tobias
sed 's/[A-Z]//g' to email

[pete]

Re: struct and union alignment

S.Tobias wrote:[color=blue]
>
> pete <pfiland@mindsp ring.com> wrote:[color=green]
> > S.Tobias wrote:[color=darkred]
> > >
> > > pete <pfiland@mindsp ring.com> wrote:
> > > > S.Tobias wrote:
> > > > >
> > > > > int *ip;
> > > > > char *cp;
> > > > > void *vp;
> > > > >
> > > > > cp = malloc(2 * sizeof(int));
> > > > > cp++; //defined
> > > > > vp = cp; //defined
> > > > > ip = vp; //???
> > >
> > > > Assuming that sizeof(int) is greater than one,
> > > > ip = vp;
> > > > is not defined.
> > >
> > > Why?[/color][/color]
>[color=green]
> > Because vp holds an address which is not aligned for type int.[/color]
>
> Which point in the Std did you apply to explain this?[/color]

It would be from C89
C89 Last public draft

3.3.4 Cast operators

A pointer to an object or incomplete type may be converted to a
pointer to a different object type or a different incomplete type.
The resulting pointer might not be valid if it is improperly
aligned for the type pointed to.

--
pete

[pete]

Re: struct and union alignment

pete wrote:[color=blue]
>
> S.Tobias wrote:[color=green]
> >
> > pete <pfiland@mindsp ring.com> wrote:[color=darkred]
> > > S.Tobias wrote:
> > > >
> > > > pete <pfiland@mindsp ring.com> wrote:
> > > > > S.Tobias wrote:
> > > > > >
> > > > > > int *ip;
> > > > > > char *cp;
> > > > > > void *vp;
> > > > > >
> > > > > > cp = malloc(2 * sizeof(int));
> > > > > > cp++; //defined
> > > > > > vp = cp; //defined
> > > > > > ip = vp; //???
> > > >
> > > > > Assuming that sizeof(int) is greater than one,
> > > > > ip = vp;
> > > > > is not defined.
> > > >
> > > > Why?[/color]
> >[color=darkred]
> > > Because vp holds an address which is not aligned for type int.[/color]
> >
> > Which point in the Std did you apply to explain this?[/color]
>
> It would be from C89
> C89 Last public draft
>
> 3.3.4 Cast operators
>
> A pointer to an object or incomplete type may be converted to a
> pointer to a different object type or a different incomplete type.
> The resulting pointer might not be valid if it is improperly
> aligned for the type pointed to.[/color]

N869
6.3.2.3 Pointers
[#7] A pointer to an object or incomplete type may be
converted to a pointer to a different object or incomplete
type. If the resulting pointer is not correctly aligned
for the pointed-to type, the behavior is undefined.

--
pete

[S.Tobias]

Re: struct and union alignment

pete <pfiland@mindsp ring.com> wrote:[color=blue]
> pete wrote:[color=green]
> >
> > S.Tobias wrote:[color=darkred]
> > >
> > > pete <pfiland@mindsp ring.com> wrote:
> > > > S.Tobias wrote:
> > > > >
> > > > > pete <pfiland@mindsp ring.com> wrote:
> > > > > > S.Tobias wrote:
> > > > > > >
> > > > > > > int *ip;
> > > > > > > char *cp;
> > > > > > > void *vp;
> > > > > > >
> > > > > > > cp = malloc(2 * sizeof(int));
> > > > > > > cp++; //defined
> > > > > > > vp = cp; //defined
> > > > > > > ip = vp; //???
> > > > >
> > > > > > Assuming that sizeof(int) is greater than one,
> > > > > > ip = vp;
> > > > > > is not defined.
> > > > >
> > > > > Why?
> > >
> > > > Because vp holds an address which is not aligned for type int.
> > >
> > > Which point in the Std did you apply to explain this?[/color]
> >
> > It would be from C89
> > C89 Last public draft
> >
> > 3.3.4 Cast operators
> >
> > A pointer to an object or incomplete type may be converted to a
> > pointer to a different object type or a different incomplete type.
> > The resulting pointer might not be valid if it is improperly
> > aligned for the type pointed to.[/color][/color]
[color=blue]
> N869
> 6.3.2.3 Pointers
> [#7] A pointer to an object or incomplete type may be
> converted to a pointer to a different object or incomplete
> type. If the resulting pointer is not correctly aligned
> for the pointed-to type, the behavior is undefined.[/color]

Now, why do you think (again, I mean C&V) that a reverse conversion
must _always_ work:
int *ip;
void *vp;
ip = ...; //assume valid pointer
vp = ip;

--
Stan Tobias
sed 's/[A-Z]//g' to email

[pete]

Re: struct and union alignment

S.Tobias wrote:[color=blue]
>
> pete <pfiland@mindsp ring.com> wrote:[color=green]
> > pete wrote:[color=darkred]
> > >
> > > S.Tobias wrote:
> > > >
> > > > pete <pfiland@mindsp ring.com> wrote:
> > > > > S.Tobias wrote:
> > > > > >
> > > > > > pete <pfiland@mindsp ring.com> wrote:
> > > > > > > S.Tobias wrote:
> > > > > > > >
> > > > > > > > int *ip;
> > > > > > > > char *cp;
> > > > > > > > void *vp;
> > > > > > > >
> > > > > > > > cp = malloc(2 * sizeof(int));
> > > > > > > > cp++; //defined
> > > > > > > > vp = cp; //defined
> > > > > > > > ip = vp; //???
> > > > > >
> > > > > > > Assuming that sizeof(int) is greater than one,
> > > > > > > ip = vp;
> > > > > > > is not defined.
> > > > > >
> > > > > > Why?
> > > >
> > > > > Because vp holds an address which is not aligned for type int.
> > > >
> > > > Which point in the Std did you apply to explain this?
> > >
> > > It would be from C89
> > > C89 Last public draft
> > >
> > > 3.3.4 Cast operators
> > >
> > > A pointer to an object or incomplete type may be converted to a
> > > pointer to a different object type or a different incomplete type.
> > > The resulting pointer might not be valid if it is improperly
> > > aligned for the type pointed to.[/color][/color]
>[color=green]
> > N869
> > 6.3.2.3 Pointers
> > [#7] A pointer to an object or incomplete type may be
> > converted to a pointer to a different object or incomplete
> > type. If the resulting pointer is not correctly aligned
> > for the pointed-to type, the behavior is undefined.[/color]
>
> Now, why do you think (again, I mean C&V) that a reverse conversion
> must _always_ work:
> int *ip;
> void *vp;
> ip = ...; //assume valid pointer
> vp = ip;[/color]

Because, according to the requoted standard text,
ip == vp
is guaranteed to be true after that.

(C&V, again)

N869
6.3.2.3 Pointers
[#1] A pointer to void may be converted to or from a pointer
to any incomplete or object type. A pointer to any
incomplete or object type may be converted to a pointer to
void and back again; the result shall compare equal to the
original pointer.

--
pete

[S.Tobias]

Re: struct and union alignment

pete <pfiland@mindsp ring.com> wrote:[color=blue]
> S.Tobias wrote:[color=green]
> >
> > pete <pfiland@mindsp ring.com> wrote:[color=darkred]
> > > pete wrote:
> > > >
> > > > S.Tobias wrote:
> > > > >
> > > > > pete <pfiland@mindsp ring.com> wrote:
> > > > > > S.Tobias wrote:
> > > > > > >
> > > > > > > pete <pfiland@mindsp ring.com> wrote:
> > > > > > > > S.Tobias wrote:
> > > > > > > > >
> > > > > > > > > int *ip;
> > > > > > > > > char *cp;
> > > > > > > > > void *vp;
> > > > > > > > >
> > > > > > > > > cp = malloc(2 * sizeof(int));
> > > > > > > > > cp++; //defined
> > > > > > > > > vp = cp; //defined
> > > > > > > > > ip = vp; //???
> > > > > > >
> > > > > > > > Assuming that sizeof(int) is greater than one,
> > > > > > > > ip = vp;
> > > > > > > > is not defined.
> > > > > > >
> > > > > > > Why?
> > > > >
> > > > > > Because vp holds an address which is not aligned for type int.
> > > > >
> > > > > Which point in the Std did you apply to explain this?
> > > >
> > > > It would be from C89
> > > > C89 Last public draft
> > > >
> > > > 3.3.4 Cast operators
> > > >
> > > > A pointer to an object or incomplete type may be converted to a
> > > > pointer to a different object type or a different incomplete type.
> > > > The resulting pointer might not be valid if it is improperly
> > > > aligned for the type pointed to.[/color]
> >[color=darkred]
> > > N869
> > > 6.3.2.3 Pointers
> > > [#7] A pointer to an object or incomplete type may be
> > > converted to a pointer to a different object or incomplete
> > > type. If the resulting pointer is not correctly aligned
> > > for the pointed-to type, the behavior is undefined.[/color]
> >
> > Now, why do you think (again, I mean C&V) that a reverse conversion
> > must _always_ work:
> > int *ip;
> > void *vp;
> > ip = ...; //assume valid pointer
> > vp = ip;[/color][/color]
[color=blue]
> Because, according to the requoted standard text,
> ip == vp
> is guaranteed to be true after that.[/color]
[color=blue]
> (C&V, again)[/color]
[color=blue]
> N869
> 6.3.2.3 Pointers
> [#1] A pointer to void may be converted to or from a pointer
> to any incomplete or object type. A pointer to any
> incomplete or object type may be converted to a pointer to
> void and back again; the result shall compare equal to the
> original pointer.[/color]

1. Why do you think that #7 does not apply this time?

2. Why #1 does not apply to void* -> int* conversion?

What are the rules by which you choose which point applies to what
conversion type?

--
Stan Tobias
sed 's/[A-Z]//g' to email

[Dan Pop]

Re: [OT] struct and union alignment

In <2rk0c8F1bpl5cU 2@uni-berlin.de> "S.Tobias" <sNOiSPAMt@amu. edu.pl> writes:
[color=blue]
>Dan Pop <Dan.Pop@cern.c h> wrote:
>[color=green]
>> struct { ... } foo, bar, baz;[/color]
>[color=green]
>> foo, bar and baz are all the objects of this type needed by the program
>> and they are used in a single translation unit. What would a tag buy you?[/color]
>
>I think we're talking of two different things. I used the definition
>of an anonymous structure I found in:
>http://docs.sun.com/source/816-2460/...xtensions.html[/color]

Which, being a C++ compiler extension is certainly topical here, right?

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Dan.Pop@ifh.de
Currently looking for a job in the European Union

[Dan Pop]

Re: struct and union alignment

In <2rqlf0F1d1o69U 1@uni-berlin.de> "S.Tobias" <sNOiSPAMt@amu. edu.pl> writes:
[color=blue]
>pete <pfiland@mindsp ring.com> wrote:[color=green]
>> S.Tobias wrote:[color=darkred]
>> >
>> > pete <pfiland@mindsp ring.com> wrote:
>> > > pete wrote:
>> > > >
>> > > > S.Tobias wrote:
>> > > > >
>> > > > > pete <pfiland@mindsp ring.com> wrote:
>> > > > > > S.Tobias wrote:
>> > > > > > >
>> > > > > > > pete <pfiland@mindsp ring.com> wrote:
>> > > > > > > > S.Tobias wrote:
>> > > > > > > > >
>> > > > > > > > > int *ip;
>> > > > > > > > > char *cp;
>> > > > > > > > > void *vp;
>> > > > > > > > >
>> > > > > > > > > cp = malloc(2 * sizeof(int));
>> > > > > > > > > cp++; //defined
>> > > > > > > > > vp = cp; //defined
>> > > > > > > > > ip = vp; //???
>> > > > > > >
>> > > > > > > > Assuming that sizeof(int) is greater than one,
>> > > > > > > > ip = vp;
>> > > > > > > > is not defined.
>> > > > > > >
>> > > > > > > Why?
>> > > > >
>> > > > > > Because vp holds an address which is not aligned for type int.
>> > > > >
>> > > > > Which point in the Std did you apply to explain this?
>> > > >
>> > > > It would be from C89
>> > > > C89 Last public draft
>> > > >
>> > > > 3.3.4 Cast operators
>> > > >
>> > > > A pointer to an object or incomplete type may be converted to a
>> > > > pointer to a different object type or a different incomplete type.
>> > > > The resulting pointer might not be valid if it is improperly
>> > > > aligned for the type pointed to.
>> >
>> > > N869
>> > > 6.3.2.3 Pointers
>> > > [#7] A pointer to an object or incomplete type may be
>> > > converted to a pointer to a different object or incomplete
>> > > type. If the resulting pointer is not correctly aligned
>> > > for the pointed-to type, the behavior is undefined.
>> >
>> > Now, why do you think (again, I mean C&V) that a reverse conversion
>> > must _always_ work:
>> > int *ip;
>> > void *vp;
>> > ip = ...; //assume valid pointer
>> > vp = ip;[/color][/color]
>[color=green]
>> Because, according to the requoted standard text,
>> ip == vp
>> is guaranteed to be true after that.[/color]
>[color=green]
>> (C&V, again)[/color]
>[color=green]
>> N869
>> 6.3.2.3 Pointers
>> [#1] A pointer to void may be converted to or from a pointer
>> to any incomplete or object type. A pointer to any
>> incomplete or object type may be converted to a pointer to
>> void and back again; the result shall compare equal to the
>> original pointer.[/color]
>
>1. Why do you think that #7 does not apply this time?[/color]

Because the standard says so: character and void pointer values have
no alignment restrictions. Therefore, when converting something to
pointer to void, the resulting value is, by definition, correctly aligned.
[color=blue]
>2. Why #1 does not apply to void* -> int* conversion?[/color]

It does. But #7 applies too. There is no contradiction between the two.
#1 says that you can do the conversion and #7 says what happens if the
original pointer value was not properly aligned for the type of the result
of the conversion.
[color=blue]
>What are the rules by which you choose which point applies to what
>conversion type?[/color]

The rest of the standard. Unfortunately, the standard is very poorly
written as a reference document: you can't only read the paragraph(s)
of immediate interest and expect to get a complete picture of the issue.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Dan.Pop@ifh.de
Currently looking for a job in the European Union

[S.Tobias]

Re: [OT] struct and union alignment

Dan Pop <Dan.Pop@cern.c h> wrote:[color=blue]
> In <2rk0c8F1bpl5cU 2@uni-berlin.de> "S.Tobias" <sNOiSPAMt@amu. edu.pl> writes:[/color]
[color=blue][color=green]
> >Dan Pop <Dan.Pop@cern.c h> wrote:
> >[color=darkred]
> >> struct { ... } foo, bar, baz;[/color]
> >[color=darkred]
> >> foo, bar and baz are all the objects of this type needed by the program
> >> and they are used in a single translation unit. What would a tag buy you?[/color]
> >
> >I think we're talking of two different things. I used the definition
> >of an anonymous structure I found in:
> >http://docs.sun.com/source/816-2460/...xtensions.html[/color][/color]
[color=blue]
> Which, being a C++ compiler extension is certainly topical here, right?[/color]

I took the first plausible definition I found. C++ is not topical here,
but C++ shares many ideas with C. I thought the definition in above
document was not C++ specific. Anonymous structures aren't topical
here either. I was casually interested, since someone mentioned them.
I think the above example you provided is called "unnamed structure".

--
Stan Tobias
sed 's/[A-Z]//g' to email