Initialising an union.

Initialising an union.

[Aurel Balmosan]

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Hello,

I noticed a problem while initialising an union.
Here a test source:

struct a {
        int a;
};

struct b {
        int a;
        double b;
};

struct c {
        int a;
        double b;
        char c[16];
};

union x {
        int a;
        struct a as;
        struct b bs;
        struct c cs;
};

void test()
{       union x X = { 0 };
        for(;;);
        X.a++;
}

void main()
{       char    c[]="1123124213";
        test();

}

You will notice that the union X is not fully initialised. At least
I noticed that
on DecUNIX-4.0b (4.0d) Solaris-2.5.1 with the egcs version

  egcs-2.90.29 980515 (egcs-1.0.3 release)

 
-- 
Aurel Balmosan           | Department SE-SW
ORGA Kartensysteme GmbH  | phone: +49 5254 991 824
An der Kapelle 2         | fax  : +49 5254 991 749
33104 Paderborn, Germany | mailto:ab@orga.com
 

Re: Initialising an union.

[Jeroen Dobbelaere]

Hi,

The initialisation of a union only initializes the first member
of the union, not the 'largest' member. (the remaining part can be
uninitialized.


(from the december 1997 draft working paper :)

  8.5  Initializers                                           [dcl.init]

1 A declarator can specify an initial value  for  the  identifier  being

[...]

5 To zero-initialize storage for an object of type T means:
[..]

  --if  T  is  a  union type, the storage for its first data member5) is
    zero-initialized;

[.....]

15When  a  union  is  initialized with a brace-enclosed initializer, the
  braces shall only contain an initializer for the first member  of  the
  union.  [Example:
          union u { int a; char* b; };

          u a = { 1 };
          u b = a;
          u c = 1;              // error
          u d = { 0, "asdf" };  // error
          u e = { "asdf" };     // error
    --end example] [Note: as described above, the braces around the ini-
  tializer for a union member can be omitted if the union is a member of
  another aggregate.  ]

Greetings,

Jeroen Dobbelaere

Re: Initialising an union.

[Aurel Balmosan]

Jeroen Dobbelaere wrote:

> 5 To zero-initialize storage for an object of type T means:
> [..]
>
>   --if  T  is  a  union type, the storage for its first data member5) is
>     zero-initialized;
>
> [.....]
>
> 15When  a  union  is  initialized with a brace-enclosed initializer, the
>   braces shall only contain an initializer for the first member  of  the
>   union.  [Example:

Well in the ANSI-C books I have this is not defined in this way. It is only
said that unions can now be initialised (ANSI-C version 2)

So it seems to not a bug in egcs-1.0.3a but what draft is it? C++ or ANSI-C?

Bye,

        Aurel.

--
Aurel Balmosan           | Department SE-SW
ORGA Kartensysteme GmbH  | phone: +49 5254 991 824
An der Kapelle 2         | fax  : +49 5254 991 749
33104 Paderborn, Germany | mailto:ab@orga.com

Re: Initialising an union.

[Jeroen Dobbelaere]

Yes, I forgot to say that this is only true for C++

(See http://www.cs.sbcc.net/~shouk/cppdraft/cd2/ for a copy
of the working paper)

(maybe something to add to the faq, although it is not the definitive
standard :( )

I don't know what it should be for C++.

Also note that in your example, you are using an **auto-variable** :

void test() 
{       union x X = { 0 }; 
        for(;;); 
        X.a++; 
} 

It also seems that I mixed up static/global variables and auto :

This is the relevant section for this problem :

 8.5.1  Aggregates                                      [dcl.init.aggr]
[..]

15When  a  union  is  initialized with a brace-enclosed initializer, the
  braces shall only contain an initializer for the first member  of  the
  union.  [Example:
          union u { int a; char* b; };

          u a = { 1 };
          u b = a;
          u c = 1;              // error
          u d = { 0, "asdf" };  // error
          u e = { "asdf" };     // error
    --end example] [Note: as described above, the braces around the ini-
  tializer for a union member can be omitted if the union is a member of
  another aggregate.  ]


It seems not to say anything about the initialization of remaining
bytes. Because those are on the stack, those (probably) wont be
initialized -> so you should explicitly initialize the member you need.


Aurel Balmosan wrote:
> 
[..]
> 
> Well in the ANSI-C books I have this is not defined in this way. It is only
> said that unions can now be initialised (ANSI-C version 2)
> 
> So it seems to not a bug in egcs-1.0.3a but what draft is it? C++ or ANSI-C?
> 
> Bye,
> 
>         Aurel.
> 
> --
> Aurel Balmosan           | Department SE-SW
> ORGA Kartensysteme GmbH  | phone: +49 5254 991 824
> An der Kapelle 2         | fax  : +49 5254 991 749
> 33104 Paderborn, Germany | mailto:ab@orga.com

Initialising an union

[Aurel Balmosan]

Hello,

I noticed a problem while initialising an union.
Here a test source:

struct a {
        int a;
};

struct b {
        int a;
        double b;
};

struct c {
        int a;
        double b;
        char c[16];
};

union x {
        int a;
        struct a as;
        struct b bs;
        struct c cs;
};

void test()
{       union x X = { 0 };
        for(;;);
        X.a++;
}

void main()
{       char    c[]="1123124213";
        test();

}

You will notice that the union X is not fully initialised. At least I noticed
that
on DecUNIX-4.0b (4.0d) Solaris-2.5.1 with the egcs version

  egcs-2.90.29 980515 (egcs-1.0.3 release)

--
Aurel Balmosan           | Department SE-SW
ORGA Kartensysteme GmbH  | phone: +49 5254 991 824
An der Kapelle 2         | fax  : +49 5254 991 749
33104 Paderborn, Germany | mailto:ab@orga.com

Re: Initialising an union

[Alexandre Oliva]

Aurel Balmosan <ab@orga.com> writes:

> You will notice that the union X is not fully initialised.

What do you mean by fully initializing a union?  You have initialized
it with only an integer, so only the first integer field will be
initialized; the remaining fields need not be, because you have not
initialized them.

-- 
Alexandre Oliva
mailto:oliva@dcc.unicamp.br mailto:aoliva@acm.org
http://www.dcc.unicamp.br/~oliva
Universidade Estadual de Campinas, SP, Brasil

Re: Initialising an union

[Aurel Balmosan]

Alexandre Oliva wrote:

> Aurel Balmosan <ab@orga.com> writes:
>
> > You will notice that the union X is not fully initialised.
>
> What do you mean by fully initializing a union?  You have initialized
> it with only an integer, so only the first integer field will be
> initialized; the remaining fields need not be, because you have not
> initialized them.

As it is defined in ANSI-C a struct where only the first part is initialised
is filled with zero's. As for gcc-2.7.2.3 and older versions the union was
fully initialised. This is not any more with egcs-1.0.3a which we now use.

Either the gcc-2.7.2.3 and older used a not ANSI-C comform initialising
for unions or the egcs-1.0.3a does not follow ANSI-C in this part.

structs are fully initialised to zero when one say ie:

struct c {
      int a;
      double b;
      char c[123];
};

struct d {
     char c[234];
     char d[231];
     int e;
};

struct c Y = { 0 };
struct d Z = { 0 };

This is with gcc-2.7.2.3 and older as well as egcs-1.0.3a. Only the union
is not! And thats my problem and a bug in egcs-1.0.3a

This is defined in the ANSI-C  Language Reference A.8.7: Initialisation
I have currently only the german version but here the sentence which
describes that behaviour and applies for struct's as well as for union's:

initializer:
        assignment-expression
        { initializer-list }
        { initializer-list, }

initializer-list:
    initializer,
    initializer-list, initializer

Contains the initializer-list less initializer then the structure component
has the rest values will be set to zero. There must not be more initializer
than values in a structure component.


I think that makes it clear.

Bye,

    Aurel.

--
Aurel Balmosan           | Department SE-SW
ORGA Kartensysteme GmbH  | phone: +49 5254 991 824
An der Kapelle 2         | fax  : +49 5254 991 749
33104 Paderborn, Germany | mailto:ab@orga.com

Re: Initialising an union

[Alexandre Oliva]

Aurel Balmosan <ab@orga.com> writes:

> As it is defined in ANSI-C a struct where only the first part is initialised
> is filled with zero's.

> This is defined in the ANSI-C  Language Reference A.8.7:

> Contains the initializer-list less initializer then the structure component
> has the rest values will be set to zero. There must not be more initializer
> than values in a structure component.

Does it apply to local non-static variables?  I'm pretty sure this is
true for variables in static storage, but how about local ones?
Unfortunately, I don't have access to the ANSI-C Standard :-(

-- 
Alexandre Oliva
mailto:oliva@dcc.unicamp.br mailto:aoliva@acm.org
http://www.dcc.unicamp.br/~oliva
Universidade Estadual de Campinas, SP, Brasil

Re: Initialising an union

[Aurel Balmosan]

Alexandre Oliva wrote:

> Does it apply to local non-static variables?  I'm pretty sure this is
> true for variables in static storage, but how about local ones?
> Unfortunately, I don't have access to the ANSI-C Standard :-(

It does. In the ANSI-C Language Referenze Version 2 A.8.7
from 31.Oct.1988 ANSI Programming Langues C, X.3.159-1989
you find the sentence:

    In the first edition the initialisation of auto and register
    struct's, union's or vektors was not allowed. The ANSI-C
    standard allowes to use const constructors or simple
    initilizer expresions.

This make clear that auto and register structs/unions/vektors
can be initialised.

Also I used the same code on different platforms i.e DEC-Unix
where we use the standard C compiler which comes with the OS.
There the code works as I expected.

So now I am back that the egcs-1.0.3a has a bug here.

Bye,

    Aurel.

--
Aurel Balmosan           | Department SE-SW
ORGA Kartensysteme GmbH  | phone: +49 5254 991 824
An der Kapelle 2         | fax  : +49 5254 991 749
33104 Paderborn, Germany | mailto:ab@orga.com

Re: Initialising an union

[Alexandre Oliva]

Aurel Balmosan <ab@orga.com> writes:

> Alexandre Oliva wrote:
>> Does it apply to local non-static variables?  I'm pretty sure this is
>> true for variables in static storage, but how about local ones?
>> Unfortunately, I don't have access to the ANSI-C Standard :-(

[snip]

> This make clear that auto and register structs/unions/vektors
> can be initialised.

That was not my question.  I asked whether the standard explicitly
stated that uninitialized members of an automatic union would be
implicitly zero-initialized.  A summary of the related posts to the
list follows:

1) automatic unions can be explicitly initialized

2) if a initializer-list contains less elements than the initialized
object, the remaining elements are zero-initialized

3) when a union is explicitly initialized, the initializer-list will
be used to initialize the first element of the union

so, given the following code snippet:

union t {
  int i;
  char c[sizeof(int)+1];
};

void foo() {
  union t x = { 0 };
  assert(c[sizeof(int)]==0);
}

Given only the three statements listed above, the assertion *can*
fail, because the second type of the union (c) is *not* initialized,
only the first one (i) is.  Thus, it needs not be zero initialized,
based on the claims above.

As I have stated earlier, I don't have a copy of the C Standard, and
I'd be happy to find out that the Standard does specify that union
initialization must be performed as if the whole union were
zero-initialized before explicit initialization takes place.  It's
just that I haven't read (or understood :-) any claim that supports
this interpretation, and it doesn't look much like C to me :-)

-- 
Alexandre Oliva
mailto:oliva@dcc.unicamp.br mailto:aoliva@acm.org
http://www.dcc.unicamp.br/~oliva
Universidade Estadual de Campinas, SP, Brasil