Re: Initialising an union

[Aurel Balmosan <ab at orga dot com>]

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