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Re: Extending unions and ABI?

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On 5/16/2011 14:54, Morris Keesan wrote:
> On Mon, 16 May 2011 14:38:02 -0400, Shao Miller <sha0.miller@gmail.com>
> wrote:
>
>> On 5/16/2011 14:06, Jef Driesen wrote:
>>> Suppose I have defined a union:
>>>
>>> typedef struct foo_t {
>>> ...
>>> } foo_t;
>>>
>>> typedef struct bar_t {
>>> ...
>>> } bar_t;
>>>
>>> typedef union foobar_t {
>>> foo_t foo;
>>> bar_t bar;
>>> } foobar_t;
>>>
>>> Which is part of the public api of my project.
>>
>> As in, these are in a header file and the struct and union definitions
>> will thus be available within all users' translation units, assuming
>> they include the header?
>>
>>> The union is used to pass
>>> different types of data (e.g. either a foo or a bar struct) back to the
>>> caller through a callback function. The type is used distinguish between
>>> the two:
>>>
>>> typedef enum foobar_type_t {
>>> FOO;
>>> BAR;
>>> } foobar_type_t;
>>>
>>
>> Did you mean to use commas instead of semi-colons for the two values?
>>
>>> typedef void (*callback_t)(foobar_type_t type, const foobar_t *foobar);
>>>
>>> void
>>> dosomething (callback_t callback)
>>> {
>>>
>>> foobar_t foobar;
>>>
>>> foobar.foo = ...
>>> callback (FOO, &foobar);
>>>
>>> foobar.bar = ...
>>> callback (BAR, &foobar);
>>> }
>>>
>>> Note that I only use pointers to the union in the public api. The union
>>> itself appears only in the implementation (e.g. in the dosomething()
>>> function).
>>>
>>
>> Do you mean that the union definition you gave closer to the beginning
>> of your post is only within the translation unit with the
>> 'dosomething()' definition? Or do you mean that the only code which
>> will use the 'sizeof' or the members of the union will be within the
>> translation unit with the 'dosomething()' definition?
>>
>>> Now suppose I need to extend my api to support a third type of data. Is
>>> it portable to just add a third struct to the union? Will it break the
>>> ABI rules and thus backwards compatibility. Is this a violation of the C
>>> standard?
>>>
>>
>> From n1256.pdf, section 6.2.5, point 27:
>>
>> "...All pointers to union types shall have the same representation and
>> alignment requirements as each other..."
>>
>> :)
>>
>>> The alternative would be to use a void pointer and require the user to
>>> cast to the appropriate data type,
>>
>> No need to cast a 'void *' to another pointer-to-object type.
>
> There is if you're going to do what he does here:
>
>>> ((foo_t *)foobar)->member
>
> Unless you want to declare an extra pointer to hold the converted type:
> (foo_t *foo = foobar)->member

Oops.  Good point. :)  I didn't understand the need for a cast, since 
one could use:

   ... foobar->foo.member ...

Assuming the union definition is available to the users... 'foo_t' 
obviously must be.

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Thread

Extending unions and ABI? Jef Driesen <jefdriesen@hotmail.com.invalid> - 2011-05-16 20:06 +0200
  Re: Extending unions and ABI? Tom St Denis <tom@iahu.ca> - 2011-05-16 11:20 -0700
    Re: Extending unions and ABI? jt@toerring.de (Jens Thoms Toerring) - 2011-05-16 19:12 +0000
  Re: Extending unions and ABI? Shao Miller <sha0.miller@gmail.com> - 2011-05-16 14:38 -0400
    Re: Extending unions and ABI? "Morris Keesan" <mkeesan@post.harvard.edu> - 2011-05-16 14:54 -0400
      Re: Extending unions and ABI? Shao Miller <sha0.miller@gmail.com> - 2011-05-16 15:13 -0400
    Re: Extending unions and ABI? Jef Driesen <jefdriesen@hotmail.com.invalid> - 2011-05-16 20:55 +0200
      Re: Extending unions and ABI? Shao Miller <sha0.miller@gmail.com> - 2011-05-16 15:43 -0400
        Re: Extending unions and ABI? Shao Miller <sha0.miller@gmail.com> - 2011-05-16 15:48 -0400
          Re: Extending unions and ABI? Jef Driesen <jefdriesen@hotmail.com.invalid> - 2011-05-17 20:10 +0200
            Re: Extending unions and ABI? Shao Miller <sha0.miller@gmail.com> - 2011-05-18 00:28 -0500
        Re: Extending unions and ABI? Jef Driesen <jefdriesen@hotmail.com.invalid> - 2011-05-16 21:54 +0200
  Re: Extending unions and ABI? Columbus sailed the ocean China Blue <chine.bleu@yahoo.com> - 2011-05-17 12:53 -0700
  Re: Extending unions and ABI? "io_x" <a@b.c.invalid> - 2011-05-18 19:31 +0200
  Re: Extending unions and ABI? gordonb.4d208@burditt.org (Gordon Burditt) - 2011-05-23 04:03 -0500
  Re: Extending unions and ABI? Tim Rentsch <txr@alumni.caltech.edu> - 2011-05-30 23:47 -0700

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