Re: A basic (?) problem with addresses (gcc)



"Seebs" <usenet-nospam@xxxxxxxxx> wrote in message
news:slrnigkle6.26q4.usenet-nospam@xxxxxxxxxxxxxxxxxx
On 2010-12-16, BartC <bc@xxxxxxxxxx> wrote:
OK. But apart from the inconvenience of wrapping these things in unions
then
having to use field selection to access the data, how do you do something
like this:

integer*4 i(20)
real*8 a
equivalance (a,i(7))

(So the 8 bytes at i(7..8) are shared with the floating point number.)

You don't. C doesn't support or allow for overlap like this, so far as I
know.

The Fortran example was to demonstrate what I meant. But in fact I use that
construction in another language, and it looks like this:

[20]int i
real a @ &i[7]

(although actual uses are usually more straightforward).

From time to time it might be necessary to rewrite that in C, and I relied
on the OP's trick to do the conversion. Now it seems that that may not work,
not because there's some weird hardware involved, but because some compilers
might exploit a 'loophole' in the standard so that they can get away with a
cheap optimisation, even thought the hardware wouldn't have a problem with
it.

That sounds wrong. It also seems like something that C ought to be capable of: directly specifying an address where you want something stored, even if relative to the address of another object.

--
Bartc

.

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