Re: egcs and bitfields

[Robert Lipe <robertlipe at usa dot net>]

> struct baz { unsigned int a:2, b:4, c:32;} y;
> 
> void foo()
> {
>   y.c = 0x56789;
> }
> 
> int main()
> {
>   unsigned int  *b;
>   bzero(&y,sizeof(y));
>   foo();
>   b=&y;
>   printf("%x %x\n",b[0],b[1]);
> }
> 
> Is this the case you are talking about? Now, you claim that the SysV
> ABI requires y.c to be at offset 4, right? (this is a claim I can't
> verify since I don't have that specification)

"System V ABI" is too vague for purposes of this discussion.  Do you
refer to the iBCS2 spec or the SVR4 ABI?  I don't know that they differ
in this regard, but it isn't an unreasonable question before tearing
into large books.

> Anyway, when I run the program on i586-pc-linux-gnu, with
> egcs-2.93.11, I get as output
> 
> 0 56789
> 
> From your explanation, I understand that this result is
> ABI-compliant. So where is the problem?

I fed this example into the following compilers on a system V/386 host:
	SCO OpenServer /bin/cc
	UDK cc (essentially the SVR4 ABI)
	EGCS 1.1.1
	Intel's optimizing compiler
It produced identical output on all of them.



I think (but am no authority) that Linux uses substantially the SVR4 ABI
but it does have some deviations.

RJL