Char bit fields

[Alex Vinokur]

struct Foo
{
unsigned char : 2;
unsigned char : 6;
};

What is correct sizeof(Foo) ?

Four compilers:
aCC: HP C/aC++ B3910B A.06.15 [May 16 2007]
CC: Sun C++ 5.9 SunOS_sparc Patch 124863-01 2007/07/25
Intel(R) C++ Version 11.0.083
Microsoft C/C++ 14.00.50727
produce sizeof(Foo) == 1

IBM XL C/C++ Enterprise Edition V8.0 for AIX Version: 08.00.0000.0014
IBM XL C/C++ Enterprise Edition for AIX, V9.0 Version: 09.00.0000.0008
produce sizeof(Foo) == 4.

Alex Vinokur

 

[Alf P. Steinbach]

* Alex Vinokur:

struct Foo
{
unsigned char : 2;
unsigned char : 6;
};

What is correct sizeof(Foo) ?

At least 1.

Four compilers:
aCC: HP C/aC++ B3910B A.06.15 [May 16 2007]
CC: Sun C++ 5.9 SunOS_sparc Patch 124863-01 2007/07/25
Intel(R) C++ Version 11.0.083
Microsoft C/C++ 14.00.50727
produce sizeof(Foo) == 1

IBM XL C/C++ Enterprise Edition V8.0 for AIX Version: 08.00.0000.0014
IBM XL C/C++ Enterprise Edition for AIX, V9.0 Version: 09.00.0000.0008
produce sizeof(Foo) == 4.

So?


Cheers,

- Alf

 

[Richard Herring]

In message

struct Foo
{
unsigned char : 2;
unsigned char : 6;
};

What is correct sizeof(Foo) ?

Anything greater than 0.

Four compilers:
aCC: HP C/aC++ B3910B A.06.15 [May 16 2007]
CC: Sun C++ 5.9 SunOS_sparc Patch 124863-01 2007/07/25
Intel(R) C++ Version 11.0.083
Microsoft C/C++ 14.00.50727
produce sizeof(Foo) == 1

IBM XL C/C++ Enterprise Edition V8.0 for AIX Version: 08.00.0000.0014
IBM XL C/C++ Enterprise Edition for AIX, V9.0 Version: 09.00.0000.0008
produce sizeof(Foo) == 4.

It's hardly surprising that alignment and padding requirements of
different architectures tend to differ.

 

[James Kanze]

struct Foo
{
unsigned char : 2;
unsigned char : 6;
};

What is correct sizeof(Foo) ?

Either implementation defined or unspecified, I'm not sure
which. (Implementation defined means that the implementation
must document what it does---in this case, how it lays out bit
fields, etc. Unspecified frees the implementation of the
documentation requirement.)

 

[Jerry Coffin]

Either implementation defined or unspecified, I'm not sure
which. (Implementation defined means that the implementation
must document what it does---in this case, how it lays out bit
fields, etc. Unspecified frees the implementation of the
documentation requirement.)

I believe documentation is required:

§9.6/1: "Allocation of bit-fields within a class object is
implementation-defined. Alignment of bit-fields is implementation-
defined."

 

[James Kanze]

I believe documentation is required:

§9.6/1: "Allocation of bit-fields within a class object is
implementation-defined. Alignment of bit-fields is
implementation- defined."

Then documentation is required. (Good luck on finding it,
however.)

Thanks for looking it up for me. (I could have looked it up as
well, but was just too lazy to.)