JTC1/SC22/WG14 N743

WG14 N743  J11 97-106


			Bit-field Enhancements
			     David Keaton
			      1997-06-27


All relevant parts of the standard have been listed, whether they
require changes or not.  Caveat:  The changes are made against C9X
draft 10-pre3 and must be merged with other changes made at the London
meeting.


6.1.2.6 Compatible type and composite type, paragraph 1

This already specifies the correct rules for extended bit-fields.  The
types and widths must match.


6.2.1.1 Characters and integers, paragraph 1

Replace with the following:

The following conversions are called the "integral promotions."37

Any of these types may be used in an expression wherever an "int" or
"unsigned int" may be used.
	- a "bool" or a "bool" bit-field
	- a "char," "signed char," or "unsigned char," or bit-fields
	  of these types
	- a "short," "signed short," or "unsigned short," or
	  bit-fields of these types
	- an "int," "signed int," or "unsigned int" bit-field
	- an enumeration type
If an "int" can represent all the values of the original type, the
value is converted to "int;"  otherwise, it is converted to "unsigned
int."

A "long," "signed long," or "unsigned long" bit-field may be used in
an expression wherever a "long" or "unsigned long" may be used.  If a
"long" can represent all values of the original type, the value is
converted to a "long;"  otherwise, it is converted to an "unsigned
long."

A "long long," "signed long long," or "unsigned long long" bit-field
may be used in an expression wherever a "long long" or "unsigned long
long" may be used.  If a "long long" can represent all the values of
the original type, the value is converted to a "long long;"
otherwise, it is converted to an "unsigned long long."

All other arithmetic types are unchanged by the integral promotions.

				- 2 -


6.5.2 Type specifiers, paragraph 4

Replace with the following:

Each of the above comma-separated sets designates the same type,
except that for bit-fields, the types "short," "int," "long," and
"long long" may differ from their signed varieties.


6.5.2.1 Structure and union specifiers, paragraph 8

Replace with the following (but merge with other changes from this
meeting first):

A bit-field shall have a type that is a qualified or unqualified
version of one of the following.
	- "bool"
	- "char," "signed char," or "unsigned char"
	- "short," "signed short," or "unsigned short"
	- "int," "signed int," or "unsigned int"
	- "long," "signed long," or "unsigned long"
	- "long long," "signed long long," or "unsigned long long"
Whether the high-order bit position of a (possibly qualified)
``plain'' "char," "short," "int," "long," or "long long" bit-field is
interpreted as a sign bit is implementation-defined.  A bit-field is
interpreted as an integral type consisting of the specified number of
bits.

A "bool" value can successfully be stored in a "bool" bit-field of any
nonzero size.

If the value "true" or "false" is stored into a bit-field of type
"bool" of any nonzero size (including a one bit bit-field), the
original bool value and the value of the bit-field shall compare
equal.


6.5.2.1 Structure and union specifiers, paragraph 9

The existing packing rules suffice.  Note that two bit-fields need not
have the same type to be packed together.  This is what we want.

				- 3 -


I.2 Undefined behavior, paragraph 1

Replace this:

- A bit-field is declared with a type other than "int," "signed int,"
or "unsigned int" (6.5.2.1).

with the following:

- A bit-field is declared with a type other than the following
  (6.5.2.1).
	- "bool"
	- "char," "signed char," or "unsigned char"
	- "short," "signed short," or "unsigned short"
	- "int," "signed int," or "unsigned int"
	- an enumeration type
	- "long," "signed long," or "unsigned long"
	- "long long," "signed long long," or "unsigned long long"


I.3.9 [Implementation-defined behavior] Structures, unions,
enumerations, and bit-fields; paragraph 1

Replace this:

- Whether a ``plain'' "int" bit-field is treated as a "signed int"
bit-field or as an "unsigned int" bit-field (6.5.2.1).

with the following:

- Whether a ``plain'' "char," "short," "int," "long," or "long long"
bit-field is treated as being signed (6.5.2.1).


I.5.8 [Common extensions] Non-"int" bit-field types

Change the title to the following.
Extended bit-field types.

Replace paragraph 1 with the following.

Types other than standard integer types can be declared as bit-fields,
with appropriate maximum widths (6.5.2.1).