What is the type of a bitfield?

Question

I can't find anywhere in the C standard where this is specified. For example, in

struct { signed int x:1; } foo;

is foo.x an lvalue of type int, or something else? It seems unnatural for it to be an lvalue of type int since you cannot store any value of type int in it, only 0 or -1, but I can't find any language that would assign it a different type. Of course, used in most expressions, it would get promoted to int anyway, but the actual type makes a difference in C11 with _Generic, and I can't find any language in the standard about how bitfields interact with _Generic either.

Answer 1

As Jonathan already cited, p5 clearly states what the type a bit-field has.

What you should have also in mind is that there is a special rule for bit-field arithmetic conversions in 6.3.1.1, basically stating that if an int can represent all values such a bit-field converts to an int in most expressions.

What the type would be in a _Generic should be the declared type (modulo the sign glitch), since it seems to be consensus that arithmetic conversions don't apply, there. So

_Generic((X), int: toto, unsigned: tutu)
_Generic(+(X), int: toto, unsigned: tutu)

could give you different results if X is an unsigned bit-field with a width that has all values fit into an int.

Answer 2

Given that you included the signed qualifier, then the only values that can be stored in the 1-bit bit field are indeed -1 and 0. If you'd omitted the qualifier, it would be implementation defined whether the 'plain' int bit field was signed or unsigned. If you'd specified unsigned int, of course, the values would be 0 and +1.

The relevant sections of the standard are:

§6.7.2.1 Structure and union specifiers

¶4 The expression that specifies the width of a bit-field shall be an integer constant expression with a nonnegative value that does not exceed the width of an object of the type that would be specified were the colon and expression omitted.¹²²⁾ If the value is zero, the declaration shall have no declarator.

¶5 A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation-defined type. It is implementation-defined whether atomic types are permitted.

¶10 A bit-field is interpreted as having a signed or unsigned integer type consisting of the specified number of bits.¹²⁵⁾ If the value 0 or 1 is stored into a nonzero-width bit-field of type _Bool, the value of the bit-field shall compare equal to the value stored; a _Bool bit-field has the semantics of a _Bool.

¹²²⁾ While the number of bits in a _Bool object is at least CHAR_BIT, the width (number of sign and value bits) of a _Bool may be just 1 bit.

¹²⁵⁾ As specified in 6.7.2 above, if the actual type specifier used is int or a typedef-name defined as int, then it is implementation-defined whether the bit-field is signed or unsigned.

The footnote 125 points to:

§6.7.2 Type Specifiers

¶5 Each of the comma-separated multisets designates the same type, except that for bitfields, it is implementation-defined whether the specifier int designates the same type as signed int or the same type as unsigned int.