Taking the address of a bitfield

Question

I'm going through few C aptitude questions. This one seemed tricky, can any one explain?

struct {
    int foo : 4;
} baz;

int *example()
{
    return &baz.foo;
}

This is invalid code, but i couldn't figure out the reason.

If you use bitfield, never use address of (or pointers) – Krishnabhadra Mar 19 '13 at 12:17 — Krishnabhadra, Mar 19 '13 at 12:17

score 7 · Answer 1 · answered Mar 19 '13 at 12:17

int foo : 4;

declares foo as a bit-field. One must not take the address of a bit-field.

Section 6.5.3.2 starts:

The operand of the unary & operator shall be either a function designator, the result of a [] or unary * operator, or an lvalue that designates an object that is not a bit-field and is not declared with the register storage-class specifier.

So applying the address operator to a bit-field is a constraint violation. (It wouldn't even make much sense, since a bit-field need not start at a byte boundary.)

score 4 · Answer 2 · answered Mar 19 '13 at 12:18

The colon syntax in the struct declares foo as a bit-field.

Since a bit-field might be smaller than the architecture's smallest addressable piece of memory (typically the byte on many modern CPU architectures), it's not allowed to take the address of such a member.

Thus, this code fails to compile since it violates that rule.

score 2 · Accepted Answer · answered Mar 19 '13 at 12:18

2

The foo field is a 4-bit wide bit-field – smaller than a single byte. Pointers can only address whole bytes, so this is invalid. Even if foo was 8 or 32 bits wide (full/aligned bytes on modern architectures) this would still be invalid.

answered Mar 19 '13 at 12:18

Gokul Nath KP

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score 1 · Answer 4 · answered Mar 19 '13 at 12:21

1

I think if you compile your file you will have solution:

cannot take address of bit-field

Because in memory architecture, each time you use pointer it will to take address where it points to. but baz is just a bit field, so it will be invalid.

answered Mar 19 '13 at 12:21

hqt

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Taking the address of a bitfield

4 Answers4