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c++cgccattributes

Can __attribute__ ((packed)) create a 1-bit struct when the only member is a bit-field?

I was reading through the documentation of GNU/GCC and I stopped at the description of __attribute__ ((packed)). It says:

packed

The packed attribute specifies that a variable or structure field should have the smallest possible alignment—one byte for a variable, and one bit for a field, unless you specify a larger value with the aligned attribute.

Here is a structure in which the field x is packed, so that it immediately follows a:

struct foo
{
    char a;
    int x[2] __attribute__ ((packed));
};

I wonder how this attribute works exactly and whether this code:

struct bar
{
    char a:1 __attribute__ ((packed));
};

would create a 1-bit structure instead of 1-byte.

Solution

  • A byte is the smallest addressable unit. As such, structs can only occupy a whole number of bytes, not fractional bytes.

    Any unused bits in bitfields (or more accurately, the storage units containing bitfields) become padding.

    So in your example of struct bar, the size of the struct will still be 1, with 1 bit used and 7 bits unused.