Since beginning of the times I've used to believe that __attrbute__((packed)) can be put on a struct or typedef, like that:

typedef struct __attribute__((packed)) {
    uint8_t   m1;
    uint16_t  m2;
    uint8_t   m3;
    uint32_t  m4;
    uint8_t   m5;
} junk;

But then someone pointed out that this obvious usage is not documented in the recent gcc manuals, for already several releases. Rather it says (in v.12): "The [packed] attribute does not apply to non-member objects."
A whole struct is not a member object, correct?
So is the above example legal, or relies on UB (and can bite later)?

CodePudding user response：

The above example is legal. The documentation that you linked to is a listing of variable attributes, which are attached to individual variables, not whole structures. When the documentation says that the attribute "does not apply to non-member objects", it is saying that it does not apply to non-member variables. Nothing on that page applies to the usage of the packed attribute shown here.

Instead, your example of using the packed attribute is using a type attribute, which happens to have the same name as the packed variable attribute; see here for the documentation. packed is one of the attributes listed there; besides the name and somewhat related functionality (of controlling how fields are arranged in a structure), these two attributes are not related to each other.