Is __attribute__ ((__packed__)) ignored on a typedef declaration?

Question

Though __attribute__ ((aligned)) works well with the typedef declaration such as :

typedef struct __attribute__((__aligned__(8))) A {

     xxx ip    ;
     xxx udp   ;
     xxx ports ;
} table ;

I have come across statements which say that this is not the case with __attribute__ ((__packed__)) with typedef ! I was going through some of the related question and some of them have used the packed attribute with typedef which tallies with our code.

Now , In our code we define

typedef struct {
     xxx ip    ;
     xxx udp   ;
     xxx ports ;
}__attribute__((packed)) table ;

Does the above declaration makes the compiler to silently dump the packed attribute declaration?

PS : Yes , I could have verified it ,but my circumstances are different at the moment .Lets say holidays and a smartphone !

Answer 1

The declaration Seems okay to me !

It can be declared in one of these ways.

#include <stdio.h>

typedef struct __attribute__((packed)) {
    char old;
    int ip;
    int new;
} NCO;

int main(void)
{ 
    printf("%zu", sizeof(NCO));
} 

or

#include <stdio.h> 

typedef struct {
    char old;
    int ip;
    int new;
} __attribute__((packed)) 

int main(void)
{
    printf("%zu", sizeof(NCO));
} 

Ensure that the __attribute__((packed)) keyword and attribute specification immediately follows the right brace (}) of the structure declaration. If it is in any other position (such as, following a structure instance instead of preceding a structure instance), the compiler will ignore__attribute__((packed)) and issue a warning message.

Although it gives us the packed size 9 , I think it is better to avoid it as stated here and try the old school structure declaration style .