I don't understand the -Wpacked description from the GCC manual page, it reads:
-Wpacked
Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be
mis-aligned for little benefit. For instance, in this
code, the variable "f.x" in "struct bar" will be misaligned even though "struct bar" does not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
why does the "__attribute__((packed))" for struct foo cause misalignment for struct bar ?
I would expect misalignment only if struct bar is defined with __attribute__((packed))
or its member f.
I thought __attribute__((packed)) only works for (the members of) the struct, but apparently
it tries to avoid 3 bytes extra padding in struct bar. However, sizeof(struct bar) = 12 !?
so there still is 3 bytes padding *and* misalignment. Why would anyone ever want this gcc behaviour ?
This case corresponds to case 3 of my simple test program, see below:
linpc166:~/c/alignment>cat -n t.c
1 #include <stdio.h>
2
3 struct foo {
4 int x;
5 char a, b, c, d;
6 };
7
8 struct foo2 {
9 int x;
10 char a, b, c, d;
11 } __attribute__ ((__packed__));
12
13
14 struct bar {
15 char z;
16 struct foo f;
17 };
18
19 struct barp {
20 char z;
21 struct foo f;
22 } __attribute__ ((__packed__));
23
24 struct bar2 {
25 char z;
26 struct foo2 f;
27 };
28
29 struct bar2p {
30 char z;
31 struct foo2 f;
32 } __attribute__ ((__packed__));
33
34
35 int main(void)
36 {
37 struct bar b;
38 struct barp bp;
39 struct bar2 b2;
40 struct bar2p b2p;
41
42 printf("case 1(b): sizeof(struct foo)=%ld, sizeof(struct bar)=%ld\n", sizeof(struct foo), sizeof(struct bar));
43 if ((((unsigned long)&b.f.x) - ((unsigned long)&b.z)) & 0x3) {
44 printf("case 1(b): MIS-ALGND\n");
45 } else {
46 printf("case 1(b): ALGND\n");
47 }
48 printf("\ncase 2(bp): sizeof(struct foo)=%ld, sizeof(struct barp)=%ld\n", sizeof(struct foo), sizeof(struct barp));
49 if ((((unsigned long)&bp.f.x) - ((unsigned long)&bp.z)) & 0x3) {
50 printf("case 2(bp): MIS-ALGND\n");
51 } else {
52 printf("case 2(bp): ALGND\n");
53 }
54 printf("\ncase 3(b2): sizeof(struct foo2)=%ld, sizeof(struct bar)=%ld\n", sizeof(struct foo2), sizeof(struct bar));
55 if ((((unsigned long)&b2.f.x) - ((unsigned long)&b2.z)) & 0x3) {
56 printf("case 3(b2): MIS-ALGND\n");
57 } else {
58 printf("case 3(b2): ALGND\n");
59 }
60 printf("\ncase 4(b2p): sizeof(struct foo2)=%ld, sizeof(struct barp)=%ld\n", sizeof(struct foo2), sizeof(struct barp));
61 if ((((unsigned long)&b2p.f.x) - ((unsigned long)&b2p.z)) & 0x3) {
62 printf("case 4(b2p): MIS-ALGND\n");
63 } else {
64 printf("case 4(b2p): ALGND\n");
65 }
66
67 return 0;
68 }
linpc166:~/c/alignment>gcc -g -Wpacked -Wall t.c
t.c:11:1: warning: packed attribute is unnecessary for 'foo2'
t.c:32:1: warning: packed attribute is unnecessary for 'bar2p'
linpc166:~/c/alignment>./a.out
case 1(b): sizeof(struct foo)=8, sizeof(struct bar)=12
case 1(b): ALGND
case 2(bp): sizeof(struct foo)=8, sizeof(struct barp)=9
case 2(bp): MIS-ALGND
case 3(b2): sizeof(struct foo2)=8, sizeof(struct bar)=12
case 3(b2): MIS-ALGND
case 4(b2p): sizeof(struct foo2)=8, sizeof(struct barp)=9
case 4(b2p): MIS-ALGND
