Jan Blunck wrote:
On 12/6/06, Phil Endecott <[email protected]> wrote:
I used to think that this:
struct foo {
int a __attribute__((packed));
char b __attribute__((packed));
... more fields, all packed ...
};
was exactly the same as this:
struct foo {
int a;
char b;
... more fields ...
} __attribute__((packed));
but it is not, in a subtle way.
The same code is generated. [...]
I don't think so. Example:
struct test {
int a __attribute__((packed));
int b __attribute__((packed));
};
char c = 1;
struct test t = { .a=2, .b=3 };
$ arm-linux-gnu-gcc -O2 -S -W -Wall test1.c
.file "test2.c"
.global c
.data
.type c, %object
.size c, 1
c:
.byte 1
.global t
.align 2 <<<<<<<<===== t is aligned
.type t, %object
.size t, 8
t:
.word 2
.word 3
.ident "GCC: (GNU) 4.1.2 20061028 (prerelease) (Debian 4.1.1-19)"
Compare with:
struct test {
int a;
int b;
} __attribute__((packed));
char c = 1;
struct test t = { .a=2, .b=3 };
$ arm-linux-gnu-gcc -O2 -S -W -Wall test2.c
.file "test1.c"
.global c
.data
.type c, %object
.size c, 1
c:
.byte 1
.global t <<<<<< "align" has gone, t is unaligned
.type t, %object
.size t, 8
t:
.4byte 2
.4byte 3
.ident "GCC: (GNU) 4.1.2 20061028 (prerelease) (Debian 4.1.1-19)"
Phil.
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