关于C struct / union的可移植性问题

union foreign_t {
    struct {
        enum enum_t an_enum;
        int an_int;
    } header;
    struct {
        double x,y;
    } point;
};

假设以下代码片段在不同平台和不同编译器上按预期工作是否安全？

struct pair_t {
    double x,y;
};

union foreign_t foreign;
struct pair_t *p_pair;

p_pair = (struct pair_t *) &foreign;
p_pair->x = 1234;
p_pair->y = 4321;

/* Expected result: (1234,4321) or something like that */
printf("(%lf,%lf)",foreign.point.x,foreign.point.y);

编辑：

按照严格的别名建议,我做了以下测试：

#include <stdint.h>
#include <stdio.h>

int main()
{
    uint16_t word = 0xabcd;
    uint8_t tmp;
    struct {
        uint8_t low;
        uint8_t high;
    } *byte = (void *) &word;

    tmp = byte->low;
    byte->low = byte->high;
    byte->high = tmp;

    printf("%xn",word);

    return 0;
}

以上显然是无辜的代码是不可靠的：

$gcc -O3 -fno-strict-aliasing -otest test.c
$./test
cdab
$gcc -O3 -fstrict-aliasing -otest test.c
$./test
abcd

开发商没有和平……