用于字节序测试和字节交换的Union hack

//snippet 1(testing for endianess): 

union
{
    int  i;
    char c[sizeof(int)];
} x;

x.i = 1;                     // writing to i
if(x.c[0] == 1)              // reading from c[0]
{   printf(\"little-endian\\n\");
}
else
{   printf(\"big-endian\\n\");
}

//snippet 2(swap bytes using union):

int swapbytes()
{
    union                   // assuming 32bit, sizeof(int)==4
    {        
        int  i;
        char c[sizeof(int)];
    } x;
    x.i = 0x12345678;       // writing to member i
    SWAP(x.ch[0],x.ch[3]);  // writing to char array elements
    SWAP(x.ch[1],x.ch[2]);  // writing to char array elements
    return x.i;             // reading from x.i 
}