Question

For a union, writing to one member and reading from other member (except for char array) is UB.

//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 
}   

Snippet 1 is legal C or C++ but not snippet 2. Am I correct? Can some one point to the section of standard where it says its OK to write to a member of union and read from another member which is a char array.

Answer 1

I believe it (snippet 1) is technically not allowed, but most compilers allow it anyway because people use this kind of code. GCC even documents that it is supported.

You will have problems on some machines where sizeof(int) == 1, and possibly on some that are neither big endian nor little endian.

Either use available functions that change words to the proper order, or set this with a configuration macro. You probably need to recognize compiler and OS anyway.