unsafe_cast

README

Unsafe Cast

This is an example of using unsafe.Pointer to convert a type between two different forms (like a C union). This is legal because the two types are the same memory size, so when you dereference them, you are not extending into unallocated memory.

You can always cast to a value where the size is equal to or less than the original size, but the result may be surprising.

For example, take note of the values you get when it is run. Given t1.a=0xC0DECAFE, you might expect t2.a to be 0xC0DE and t2.b to be 0xCAFE, but instead you get:

t1.a=0xC0DECAFE

t2.a=0xCAFE
t2.b=0xC0DE

Why?

This is the Endianness of the memory sequencing mode of the CPU leaking through. The memory for T1 is actually laid-out physically as "Little Endian", or Lower-order bytes first (Intel x86/amd64 are LE).

bytes   0  1  2  3
        FE CA DE 0C
       |--|--|--|--|
       \           /
        \_________/
             |
           t1.a
        0xC0DECAFE

So when we access them as T2, those fields correspond to

bytes   0  1  2  3
        FE CA DE 0C
       |--|--|--|--|
      /     / \     \
     /_____/   \_____\
        |         |
      t2.a      t2.b
     0xCAFE    0xCODE

When printed, they display as "Big Endian" (Higher order bytes first) which is more natural for a human to read. This mode also happens to be "Network Byte Order", the agreed-upon byte ordering when transmitting data over a network.