Converting a UINT32 value into a UINT8 array[4]

My question is how do you convert a UINT32 value to a UINT8 array[4] (C/C++) preferably in a manner indepen开发者_如何学Godent of endianness? Additionally, how would you reconstruct the UINT32 value from the UINT8 array[4], to get back to where you started?

You haven't really said what you mean by independent of endianness - it's unclear since the byte array must have some endianness. That said, one of the below must answer your requirements:

Given UINT32 v and UINT8 a[4]:

"Host" endian

(use the machine's native byte order):

UINT8 *vp = (UINT8 *)&v;
a[0] = vp[0];
a[1] = vp[1];
a[2] = vp[2];
a[3] = vp[3];

or:

memcpy(a, &v, sizeof(v));

or:

*(UINT32 *)a = v;

Big endian

(aka "network order"):

a[0] = v >> 24;
a[1] = v >> 16;
a[2] = v >>  8;
a[3] = v;

Little endian

a[0] = v;
a[1] = v >>  8;
a[2] = v >> 16;
a[3] = v >> 24;

E.g. like this:

UINT32 value;
UINT8 result[4];

result[0] = (value & 0x000000ff);
result[1] = (value & 0x0000ff00) >> 8;
result[2] = (value & 0x00ff0000) >> 16;
result[3] = (value & 0xff000000) >> 24;

Edit: added parenthesis (>> seems to have higher precedence than &)

If you don't want to code it yourself, you can use the C library function htonl() to convert the 32-bit int to network byte order. There is also the function ntohl() to convert them back to host order.

Once they're in network byte order, it's simply a matter of accessing the int/long as a byte array.

All in all that's are probably the most portable and tested way of achieving your goal.

One could also do it with pointers. (This is little endian, but if you use the same reconstruction method it won't matter)

uint32 in = 0x12345678;
uint8 out[4];
*(uint32*)&out = in;

This assigns the value of the uint32 to the 4 bytes after the memory address of the uint8, doing exactly what you need.

To go the other way:

uint8 in[4] = {0x78, 0x56, 0x34, 0x12};
uint32 out;
out = *(uint32*)&in

use a Union consisting of an Array with 4 time uint8 and an uint32.

So it sorts automatically by c inherent pointer Magic (Arrays are pointers to start of array)