Big endian or little endian?

Question

I need to pass a 16-bit long address (for an external EEPROM) to a function (twi_writeTo) in a buffer. I am currently doing this

byte buffer[2]; buffer[0] = (byte)(eeaddress >> 8); buffer[1] = (byte)(eeaddress & 0xFF);

where eeaddress is an unsigned int with the address.

I then do

int8_t ret = twi_writeTo(eeprom_i2c_addr, buffer, 2, 1);

Before I tried

int8_t ret = twi_writeTo(eeprom_i2c_addr, (uint8_t*)&eeaddress, 2, 1);

but that returns garbage. Probably the byte order is swapped. Is that the case? And if so, is there a better way than copying the two bytes to a buffer?

If you think endianness is the problem, just change buffer[0] to buffer[1] and vice versa, and check if that fixes your problem. — Gerben
– Gerben, Commented May 10, 2015 at 11:30
The use of (uint8_t*)&eeaddress and buffer is mutually exclusive, so I don't understand your comment. — fuenfundachtzig
– fuenfundachtzig, Commented May 10, 2015 at 12:49
Swap the buffer[] bytes over and see if that creates the same garbage that (uint8_t*)&eeaddress does. If so, then yes it is an endian problem. — Majenko
– Majenko, Commented May 10, 2015 at 13:05
What do you mean exactly by it returns garbage? Please give some examples with hex numbers. — rslite
– rslite, Commented May 10, 2015 at 23:06
Garbage as in it reads "arbitrary" addresses so that the result doesn't make sense. I checked and indeed the first approach swaps the bytes w.r.t. the second. So, yes, it's the byte order. — fuenfundachtzig
– fuenfundachtzig, Commented May 11, 2015 at 16:16

rslite · Accepted Answer · 2015-05-13 01:50:23Z

If it's the byte order that causes a problem that's likely compiler specific and I don't think one can change it. Even if it were possible, changing the global endianness might break libraries that depend on it, so you likely can't escape using a function to swap the variable.

If you know you only need one value at a time you could write something like the following code, with a global buffer (untested but in principle should work (and I know global variables are evil, but should work for an Arduino project)):

byte buf[2]; byte* reverse(int val) { buf[0] = (byte)(val >> 8); but[1] = (byte)val; return buf; }

So you could use it like:

int8_t ret = twi_writeTo(eeprom_i2c_addr, reverse(eeaddress), 2, 1);

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Big endian or little endian?

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