1

You can extract

the low-order (rightmost) byte of a variable

or

the high-order (leftmost) byte of a word

with the functions lowByte() and highByte() respectively (the quotes are from the Arduino Reference).

Does Arduino provide a way to extract any byte from a number with a similar function?

And does it provide a way to set any individual bytes of a number (which would be the counterparts to these functions, if they exist)?

6

Any more sophisticated byte exchange can be done with binary operators.

Extracting information

You'll need some constants like

MASK         SHIFT                                    
0xFF000000   24       (first byte, most significant)
0x00FF0000   16       (second byte)
0x0000FF00    8       (third byte)
0x000000FF    0       (last byte, least significant)

You would binary AND the original value with MASK

     0x12345678
AND  0x00FF0000
 =   0x00340000

and then bit shift it left, e.g.

   0x00340000
>> 16
 = 0x00000034

So from the original value 0x12345678 you have gotten 0x34.

Setting information

The opposite direction is also possible with the same constants but opposite operators and in opposite order:

   0x00000034
<< 16
 = 0x00340000

and then

   0x12005678
OR 0x00340000
 = 0x12345678

Note that the OR operation only works reliably if the corresponding positions are 0x00. That's fine with a starting value of 0x00000000.

If you don't know that or you want to process an arbitrary number, you can introduce a step in between

    0x12??5678   (?? could be anything)
AND 0xFF00FFFF   (which is the inverse of 0x00FF0000 and can be expressed as ~0x00FF0000)
 =  0x12005678   (whatever ?? was, it'll be cleared out)

Actual Code

Tested on Arduino Uno. The long data type is 32 bits or 4 bytes.

The code here may not be the most efficient. Advanced developers would likely not use MASK and SHIFT as arrays like this. This answer focuses more on the educational point.

long MASK[]  = {0xFF000000, 0x00FF0000, 0x0000FF00, 0x000000FF};
byte SHIFT[] = {24        , 16        , 8         , 0};

// Extracts a byte from a larger data type
// value: larger data type to extract a byte from
// position: 0 based number, 0 = MSB, 3 = LSB
byte getByteAt(long value, byte position)
{
  long result = value & MASK[position];  // binary AND
  result = result >> SHIFT[position];    // Shift right, moving all bits
  byte resultAsByte = (byte) result;     // Convert to an actual byte
  return resultAsByte;  
}

// Sets a byte in a larger data type
// value: larger data type where to set the byte
// position: 0 based number, 0 = MSB, 3 = LSB
// newPartialValue: the byte to be inserted
long setByteAt(long value, byte position, byte newPartialValue)
{
  long result = value & ~MASK[position];                       // clear the affected byte so that it is 0x00
  long valueToSet = (long) newPartialValue << SHIFT[position]; // Shift left, moving all bits
  result = result | valueToSet;                                // binary OR
  return result;  
}

void setup() {
  Serial.begin(9600);
  Serial.println(getByteAt(0x12345678, 1));       // Prints 52, which is 0x34
  Serial.println(setByteAt(0x87654321, 1, 0xBB)); // Prints -2017770719 which is 0x87BB4321
}

void loop() {

}
  • Worth mentioning that the value you AND the number with to clear the bits prior to OR-ing is the inverse of the mask. You can use ~MASK to create it. – Majenko Nov 28 '19 at 11:32
  • @Majenko: used and added in the example – Thomas Weller Nov 28 '19 at 11:50
  • Thanks for the extensive explanation, but I already knew about this possibility. So there is no inherent way to do this with built in functions? – LukasFun Nov 28 '19 at 12:04
  • You don't need to load MASK and SHIFT from an array! shift = position * 8 = position << 3 (convert a bit offset to a byte offset. Also, mask after shifting so it's just return (uint8_t)(value >> shift);. e.g. on Godbolt for AVR and ARM cortex-M0 get and set byte functions: godbolt.org/z/2uFXN5 Also included are versions that store/reload and use a byte index, avoiding shift loops on AVR which can only shift 1 byte at a time. (Will post an answer if I get back to this.) – Peter Cordes Nov 28 '19 at 23:17
  • @PeterCordes: my primary concern was to explain it thoroughly and in an understandable, visible way. I am also participating in code golf, so yeah, it's definitely not the shortest form :-) In general I am not optimizing for speed as long as there is no speed requirement. – Thomas Weller Nov 28 '19 at 23:53
1

The header Arduino.h defines the macro word(high_byte, low_byte). The resulting value is an uint16_t.

  • Thank you, that answers the question about the counterpart of lowByte() and highByte(). Are there any functions for the rest of my question though? – LukasFun Nov 28 '19 at 15:38
  • @LukasFun: I couldn't find any. But note that on the AVR-based Arduinos an int is only two bytes. – Edgar Bonet Nov 28 '19 at 15:58
  • 1
    You should simply treat those definitions as samples, learn enough C/C++ to understand them, and extend them to whatever you need. If uint16_t is not what you need, why should exactly uint32_t be your solution? – DataFiddler Nov 28 '19 at 16:11
1

There are two alternative methods, and they can both be easily used "in reverse" as well:

Unions

union byte_extract {
    uint32_t ival;
    struct {
        uint8_t byte_0; // least significant byte
        uint8_t byte_1;
        uint8_t byte_2;
        uint8_t byte_3; // most significant byte
    } bval;
};

... later ...

union byte_extract x;

x.ival = 65536 * 99; // change to whatever you want
Serial.println(x.bval.byte_2); //3rd byte

Unions are a way to define multiple possible ways to interpret a piece of memory, usually to save memory when you know something can't be 2 things at once. Strictly speaking it is not recommended to use them in this way but it works.

Pointer arithmetic

uint32_t *xaddr;
uint8_t *xbyteaddr;

uint32_t x = 65536 * 99; // change to whatever you want
xaddr = &x;
xbyteaddr = (uint8_t *) xaddr;

Serial.println(xbyteaddr[2]); //3rd byte

Memory addresses (on Arduino as well as on PCs) refer to the first byte of a variable. If you have an (uint32_t *) you will read 4 bytes starting at that address. But if you have a (uint8_t *) or a (char *) you will only read 1 byte. You can shift this pointer-to-uint8_t forwards by as many bytes as you want.

Apologies if this doesn't quite work on Arduino, I have only tested it on PC.

  • This use of unions is well-defined by C99. An array of uint8_t bytes[4] would be more readable and allow runtime indexing, though. Your pointer-arithmetic version is only safe because uint8_t is going to be unsigned char, and unsigned char* is allowed to alias anything without violating strict-aliasing. Note that both of these are endianness-dependent. – Peter Cordes Dec 11 '19 at 0:18

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