C Function to Convert float to byte array

Question

I'm trying to make a function that will accept a float variable and convert it into a byte array. I found a snippet of code that works, but would like to reuse it in a function if possible.

I'm also working with the Arduino environment, but I understand that it accepts most C language.

Currently works:

float_variable = 1.11;
byte bytes_array[4];

*((float *)bytes_array) = float_variable;

What can I change here to make this function work?

float float_test = 1.11;
byte bytes[4];

// Calling the function
float2Bytes(&bytes,float_test);

// Function
void float2Bytes(byte* bytes_temp[4],float float_variable){ 
     *(float*)bytes_temp = float_variable;  
}

I'm not so familiar with pointers and such, but I read that (float) is using casting or something?

Any help would be greatly appreciated!

Cheers

*EDIT: SOLVED

Here's my final function that works in Arduino for anyone who finds this. There are more efficient solutions in the answers below, however I think this is okay to understand.

Function: converts input float variable to byte array

void float2Bytes(float val,byte* bytes_array){
  // Create union of shared memory space
  union {
    float float_variable;
    byte temp_array[4];
  } u;
  // Overite bytes of union with float variable
  u.float_variable = val;
  // Assign bytes to input array
  memcpy(bytes_array, u.temp_array, 4);
}

Calling the function

float float_example = 1.11;
byte bytes[4];

float2Bytes(float_example,&bytes[0]);

Thanks for everyone's help, I've learnt so much about pointers and referencing in the past 20 minutes, Cheers Stack Overflow!

Answer 1

Easiest is to make a union:

#include <stdio.h>

int main(void) {
  int ii;
  union {
    float a;
    unsigned char bytes[4];
  } thing;

  thing.a = 1.234;
  for (ii=0; ii<4; ii++) 
    printf ("byte %d is %02x\n", ii, thing.bytes[ii]);
  return 0;
}

Output:

byte 0 is b6
byte 1 is f3
byte 2 is 9d
byte 3 is 3f

Note - there is no guarantee about the byte order… it depends on your machine architecture.

To get your function to work, do this:

void float2Bytes(byte bytes_temp[4],float float_variable){ 
  union {
    float a;
    unsigned char bytes[4];
  } thing;
  thing.a = float_variable;
  memcpy(bytes_temp, thing.bytes, 4);
}

Or to really hack it:

void float2Bytes(byte bytes_temp[4],float float_variable){ 
  memcpy(bytes_temp, (unsigned char*) (&float_variable), 4);
}

Note - in either case I make sure to copy the data to the location given as the input parameter. This is crucial, as local variables will not exist after you return (although you could declare them static, but let's not teach you bad habits. What if the function gets called again…)

Answer 2

Here's a way to do what you want that won't break if you're on a system with a different endianness from the one you're on now:

byte* floatToByteArray(float f) {
    byte* ret = malloc(4 * sizeof(byte));
    unsigned int asInt = *((int*)&f);

    int i;
    for (i = 0; i < 4; i++) {
        ret[i] = (asInt >> 8 * i) & 0xFF;
    }

    return ret;
}

You can see it in action here: http://ideone.com/umY1bB

The issue with the above answers is that they rely on the underlying representation of floats: C makes no guarantee that the most significant byte will be "first" in memory. The standard allows the underlying system to implement floats however it feels like -- so if you test your code on a system with a particular kind of endianness (byte order for numeric types in memory), it will stop working depending on the kind of processor you're running it on.

That's a really nasty, hard-to-fix bug and you should avoid it if at all possible.

Answer 3

I would recommend trying a "union".

Look at this post:

http://forum.arduino.cc/index.php?topic=158911.0

typedef union I2C_Packet_t{
 sensorData_t sensor;
 byte I2CPacket[sizeof(sensorData_t)];
};

In your case, something like:

union {
  float float_variable;
  char bytes_array[4];
} my_union;

my_union.float_variable = 1.11;

Answer 4

Yet another way, without unions: (Assuming byte = unsigned char)

void floatToByte(byte* bytes, float f){

  int length = sizeof(float);

  for(int i = 0; i < length; i++){
    bytes[i] = ((byte*)&f)[i];
  }

}

Answer 5

this seems to work also

#include <stddef.h>
#include <stdint.h>
#include <string.h>

float fval = 1.11;
size_t siz;
siz = sizeof(float);

uint8_t ures[siz];

memcpy (&ures, &fval, siz);

then

float utof;
memcpy (&utof, &ures, siz);

also for double

double dval = 1.11;
siz = sizeof(double);

uint8_t ures[siz];

memcpy (&ures, &dval, siz);

then

double utod;
memcpy (&utod, &ures, siz);

Answer 6

Although the other answers show how to accomplish this using a union, you can use this to implement the function you want like this:

byte[] float2Bytes(float val)
{
    my_union *u = malloc(sizeof(my_union));
    u->float_variable = val;
    return u->bytes_array;
}

or

void float2Bytes(byte* bytes_array, float val)
{
    my_union u;
    u.float_variable = val;
    memcpy(bytes_array, u.bytes_array, 4);
}

Answer 7

**conversion without memory reference** \
#define FLOAT_U32(x) ((const union {float f; uint32_t u;}) {.f = (x)}.u) // float->u32
#define U32_FLOAT(x) ((const union {float f; uint32_t u;}) {.u = (x)}.f) // u32->float

**usage example:**
float_t sensorVal = U32_FLOAT(eeprom_read_dword(&sensor));

Answer 8

First of all, some embedded systems 101:

Anyone telling you to use malloc/new on Arduino have no clue what they are talking about. I wrote a fairly detailed explanation regarding why here: Why should I not use dynamic memory allocation in embedded systems?

You should avoid float on 8 bit microcontrollers since it leads to incredibly inefficient code. They do not have a FPU, so the compiler will be forced to load a very resource-heavy software floating point library to make your code work. General advise here.

---

Regarding pointer conversions:

C allows all manner of wild and crazy pointer casts. However, there are lots of situations where it can lead to undefined behavior if you cast a character byte array's address into a float* and then de-reference it.

• If the address of the byte array is not aligned, it will lead to undefined behavior on systems that require aligned access. (AVR doesn't care about alignment though.)
• If the byte array does not contain a valid binary representation of a float number, it could become a trap representation. Similarly you must keep endianess in mind. AVR is an 8-bitter but it's regarded as little endian since it uses little endian format for 16 bit addresses.
• It leads to undefined behavior because it goes against the C language "effective type" system, also known as a "strict pointer aliasing violation". What is the strict aliasing rule?

Going the other way around is fine though - taking the address of a float variable and converting it to a character pointer, then de-reference that character pointer to access individual bytes. Multiple special rules in C allows this for serialization purposes and hardware-related programming.

---

Viable solutions:

• memcpy always works fine and then you won't have to care about alignment and strict aliasing. You still have to care about creating a valid floating point representation though.
• union "type punning" as demonstrated in other answers. Note that such type punning will assume a certain endianess.
• Bit shifting individual bytes and concatenating with | or masking with & as needed. The advantage of this is that it's endianess-independent in some scenarios.

Answer 9

float f=3.14;
char *c=(char *)&f;

float g=0;
char *d=(char *)&g;

for(int i=0;i<4;i++) d[i]=c[i];

/* Now g=3.14 */

Cast your float as char, and assign the address to the char pointer. Now, c[0] through c[3] contain your float.

http://justinparrtech.com/JustinParr-Tech/c-access-other-data-types-as-byte-array/