Type 7 – Floats & control tokens

This type combines two completely unrelated types of items – floating point numbers and special values such as true, false, null, etc. We refer to these special values as ‘control values’ or ‘ctrls’ for short throughout the code.

Just like integers, they have different possible width (resulting in different value ranges and precisions).

enum type cbor_float_width

Possible widths of CBOR_TYPE_FLOAT_CTRL items.

Values:

Internal use - ctrl and special values.

Half float.

Single float.

Double.

Corresponding cbor_type CBOR_TYPE_FLOAT_CTRL
Number of allocations One per lifetime
Storage requirements sizeof(cbor_item_t) + 1/4/8

Getting metadata

bool cbor_float_ctrl_is_ctrl(const cbor_item_t *item)

Is this a ctrl value?

Return
Is this a ctrl value?
Parameters
  • item[borrow]: A float or ctrl item

cbor_float_width cbor_float_get_width(const cbor_item_t *item)

Get the float width.

Return
The width.
Parameters
  • item[borrow]: A float or ctrl item

bool cbor_ctrl_is_bool(const cbor_item_t *item)

Is this ctrl item a boolean?

Return
Is this ctrl item a boolean?
Parameters
  • item[borrow]: A ctrl item

Reading data

float cbor_float_get_float2(const cbor_item_t *item)

Get a half precision float.

The item must have the corresponding width

Return
half precision value
Parameters
  • borrow]: A half precision float

float cbor_float_get_float4(const cbor_item_t *item)

Get a single precision float.

The item must have the corresponding width

Return
single precision value
Parameters
  • borrow]: A signle precision float

double cbor_float_get_float8(const cbor_item_t *item)

Get a double precision float.

The item must have the corresponding width

Return
double precision value
Parameters
  • borrow]: A double precision float

double cbor_float_get_float(const cbor_item_t *item)

Get the float value represented as double.

Can be used regardless of the width.

Return
double precision value
Parameters
  • borrow]: Any float

uint8_t cbor_ctrl_value(const cbor_item_t *item)

Reads the control value.

Return
the simple value
Parameters
  • item[borrow]: A ctrl item

Creating new items

cbor_item_t *cbor_new_ctrl()

Constructs a new ctrl item.

The width cannot be changed once the item is created

Return
new 1B ctrl

cbor_item_t *cbor_new_float2()

Constructs a new float item.

The width cannot be changed once the item is created

Return
new 2B float

cbor_item_t *cbor_new_float4()

Constructs a new float item.

The width cannot be changed once the item is created

Return
new 4B float

cbor_item_t *cbor_new_float8()

Constructs a new float item.

The width cannot be changed once the item is created

Return
new 8B float

cbor_item_t *cbor_new_null()

Constructs new null ctrl item.

Return
new null ctrl item

cbor_item_t *cbor_new_undef()

Constructs new under ctrl item.

Return
new under ctrl item

Building items

cbor_item_t *cbor_build_bool(bool value)

Constructs new boolean ctrl item.

Return
new boolen ctrl item
Parameters
  • value: The value to use

cbor_item_t *cbor_build_ctrl(uint8_t value)

Constructs a ctrl item.

Return
new ctrl item
Parameters
  • value: the value to use

cbor_item_t *cbor_build_float2(float value)

Constructs a new float.

Return
new float
Parameters
  • value: the value to use

cbor_item_t *cbor_build_float4(float value)

Constructs a new float.

Return
new float
Parameters
  • value: the value to use

cbor_item_t *cbor_build_float8(double value)

Constructs a new float.

Return
new float
Parameters
  • value: the value to use

Manipulating existing items

void cbor_set_ctrl(cbor_item_t *item, uint8_t value)

Assign a control value.

Warning

It is possible to produce an invalid CBOR value by assigning a invalid value using this mechanism. Please consult the standard before use.

Parameters
  • item[borrow]: A ctrl item
  • value: The simple value to assign. Please consult the standard for allowed values

void cbor_set_float2(cbor_item_t *item, float value)

Assigns a float value.

Parameters
  • item[borrow]: A half precision float
  • value: The value to assign

void cbor_set_float4(cbor_item_t *item, float value)

Assigns a float value.

Parameters
  • item[borrow]: A single precision float
  • value: The value to assign

void cbor_set_float8(cbor_item_t *item, double value)

Assigns a float value.

Parameters
  • item[borrow]: A double precision float
  • value: The value to assign

Half floats

CBOR supports two bytes wide (“half-precision”) floats which are not supported by the C language. libcbor represents them using float values throughout the API, which has important implications when manipulating these values.

In particular, if a user uses some of the manipulation APIs (e.g. cbor_set_float2(), cbor_new_float2()) to introduce a value that doesn’t have an exect half-float representation, the encoding semantics are given by cbor_encode_half() as follows:

size_t cbor_encode_half(float, unsigned char *, size_t)

Encodes a half-precision float.

Since there is no native representation or semantics for half floats in the language, we use single-precision floats, as every value that can be expressed as a half-float can also be expressed as a float.

This however means that not all floats passed to this function can be unambiguously encoded. The behavior is as follows:

  • Infinity, NaN are preserved
  • Zero is preserved
  • Denormalized numbers keep their sign bit and 10 most significant bit of the significand
  • All other numbers
    • If the logical value of the exponent is < -24, the output is zero
    • If the logical value of the exponent is between -23 and -14, the output is cut off to represent the ‘magnitude’ of the input, by which we mean (-1)^{signbit} x 1.0e{exponent}. The value in the significand is lost.
    • In all other cases, the sign bit, the exponent, and 10 most significant bits of the significand are kept

Return
number of bytes written
Parameters
  • value:
  • buffer: Target buffer
  • buffer_size: Available space in the buffer