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).
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enum
cbor_float_width
¶ Possible widths of CBOR_TYPE_FLOAT_CTRL items.
Values:
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CBOR_FLOAT_0
¶ Internal use - ctrl and special values.
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CBOR_FLOAT_16
¶ Half float.
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CBOR_FLOAT_32
¶ Single float.
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CBOR_FLOAT_64
¶ Double.
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Corresponding |
|
Number of allocations |
One per lifetime |
Storage requirements |
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Getting metadata¶
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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
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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¶
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float
cbor_float_get_float2
(const cbor_item_t *item)¶ Get a half precision float.
The item must have the corresponding width
- Parameters
-
float
cbor_float_get_float4
(const cbor_item_t *item)¶ Get a single precision float.
The item must have the corresponding width
- Parameters
-
double
cbor_float_get_float8
(const cbor_item_t *item)¶ Get a double precision float.
The item must have the corresponding width
- Parameters
-
double
cbor_float_get_float
(const cbor_item_t *item)¶ Get the float value represented as double.
Can be used regardless of the width.
- Parameters
-
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¶
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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
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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
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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
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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
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cbor_item_t *
cbor_new_null
()¶ Constructs new null ctrl item.
- Return
new null ctrl item
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cbor_item_t *
cbor_new_undef
()¶ Constructs new under ctrl item.
- Return
new under ctrl item
Building items¶
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cbor_item_t *
cbor_build_bool
(bool value)¶ Constructs new boolean ctrl item.
- Return
new boolen ctrl item
- Parameters
value
: The value to use
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cbor_item_t *
cbor_build_ctrl
(uint8_t value)¶ Constructs a ctrl item.
- Return
new ctrl item
- Parameters
value
: the value to use
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cbor_item_t *
cbor_build_float2
(float value)¶ Constructs a new float.
- Return
new float
- Parameters
value
: the value to use
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cbor_item_t *
cbor_build_float4
(float value)¶ Constructs a new float.
- Return
new float
- Parameters
value
: the value to use
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cbor_item_t *
cbor_build_float8
(double value)¶ Constructs a new float.
- Return
new float
- Parameters
value
: the value to use
Manipulating existing items¶
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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 itemvalue
: The simple value to assign. Please consult the standard for allowed values
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void
cbor_set_float2
(cbor_item_t *item, float value)¶ Assigns a float value.
- Parameters
item[borrow]
: A half precision floatvalue
: The value to assign
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void
cbor_set_float4
(cbor_item_t *item, float value)¶ Assigns a float value.
- Parameters
item[borrow]
: A single precision floatvalue
: The value to assign
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void
cbor_set_float8
(cbor_item_t *item, double value)¶ Assigns a float value.
- Parameters
item[borrow]
: A double precision floatvalue
: 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:
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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 bufferbuffer_size
: Available space in the buffer