PacketBuffer¶
Overview¶
The MicroBitRadio class provides direct micro:bit to micro:bit communication.
It is often beneficial in both wired and wireless communication protocols to send and receive data in a raw format, viewed as an ordered sequence of bytes.
This class provides a basic set of functions for the creation, manipulation and accessing of a managed type for byte arrays used to hold network data packets.
Note
This is a managed type. This means that it will automatically use and release memory as needed. There is no need for you to explicitly free or release memory when your done - the memory will be freed as soon as the last piece of code stops using the data.
Creating PacketBuffers¶
PacketBuffers are simple to create - just create them like a variable, and provide the size (in bytes) of the buffer you want to create.
PacketBuffer b(16);
Alternatively, if you already have an array of bytes allocated, then you can simply refer to that using a slightly different form:
uint8_t data[16];
PacketBuffer b(data,16);
Manipulating PacketBuffers¶
Once created, the data inside PacketBuffers can be freely changed at any time. The simplest way to do this is through the array operators [ and ].
You can read or write bytes to the buffer by simply dereferencing it with square bracket.
For example: to create, set and send a PacketBuffer on the micro:bit radio, you could do the following:
PacketBuffer b(2);
b[0] = 255;
b[1] = 10;
uBit.radio.datagram.send(b);
PacketBuffers can also be assigned a value:
PacketBuffer b;
b = uBit.radio.datagram.recv();
If you need more granular access, the getBytes function provides direct access to the memory buffer, presented as a byte array:
PacketBuffer b(16);
uint8_t *buf = b.getBytes();
memcpy(buf, "HI", 2);
Finally, it is also possible to extract the received signal strength of a packet from a PacketBuffer.
This can provide both a rough indication of the reliability of the link, and a crude but moderately effective mechanism to estimate the distance between two micro:bits.
PacketBuffer b;
b = uBit.radio.datagram.recv();
uBit.display.scroll(b.getRSSI());
API¶
Constructor¶
ManagedString( StringData * ptr)¶
Description¶
Constructor. Create a managed string from a specially prepared string literal.
Parameters¶
StringData *ptr - The literal - first two bytes should be 0xff, then the length in little endian, then the literal. The literal has to be 4-byte aligned.
Example¶
static const char hello[] __attribute__ ((aligned (4))) = "\xff\xff\x05\x00" "Hello";
ManagedString s((StringData*)(void*)hello);
leakData¶
StringData leakData()¶
Description¶
Get current ptr, do not decr() it, and set the current instance to empty string.
This is to be used by specialized runtimes which pass StringData around.
Constructor¶
ManagedString( const char * str)¶
Description¶
Constructor.
Create a managed string from a pointer to an 8-bit character buffer.
The buffer is copied to ensure safe memory management (the supplied character buffer may be declared on the stack for instance).
Parameters¶
const char *str - The character array on which to base the new ManagedString .
Example¶
ManagedString s("abcdefg");
ManagedString( const int value)¶
Description¶
Constructor.
Create a managed string from a given integer.
Parameters¶
const intvalue - The integer from which to create the ManagedString .
Example¶
ManagedString s(20);
ManagedString( const char value)¶
Description¶
Constructor. Create a managed string from a given char.
Parameters¶
const charvalue - The character from which to create the ManagedString .
Example¶
ManagedString s('a');
ManagedString( PacketBuffer buffer)¶
Description¶
Constructor. Create a ManagedString from a PacketBuffer . All bytes in the PacketBuffer are added to the ManagedString .
Parameters¶
PacketBufferbuffer - The PacktBuffer from which to create the ManagedString .
Example¶
ManagedString s = radio.datagram.recv();
ManagedString( const char * str, const int16_t length)¶
Description¶
Constructor. Create a ManagedString from a pointer to an 8-bit character buffer of a given length.
The buffer is copied to ensure sane memory management (the supplied character buffer may be declared on the stack for instance).
Parameters¶
const char *str - The character array on which to base the new ManagedString .const int16_tlength - The length of the character array
Example¶
ManagedString s("abcdefg",7);
ManagedString( const ManagedString & s)¶
Description¶
Copy constructor. Makes a new ManagedString identical to the one supplied.
Shares the character buffer and reference count with the supplied ManagedString .
Parameters¶
const ManagedString &s - The ManagedString to copy.
Example¶
ManagedString s("abcdefg");
ManagedString p(s);
ManagedString()¶
Description¶
Default constructor.
Create an empty ManagedString .
Example¶
ManagedString s();
operator=¶
ManagedString operator=( const ManagedString & s)¶
Description¶
Copy assign operation.
Called when one ManagedString is assigned the value of another.
If the ManagedString being assigned is already referring to a character buffer, decrement the reference count and free up the buffer as necessary.
Then, update our character buffer to refer to that of the supplied ManagedString , and increase its reference count.
Parameters¶
const ManagedString &s - The ManagedString to copy.
Example¶
ManagedString s("abcd");
ManagedString p("efgh");
p = s // p now points to s, s' ref is incremented
operator==¶
bool operator==( const ManagedString & s)¶
Description¶
Equality operation.
Called when one ManagedString is tested to be equal to another using the '==' operator.
Parameters¶
const ManagedString &s - The ManagedString to test ourselves against.
Returns¶
true if this ManagedString is identical to the one supplied, false otherwise.
Example¶
MicroBitDisplay display;
ManagedString s("abcd");
ManagedString p("efgh");
if(p == s)
display.scroll("We are the same!");
else
display.scroll("We are different!"); //p is not equal to s - this will be called
operator<¶
bool operator<( const ManagedString & s)¶
Description¶
Inequality operation.
Called when one ManagedString is tested to be less than another using the '<' operator.
Parameters¶
const ManagedString &s - The ManagedString to test ourselves against.
Returns¶
true if this ManagedString is alphabetically less than to the one supplied, false otherwise.
Example¶
MicroBitDisplay display;
ManagedString s("a");
ManagedString p("b");
if(s < p)
display.scroll("a is before b!"); //a is before b
else
display.scroll("b is before a!");
operator>¶
bool operator>( const ManagedString & s)¶
Description¶
Inequality operation.
Called when one ManagedString is tested to be greater than another using the '>' operator.
Parameters¶
const ManagedString &s - The ManagedString to test ourselves against.
Returns¶
true if this ManagedString is alphabetically greater than to the one supplied, false otherwise.
Example¶
MicroBitDisplay display;
ManagedString s("a");
ManagedString p("b");
if(p>a)
display.scroll("b is after a!"); //b is after a
else
display.scroll("a is after b!");
substring¶
ManagedString substring( int16_t start, int16_t length)¶
Description¶
Extracts a ManagedString from this string, at the position provided.
Parameters¶
int16_tstart - The index of the first character to extract, indexed from zero.int16_tlength - The number of characters to extract from the start position
Returns¶
a ManagedString representing the requested substring.
Example¶
MicroBitDisplay display;
ManagedString s("abcdefg");
display.scroll(s.substring(0,2)) // displays "ab"
charAt¶
char charAt( int16_t index)¶
Description¶
Provides a character value at a given position in the string, indexed from zero.
Parameters¶
int16_tindex - The position of the character to return.
Returns¶
the character at posisiton index, zero if index is invalid.
Example¶
MicroBitDisplay display;
ManagedString s("abcd");
display.scroll(s.charAt(1)) // scrolls "b"
toCharArray¶
const char * toCharArray()¶
Description¶
Provides an immutable 8 bit wide character buffer representing this string.
Returns¶
a pointer to the character buffer.
length¶
int16_t length()¶
Description¶
Determines the length of this ManagedString in characters.
Returns¶
the length of the string in characters.
Example¶
MicroBitDisplay display;
ManagedString s("abcd");
display.scroll(s.length()) // scrolls "4"