What is the «_t» in «uint8_t»
The Arduino language contains several easily recognizable variables, like «bool», «byte», «int» and «char». But, below the surface, the Arduino language is really a subset of the C language that works on microcontrollers. With it, you will find many specialized data types designed to ensure compatibility across devices that don’t always treat a byte the same way.
If you have looked at more advanced Arduino code, perhaps looking at Arduino sources on Github, you may have noticed that a lot of variable types end with «_t».
You probably already know that the Arduino «language» is based on the C language. I am not exaggerating when I say that our modern civilization depends on C and its object-oriented cousin, C++. No matter what gadget (computer, tablet, phone) you are reading this on, your electronic device functionality infrastructure is written in C and C++.
Because C and C++ is used on so many different platforms, from microcontrollers to supercomputers, there was a need for types (like integers, floats, etc.) that are compatible across all these platforms.
So, in the C99 standard (the ISO standard for the C language), types that are designed to be cross-platform compatible are marked with a «_t». «t» stands for «type.»
This way, the programmers know that the uint8_t is a byte with 8 bits no matter which platform the program runs on.
If you strive to write code that can be executed on different computer or microcontroller systems, then it is good practice to use data types with the «_t» extension for this reason.
For the Arduino, we tend not to use C99 cross-platform compatible types because most often our code is not meant to run on other systems.
Arduino: Difference in “Byte” VS “uint8_t” VS “unsigned char”
Byte, uint8_t and unsigned char, they are basically the same thing in Arduino. These data types often cause confusions to new programmers. So is there any difference in them?
A byte stores an 8-bit unsigned number, from 0 to 255. For example for the number 0, the binary form is 00000000, there are 8 zeros (8 bits in total). for the number 255, the binary form is 11111111.
A uint8_t data type is basically the same as byte in Arduino. Writers of embedded software often define these types, because systems can sometimes define int to be 8 bits, 16 bits or 32 bits long. The issue doesn’t arise in C# or Java, because the size of all the basic types is defined by the language.
An unsigned char data type that occupies 1 byte of memory. It is the same as the byte datatype. The unsigned char datatype encodes numbers from 0 to 255. For consistency of Arduino programming style, the byte data type is to be preferred.
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What are the difference? Which one should I use?
Not really, but It documents your intent for that variable. For example you will be storing small numbers, rather than a character, you will use byte.
Also it looks nicer to use uint8_t if you are also using other similar typedefs such as uint16_t or int32_t. With the number in the type definition, it clearly states how many bits the variable would take up. With or without the “u” at the beginning also gives people an idea it’s signed or unsigned.
The other thing that trips people up is “char”. It can be equivalent to uint8_t or int8_t. Or it might not be 8-bits at all, but that’s fairly rare. On Arduino, char is int8_t but byte is uint8_t.
Anyway, in Arduino, byte, uint8_t and unsigned short can be used interchangeably because they are literally the same type. It’s just an alias. If you are just compiling the sketch on Arduino IDE and upload to the Arduino, use byte should be enough.
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5 comments
is it possible to convert byte value into integer… and can we compare two byte values if yes then how we will do that from initialising to execution…
plzz help me out on this
An int can never be 8 bits as the C standard mandates that the minimum size for an int is 16 bits.
Also in C there are actually three “char” types.
char, signed char, and unsigned char.
For historical reasons, they are different. So while a char may default to being signed or unsigned, it is not the same as either “signed char” or “unsigned char”.
In fact you can even see this if you start to look closely at the assembler output of the avr gcc compiler.
There are certain loops that will generate different code (for each of the 3 types) if you declare the loop variable,
char vs signed char vs unsigned char.
I would discourage using the “cutsie” Arduino types like byte. and NEVER use the Arduino type “word” as that is defined to be “unsigned short” which is 16 bits on AVR and 32 bits on ARM and pic32. The “word” type is a total fail.
If you want your code to be portable, then always use the ANSI defined types from and avoid Arduino proprietary types.
There is no reason to use the “cutsie” types from Arduino when there are standard types.
That is why standard types were created and they offer more than just bit width choices as you can also specify other types that give the compiler clues for their use like minimum size or speed desires. These can help the compiler generate better or faster code. Something that the Arduino types cannot do.
One bit of useful information is that for small loops like a for loop a uint8_t or Arduino “byte” type will generate much better code on an 8 bit processor like the AVR; however, it can actually generate worse code on a larger processor.
That is where the other types in stdint can come into play.
If you were to use uint_fast8_t or uint_least8_t you would would not require a larger processor to be restricted to using only 8 bits which might use less efficient instructions to handle a smaller loop variable when the registers are larger.
That is the power of using the stdint types.
You can provider better information to the compiler so it can generate better code.
“Anyway, in Arduino, byte, uint8_t and unsigned short can be used interchangeably”
Shouldn’t that be “byte, uint8_t and unsigned char”?
Good summary 🙂
Программирование Arduino — Последовательная передача данных
Arduino/Freeduino имеет встроенный контроллер для последовательной передачи данных, который может использоваться как для связи между Arduino/Freeduino устройствами, так и для связи с компьютером. На компьютере соответствующее соединение представлено либо обычным COM-портом (в случае Arduino Single-Sided Serial Board), либо USB COM-портом, который появляется в системе после установки необходимого драйвера.
Связь происходит по цифровым портам 0 и 1, и поэтому Вы не сможете использовать их для цифрового ввода/вывода если используете функции последовательной передачи данных.
Serial.begin(long);
Описание:
Устанавливает скорость передачи информации COM порта битах в секунду для последовательной передачи данных. Для того чтобы поддерживать связь с компьютером, используйте одну из этих нормированных скоростей: 300, 1200, 2400, 4800, 9600, 14400, 19200, 38400, 57600, или 115200. Также Вы можете определить другие скорости при связи с другим микроконтроллером по портам 0 и 1.
Параметры:
скорость_передачи: скорость потока данных в битах в секунду.
Serial.available(void);
Описание:
Принимаемые по последовательному порту байты попадают в буфер микроконтроллера, откуда Ваша программа может их считать. Функция возвращает количество накопленных в буфере байт. Последовательный буфер может хранить до 128 байт.
Возвращаемое значение:
Возвращает значение типа uint8_t (typedef uint8_t byte;) – количество байт, доступных для чтения, в последовательном буфере, или 0, если ничего не доступно.
Serial.read(void);
Описание:
Считывает следующий байт из буфера последовательного порта.
Возвращаемое значение:
Первый доступный байт входящих данных с последовательного порта, или -1 если нет входящих данных.
Serial.write(uint8_t c)
Описание:
Записывает данные в последовательный порт. Данные посылаются как байт или последовательность байт; для отправки символьной информации следует использовать функцию print().
Параметры:
val: переменная для передачи, как единственный байт
str: строка для передачи, как последовательность байт
buf: массив для передачи, как последовательность байт
len: длина массива
Serial.flush(void)
Описание:
Очищает входной буфер последовательного порта. Находящиеся в буфере данные теряются, и дальнейшие вызовы Serial.read() или Serial.available() будут иметь смысл для данных, полученных после вызова Serial.flush().
Serial.print()
Функции print наследуются классом HardwareSerial от класса Print (\hardware\cores\arduino\ Print.h)
Описание:
Вывод данных на последовательный порт.
Параметры:
Функция имеет несколько форм вызова в зависимости от типа и формата выводимых данных.
jibsen / bytes.md
The C standard only specifies minimum limits for the values of character types and standard integer types. This makes it possible to generate efficient code on diverse architectures, but can pose problematic if your code expects the limits to match your development platform, or if you have to do low-level things.
Before C99, the usual way to solve this was to use typedef to declare synonyms for standard types of the right size (like u8 , u16 , u32 used in the Linux kernel). These can then easily be changed to matching types on other platforms.
C99 introduced extended integer types in <stdint.h> , which include exact-width types of the form intN_t and uintN_t , where N is the width. They contain no padding, and the signed types are two’s complement. These types are optional, but if the implementation provides a suitable integer type of any of the widths 8, 16, 32, 64, it must define the corresponding typedef.
While they are quite useful, the standard is (as ever) loose enough in its requirements, that there are (at least theoretical) pitfalls that may not be entirely obvious.
In the following, we are going to have a look at uint8_t .
From the C11 standard, 3.6 (byte) and 5.2.4.2.1 (Sizes of integer types), we gather that a byte is the smallest object that is not a bit-field. Each byte is uniquely addressable, and is composed of a contiguous sequence of CHAR_BIT bits.
From 6.2.6.1p3 (Representations of types) and footnote 49, we further see that unsigned char is required to match a byte.
Since CHAR_BIT is the number of bits of the smallest object, and it is at least 8, uint8_t can only be defined on platforms where CHAR_BIT is 8.
Thus it seems obvious to use unsigned char as the type of uint8_t , and that is usually the case, but the standard does not specify this (link, link, link).
Special properties of character types
Sometimes you need to access the individual bytes of an object, or do arithmetic on pointers. The standard describes special properties of the character types that allow this.
When two lvalues refer to the same memory location, they are said to alias. C has rules about which types are allowed to alias:
- a type compatible with the effective type of the object,
- a qualified version of a type compatible with the effective type of the object,
- a type that is the signed or unsigned type corresponding to the effective type of the object,
- a type that is the signed or unsigned type corresponding to a qualified version of the effective type of the object,
- an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), or
- a character type.
As we can see, we are only allowed to access the memory of an object using certain compatible types or a character type. This is referred to as the strict aliasing rule, and has created some controversy because compiler vendors have chosen to start enforcing this rule to be able to perform certain optimizations (link, link, link, link, link, link).
When we access the memory of an object using a pointer to a character type, we can address each byte of the object, and pointer arithmetic (within the bounds of the object) works as expected:
6.3.2.3p7 Pointers
When a pointer to an object is converted to a pointer to a character type, the result points to the lowest addressed byte of the object. Successive increments of the result, up to the size of the object, yield pointers to the remaining bytes of the object.
For pointers to non-character types, arithmetic is only defined within an array (link, link):
6.5.6p7 Additive operators
For the purposes of these operators, a pointer to an object that is not an element of an array behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type.
6.5.6p8 Additive operators
If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined.
What if uint8_t is not a character type?
Since the standard does not require uint8_t to be a character type, you could imagine a compiler vendor deliberately making it a separate type to be able to take advantage of the strict aliasing rule.
If you are fond of the fixed-width integer types, it is tempting to use uint8_t in situations where you would have used a character type. Consider this function for reading an uin32_t value in little-endian order:
Here p aliases val , but if uint8_t is not a character type, we are breaking the strict aliasing rule, and get undefined behavior.
If we make sure CHAR_BIT is 8, we can replace uint8_t with unsigned char to avoid this problem.
Here is an example that uses a pointer to uint8_t to do pointer arithmetic, in order to process memory in blocks:
We do not access memory, so the strict aliasing rule does not apply. If p were a pointer to a character type, we would be sure we could address the individual bytes of whatever object type data points to in this way. But if uint8_t is not a character type, this could be undefined behavior, unless data points to an array of uin8_t (or a single uint8_t ).