Difference between uint8_t and unsigned char, uint16_t or uint32_t and unsigned int

Have you ever wondered why people uses different keywords to define the same variable in C or C++ program?

Sometimes you would have noticed that 8 bit variable has been defined as

unsigned char a;
uint8_t       b;

Also, many a times you would notice integer has been defined as

unsigned int c;
uint16_t     d;
uint32_t     e;

Initially when I was in the learning phase of C programming with microcontrollers, I used to get confused, what to use when.

Fixed width integers

The keywords like uint8_t, int8_t and uint16_t, all are called fixed width integers. These keywords contain numbers in their keyword, which defines the width of a variable in bytes.

E. g. 

• uint8_t defines 8 bits width of variable b(in the above example).
• uint16_t defines 16 bits width of variable d(in the above example).
• uint32_t defines 32 bits width of variable e(in the above example).

The support for uint8_t, uint16_t and uint32_t has been added from C11 standard(C language standard). 

Advantage

Fixed width integers helps in eliminating the confusion in assumption of the width of variable.

I will explain how it is confusing.

Different C compilers designed and optimized for different architectures of microprocessor or microcontrollers.

Generally if microcontroller have 8 or 16 bit architecture, the width of the integer would be by default 16 bit in its C compiler and if microcontroller have 32 bit architecture, width would be 32 bits. Now when I will try to run the same program on different architectures, it might create unexpected results.

When I define unsigned int c in the program for PIC18 microcontroller, compiler by default will assign 2 bytes to variable c. But when I will use same declaration in ARM based microcontroller, the compiler by default will assign 4 bytes to variable c.

Now when you define uint8_t or int8_t, it states that the width of a variable is strictly 8 bits only in any architecture, may it be 8/16/32 bits. This will make your program easily portable across different microcontrollers.

Backward Compatibility

Sometimes, a C compiler is modified from C standard library earlier than C11, for example C99, they define typedef for the new keyword for compatibility purpose. They are mostly defined in stdint.h or cstdint.h

typedef uint8_t unsigned char

Available Fixed width integers

Keyword	Description
int8_t	Signed integer type with width of exactly 8 bit
int16_t	Signed integer type with width of exactly 16 bit
int32_t	Signed integer type with width of exactly 32 bit
int64_t	Signed integer type with width of exactly 64 bit
int_fast8_t	Fastest signed integer type with width of at least 8 bits
int_fast16_t	Fastest signed integer type with width of at least 16 bits
int_fast32_t	Fastest signed integer type with width of at least 32 bits
int_fast64_t	Fastest signed integer type with width of at least 64 bits
int_least8_t	Smallest signed integer type with width of at least 8 bits
int_least16_t	Smallest signed integer type with width of at least 16 bits
int_least32_t	Smallest signed integer type with width of at least 32 bits
int_least64_t	Smallest signed integer type with width of at least 64 bits
intmax_t	Maximum width integer type
intptr_t	Integer type capable of holding a pointer
uint8_t	Unsigned integer type with width of exactly 8 bits
uint16_t	Unsigned integer type with width of exactly 16 bits
uint32_t	Unsigned integer type with width of exactly 32 bits
uint64_t	Unsigned integer type with width of exactly 64 bits
uint_fast8_t	Fastest unsigned integer type with width of at least 8 bits
uint_fast16_t	Fastest unsigned integer type with width of at least 16 bits
uint_fast32_t	Fastest unsigned integer type with width of at least 32 bits
uint_fast64_t	Fastest unsigned integer type with width of at least 64 bits
uint_least8_t	Smallest unsigned integer type with width of at least 8 bits
uint_least16_t	Smallest unsigned integer type with width of at least 16 bits
uint_least32_t	Smallest unsigned integer type with width of at least 32 bits
uint_least64_t	Smallest unsigned integer type with width of at least 64 bits
uintmax_t	Maximum width unsigned integer type
uintptr_t	Unsigned integer type capable of holding a pointer

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