什么是无符号数据类型？

Question

I've seen this unsigned "typeless" type used a couple of times, but never seen an explanation for it. I suppose there's a corresponding signed type. Here's an example:

static unsigned long next = 1;
/* RAND_MAX assumed to be 32767 */
int myrand(void) {
    next = next * 1103515245 + 12345;
    return(()(next/65536) % 32768);
}
void mysrand() {
    next = seed;
}

What I have gathered so far:
- on my system, sizeof(unsigned) = 4 (hints at a 32-bit unsigned int)
- it might be used as a shorthand for casting another type to the unsigned version:

signed long int i = -42;
printf("%u\n", (unsigned)i);

Is this ANSI C, or just a compiler extension?

Answer 1

unsigned really is a shorthand for unsigned int , and so defined in standard C.

Answer 2

unsigned means unsigned int . signed means signed int . Using just unsigned is a lazy way of declaring an unsigned int in C. Yes this is ANSI.

Answer 3

Historically in C, if you omitted a datatype "int" was assumed. So "unsigned" is a shorthand for "unsigned int". This has been considered bad practice for a long time, but there is still a fair amount of code out there that uses it.

Answer 4

in C, unsigned is a shortcut for unsigned int .

You have the same for long that is a shortcut for long int

And it is also possible to declare a unsigned long (it will be a unsigned long int ).

This is in the ANSI standard

Answer 5

In C and C++

unsigned = unsigned int (Integer type)
signed   = signed int (Integer type)

An unsigned integer containing n bits can have a value between 0 and (2^n-1), which is 2^n different values.

An unsigned integer is either positive or zero.

Signed integers are stored in a computer using 2's complement.

Answer 6

According to C17 6.7.2 §2:

Each list of type specifiers shall be one of the following multisets (delimited by commas, when there is more than one multiset per item); the type specifiers may occur in any order, possibly intermixed with the other declaration specifiers

— void
— char
— signed char
— unsigned char
— short, signed short, short int, or signed short int
— unsigned short, or unsigned short int
— int, signed, or signed int
— unsigned, or unsigned int
— long, signed long, long int, or signed long int
— unsigned long, or unsigned long int
— long long, signed long long, long long int, or signed long long int
— unsigned long long, or unsigned long long int
— float
— double
— long double
— _Bool
— float _Complex
— double _Complex
— long double _Complex
— atomic type specifier
— struct or union specifier
— enum specifier
— typedef name

So in case of unsigned int we can either write unsigned or unsigned int , or if we are feeling crazy, int unsigned . The latter since the standard is stupid enough to allow "...may occur in any order, possibly intermixed". This is a known flaw of the language.

Proper C code uses unsigned int .

Answer 7

Bringing my answer from another question .

From the C specification , section 6.7.2:

— unsigned, or unsigned int

Meaning that unsigned , when not specified the type, shall default to unsigned int . So writing unsigned a is the same as unsigned int a .