C 常量的类型

http://bbs.csdn.net/topics/380028485

整型常量的类型是下列相应表中第一个能表示其值的类型：

int --> long int --> long long int

无后缀的十进制整数常量：int，long int，long long int
以字母u或U为后缀的十进制整型常量：unsigned int，unsigned long int，unsigned long long int

以字母l或L为后缀的十进制整型常量：long int，long long int
同时以字母u或U和字母l或L为后缀的十进制整型常量：unsigned long int，unsigned long long int

以字母ll或LL为后缀的十进制整型常量：long long int
同时以字母u或U和字母ll或LL为后缀的十进制整型常量：unsigned long long int

无后缀的八进制或十六进制常量：int，unsigned int，long int，unsigned long int，long long int，unsigned long long int
以字母u或U为后缀的八进制或十六进制常量：unsigned int，unsigned long int，unsigned long long int
以字母l或L为后缀的八进制或十六进制常量：long int，unsigned long int，long long int，unsigned long long int

同时以字母u或U和字母l或L为后缀的八进制或十六进制常量：unsigned long int，unsigned long long int
以字母ll或LL为后缀的八进制或十六进制常量：long long int，unsigned long long int

同时以字母u或U和字母ll或LL为后缀的八进制或十六进制常量：unsigned long long int

http://www.rapidtables.com/prog/cref/const.htm 

Numbers characters and string constants.

See enum end const in data types page.

Data Type Constants

Syntax	Description	Example
0xhexnum 0Xhexnum	hexadecimal number	int x=0x7FFF0000;
0 octnum	octal number	int x=07654321;
num u num U	unsigned integer number constant	unsigned int x=1000U;
num l num L	long integer number constant	long x=-99999L;
num ul num UL	unsigned long integer number constant	unsigned long x=99999L;
num ll num LL	long long  integer number constant	long long x=-888888LL;
num ull num ULL	unsigned long long integer number constant	unsigned long long x=100ULL;
num f num F	float number constant	float x=0.005F;
num l num L	long double number constant	long double x=0.005L;
num e±exp num E±exp	floating point exponent number constant	double x=5.2E-3;
'char'	character constant	char c='A';
"string"	string constant	char name[6]="Hello";

http://stackoverflow.com/questions/7036056/what-do-0ll-or-0x0ul-mean

These are constants in C and C++.
The suffix LL means the constant is of type long long, and UL means unsigned long.

In general, each L or l represents a long and
each U or u represents anunsigned. So, e.g.

1uLL    // means the constant 1 with type unsigned long long.

This also applies to floating point numbers:

1.0f    // of type 'float'
1.0     // of type 'double'
1.0L    // of type 'long double'

and strings and characters, but they are prefixes:

 'A'   // of type 'char'
L'A'   // of type 'wchar_t'
u'A'   // of type 'char16_t' (C++0x only)
U'A'   // of type 'char32_t' (C++0x only)

In C and C++ the integer constants are evaluated using their original type,
which can cause bugs due to integer overflow:

long long nanosec_wrong = 1000000000 * 600;
// ^ you'll get '-1295421440' since the constants are of type 'int'
//   which is usually only 32-bit long, not big enough to hold the result.

long long nanosec_correct = 1000000000LL * 600
// ^ you'll correctly get '600000000000' with this

int secs = 600;
long long nanosec_2 = 1000000000LL * secs;
// ^ use the '1000000000LL' to ensure the multiplication is done as 'long long's.

In Google Go, all integers are evaluated as big integers (no truncation happens),

var nanosec_correct int64 = 1000000000 * 600

and there is no "usual arithmetic promotion"

 var b int32 = 600
    var a int64 = 1000000000 * b
    // ^ cannot use 1000000000 * b (type int32) as type int64 in assignment

so the suffixes are not necessary.