這些數據類型的sizeof具體長度依賴於編譯器和操作系統（32-bit or 64-bit）
1: 首先，參見c99標準
標準中沒有定義這些數據類型的長度，而是定義了這些數據類型能表達的大小範圍的最小極限。
C99鏈接： http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf
The C++ standard does not specify the size of integral types in bytes, but it specifies minimum ranges they must be able to hold. You can infer minimum size in bits from the required range and the value of CHAR_BIT macro, that defines the number of bits in a byte (in all but the most obscure platforms it's 8).
One additional constraint for char is that its size is always 1 byte, or CHAR_BIT bits (hence the name).
Minimum ranges required by the standard (page 22) are:
signed char: -127 to 127 (note, not -128 to 127; this accommodates 1's-complement platforms)
unsigned char: 0 to 255
"plain" char: -127 to 127 or 0 to 255 (depends on default char signedness)
signed short: -32767 to 32767
unsigned short: 0 to 65535
signed int: -32767 to 32767
unsigned int: 0 to 65535
signed long: -2147483647 to 2147483647
unsigned long: 0 to 4294967295
signed long long: -9223372036854775807 to 9223372036854775807
unsigned long long: 0 to 18446744073709551615
A C++ (or C) implementation can define the size of a type in bytes sizeof(type) to any value, as long as
the expression sizeof(type) * CHAR_BIT evaluates to the number of bits enough to contain required ranges, and
the ordering of type is still valid (e.g. sizeof(int) <= sizeof(long)).
The actual implementation-specific ranges can be found in <limits.h> header in C, or <climits> in C++ (or even better, templated std::numeric_limits in <limits> header). 2: 數據類型長度需要符合2個標準
一個是數據類型能描述的範圍，一個是數據類型表達範圍之間的順序
C90 standard requires that
sizeof(short) <= sizeof(int) <= sizeof(long)C99 standard requires that
sizeof(short) <= sizeof(int) <= sizeof(long) < sizeof(long long)3: 5種標準數據類型和他們的衍生類型
signed char
short int
int
long int
long long int
There are five standard signed integer types : signed char, short int, int, long int, and long long int. In this list, each type provides at least as much storage as those preceding it in the list.
For each of the standard signed integer types, there exists a corresponding (but different) standard unsigned integer type: unsigned char, unsigned short int, unsigned int, unsigned long int, and unsigned long long int, each of which occupies the same amount of storage and has the same alignment requirements. The C++ Standard says it like this :
3.9.1, §2 :
There are five signed integer types : "signed char", "short int", "int", "long int", and "long long int". In this list, each type provides at least as much storage as those preceding it in the list. Plain ints have the natural size suggested by the architecture of the execution environment (44); the other signed integer types are provided to meet special needs.
(44) that is, large enough to contain any value in the range of INT_MIN and INT_MAX, as defined in the header <climits>.
The conclusion : it depends on which architecture you're working on. Any other assumption is false.4: 實踐中的事實標準
32-bit 操作系統中，事實標準爲 ILP32, int, long, pointer 都是4字節
64-bit 操作系統中，事實標準爲LP64， int - 4字節， long, pointer 是8字節
在linux操作系統中，參見頭文件 int-ll64.h
For 32-bit systems, the 'de facto' standard is ILP32 - that is, int, long and pointer are all 32-bit quantities.
For 64-bit systems, the primary Unix 'de facto' standard is LP64 - long and pointer are 64-bit (but int is 32-bit). The Windows 64-bit standard is LLP64 - long long and pointer are 64-bit (but long and int are both 32-bit).
At one time, some Unix systems used an ILP64 organization.
None of these de facto standards is legislated by the C standard (ISO/IEC 9899:1999), but all are permitted by it. 5: 數據模型 LP64和ILP32
數據來源： http://en.wikipedia.org/wiki/64-bit#64-bit_data_models
| Data model | short (integer) | int | long (integer) | long long |
pointers/ size_t |
Sample operating systems |
|---|---|---|---|---|---|---|
|
LLP64/ IL32P64 |
16 | 32 | 32 | 64 | 64 | Microsoft Windows (X64/IA-64) |
|
LP64/ I32LP64 |
16 | 32 | 64 | 64 | 64 | Most Unix and Unix-like systems, e.g. Solaris, Linux, BSD, and OS X; z/OS |
數據來源：http://docs.oracle.com/cd/E19620-01/805-3024/lp64-1/index.html
Table F-1 C Data Type Sizes
|
C Type |
ILP32 |
LP64 |
|---|---|---|
|
char |
8 |
8 |
|
short |
16 |
16 |
|
int |
32 |
32 |
|
long |
32 |
64 |
|
long long |
64 |
64 |
|
pointer |
32 |
64 |
In addition to the data model changes, some system-derived types, such as size_t, have been expanded to be 64-bit quantities when
compiled in the 64-bit environment.同上
6: linux 中的實際使用
#ifndef __ASSEMBLY__
/*
* __xx is ok: it doesn't pollute the POSIX namespace. Use these in the
* header files exported to user space
*/
typedef __signed__ char __s8;
typedef unsigned char __u8;
typedef __signed__ short __s16;
typedef unsigned short __u16;
typedef __signed__ int __s32;
typedef unsigned int __u32;
#ifdef __GNUC__
__extension__ typedef __signed__ long long __s64;
__extension__ typedef unsigned long long __u64;
#else
typedef __signed__ long long __s64;
typedef unsigned long long __u64;
#endif
也就是沒用到long 類型，用了char, short, int, long long 就夠了。
sizeof (void *) == sizeof (long)
數據類型分爲三種
C standard (var)
constant
subsystem
7. printf
http://www.gnu.org/software/libc/manual/html_mono/libc.html#Integer-Conversions
12.12.4 Integer Conversions
This section describes the options for the ‘%d’, ‘%i’, ‘%o’, ‘%u’, ‘%x’, and ‘%X’ conversion specifications. These conversions print integers in various formats.
The ‘%d’ and ‘%i’ conversion specifications both print an int argument as a signed decimal number; while ‘%o’, ‘%u’, and ‘%x’ print the argument as an unsigned octal, decimal, or hexadecimal number (respectively). The ‘%X’ conversion specification is just like ‘%x’ except that it uses the characters ‘ABCDEF’ as digits instead of ‘abcdef’.
‘l’
Specifies that the argument is a long int or unsigned long int, as appropriate. Two ‘l’ characters is like the ‘L’ modifier, below.
If used with ‘%c’ or ‘%s’ the corresponding parameter is considered as a wide character or wide character string respectively. This use of ‘l’ was introduced in Amendment 1 to ISO C90.
‘L’
‘ll’
‘q’
Specifies that the argument is a long long int. (This type is an extension supported by the GNU C compiler. On systems that don't support extra-long integers, this is the same as long int.)
The ‘q’ modifier is another name for the same thing, which comes from 4.4 BSD; a long long int is sometimes called a “quad” int.
8: 字節對齊
http://www.unix.org/whitepapers/64bit.html
http://software.intel.com/en-us/articles/data-alignment-when-migrating-to-64-bit-intel-architecture
https://en.wikipedia.org/wiki/Data_structure_alignment
http://csweapon.diandian.com/post/2011-08-26/4372667
自然對齊
64-bit operating environment
- Align 8-bit data at any address
- Align 16-bit data to be contained within an aligned four-byte word
- Align 32-bit data so that its base address is a multiple of four
- Align 64-bit data so that its base address is a multiple of eight
- Align 80-bit data so that its base address is a multiple of sixteen
- Align 128-bit data so that its base address is a multiple of sixteen
|
| Source: |
#include <stdio.h>
int main(void) {
struct li{
long la;
int ia;
} li;
struct lii{
long la;
int ia;
int ib;
} lii;
struct ili{
int ia;
long la;
int ib;
} ili;
printf("length li = %d\n",sizeof(li));
printf("length lii = %d\n",sizeof(lii));
printf("length ili = %d\n",sizeof(ili));
}
|
| ILP32 member lengths: |
length li = 8 length lii = 12 length ili = 12 |
| LP64 member lengths: |
length li = 16 length lii = 16 length ili = 24 |
9: float double
In main storage and in disk storage, a float is
represented with a 32-bit pattern and a double is
represented with a 64-bit pattern.
| Floating Point Primitive Data Types | ||
|---|---|---|
| Type | Size | Range |
| float | 32 bits | -3.4E+38 to +3.4E+38 |
| double | 64 bits | -1.7E+308 to 1.7E+308 |