一些基本的C/C++数据类型

• size_t

size_t. A basic unsigned integer C/C++ type. It is the type of the result returned by sizeof operator. The type's size is chosen so that it could store the maximum size of a theoretically possible array of any type. On a 32-bit system size_t will take 32 bits and on a 64-bit one - 64 bits. In other words, a pointer can be safely put inside size_t type (an exception is class-function-pointers but this is a special case). size_t type is usually used for loop, array indexing, size storage and address arithmetic. Although size_t can store a pointer, it is better to use another unsinged integer type uintptr_t for that purpose (its name reflects its capability). In some cases using size_t type is more effective and safe than using a more habitual for the programmer unsigned type.

size_t is a base unsigned integer memsize-type defined in the standard library of C/C++ languages. This type is described in the header file stddef.h for C and in the file cstddef for C++. Types defined by the header file stddef.h are located in the global namespace while cstddef places the size_t type in the namespace std. Since the standard header file stddef.h of the C language is included into C++ programs for the purpose of compatibility, in these programs you may address the type both in the global namespace (::size_t, size_t) and namespace std (std::size_t).

32：
/usr/include/asm/posix_types.h:18:typedef unsigned int     __kernel_size_t;
/usr/include/linux/types.h:39:typedef __kernel_size_t              size_t;
64：
/usr/include/asm-i386/posix_types.h:18:typedef unsigned int        __kernel_size_t;
/usr/include/asm-x86_64/posix_types.h:18:typedef unsigned long     __kernel_size_t;
/usr/include/linux/types.h:39:typedef __kernel_size_t              size_t;

• ptrdiff_t

A basic signed integer C/C++ type. The type's size is chosen so that it could store the maximum size of a theoretically possible array of any type. On a 32-bit system ptrdiff_t will take 32 bits and on a 64-bit one - 64 bits. In other words, a pointer can be safely put inside ptrdiff_t type (an exception is class-function-pointers but this is a special case). By the way the result of the expression where one pointer is subtracted from another (ptr1-ptr2) will have ptrdiff_t type. ptrdiff_t type is usually used for loop, array indexing, size storage and address arithmetic. ptrdiff_t type has its synonym intptr_t whose name indicates more clearly that it can store a pointer. In some cases using ptrdiff_t type is more effective and safe than using a more habitual for the programmer int type.

• time_t

time_t 在32和64位相同，都是用long来定义的：
/usr/include/linux/types.h:54:typedef __kernel_time_t              time_t;
/usr/include/asm/posix_types.h:21:typedef long             __kernel_time_t;

• size_type

the Standard really defines it as,

template <class T>
class allocator
{
   public:
       typedef size_t size_type;
       //.......
};

简单总结下：

long和指针char*在32位及64位下占用字节数一致，均是4和8.

在32位下要注意，long(time_t\size_t\size_type)、char*、这一阵营占用都是4，表面上与int、uint32_t、unsigned一样

uint64_t始终是8个字节的（即使在32bit下，会定义为long long来保证的，long long在64bit下同long一样8字节）如下：

#if __WORDSIZE == 64
typedef unsigned long int       uint64_t;
#else
__extension__
typedef unsigned long long int  uint64_t;
#endif

所以，在64bit下，long(time_t\size_t\size_type\uint64_t)、char*这一阵营占用都是8，与int、uint32_t、unsigned真不一样了。