问题
I noticed that UInt32 is defined differently based on the platform in MacTypes.h
#if __LP64__
typedef unsigned int UInt32;
typedef signed int SInt32;
#else
typedef unsigned long UInt32;
typedef signed long SInt32;
#endif
If unsigned int is always 32 bits on 32 and 64bit machines, why do they bother conditionally checking the platform?
回答1:
My guess is it is some old code which was initially:
typedef unsigned long UInt32;
typedef signed long SInt32;
and a developper later added the LP64 and he did by adding the lines:
#if __LP64__
typedef unsigned int UInt32;
typedef signed int SInt32;
#else
to no impact the previous platforms.
Of course, it does not make much sense to do that.
回答2:
The type UInt32 existed before 64-bit support. It has historically been defined as unsigned long. It could have been unsigned int. I don't know why long was chosen over int at that time. The choice would have been largely arbitrary.
Once that choice was made, though, it can't be changed, even though unsigned int would work for both 32- and 64-bit.
The big thing that would break if it were changed would be C++. In C++, the types of arguments are baked into the symbol names in the object files and libraries. long and int are different types, so void foo(long); and void foo(int); are separate functions with separate symbol names. If UInt32 were to change in 32-bit, then you wouldn't be able to link against libraries that were built with the old definition. If the libraries were rebuilt with the new definition, then old compiled code would not be able to load them.
回答3:
A long is guaranteed to be at least 32 bits. An int will be 16 bits on a 16 bit processor. This is discussed here http://en.wikipedia.org/wiki/C_data_types, among other places.
回答4:
The actual size of integer types varies by implementation. The standard only requires size relations between the data types and minimum sizes for each data type. Generally, the sizeof(int) reflects that "natural/native" size of the machine. On a 64-bit machine, int might be defined as 32 or 64 bits, ; 32-bits on 32-bit architecture, 16-bit on 16-bit machines. However, the standard indicates that int type will always be at least 16 bits.
回答5:
unsigned int is not always 32-bit long, it depends on the model
- In LP64 model, long and pointer are 64-bit types, int is a 32-bit type
- In ILP64 model, int, long and pointer are 64-bit types
https://en.wikipedia.org/wiki/64-bit_computing#64-bit_data_models
In other models, int can have any other number of bits. The only restriction is that it must have at least 16 bits
来源：https://stackoverflow.com/questions/23503088/why-does-apple-define-uint32-as-long-or-int-depending-on-platform