How do I split up a long value (32 bits) into four

问题:

I have a 32 bit long variable, CurrentPosition, that I want to split up into 4, 8bit characters. How would I do that most efficiently in C? I am working with an 8bit MCU, 8051 architectecture.

unsigned long CurrentPosition = 7654321;
unsigned char CP1 = 0;
unsigned char CP2 = 0;
unsigned char CP3 = 0;
unsigned char CP4 = 0;
// What do I do next? 

Should I just reference the starting address of CurrentPosition with a pointer and then add 8 two that address four times?

It is little Endian.

ALSO I want CurrentPosition to remain unchanged.

回答1:

    CP1 = (CurrentPosition & 0xff000000UL) >> 24;
    CP2 = (CurrentPosition & 0x00ff0000UL) >> 16;
    CP3 = (CurrentPosition & 0x0000ff00UL) >>  8;
    CP4 = (CurrentPosition & 0x000000ffUL)      ;

You could access the bytes through a pointer as well,

unsigned char *p = (unsigned char*)&CurrentPosition;
//use p[0],p[1],p[2],p[3] to access the bytes.

回答2:

I think you should consider using a union:

union {
   unsigned long position;
   unsigned char bytes[4];
} CurrentPosition;

CurrentPosition.position = 7654321;

The bytes can now be accessed as: CurrentPosition.bytes[0], ..., CurrentPosition.bytes[3]

回答3:

If You are using an 8 bit MCU shifting a whole 32 bit variable is a bit of work. In this case it's better to read 4 bytes of CurrentPosition using pointer arithmetic. The cast:

unsigned char *p = (unsigned char*)&CurrentPosition;

doesn't change the CurrentPosition, but if You try to write to p[0] You will change the least significant byte of the CurrentPosition. If You want a copy do this:

unsigned char *p = (unsigned char*)&CurrentPosition;
unsigned char arr[4];
arr[0] = p[0];
arr[1] = p[1];
arr[2] = p[2];
arr[3] = p[3];

and work with arr. (If you want most significant byte first change the order in those assignments).

If You prefer 4 variables You can obviously do:

unsigned char CP1 = p[0];
unsigned char CP2 = p[1];
unsigned char CP3 = p[2];
unsigned char CP4 = p[3];

回答4:

CP1 = (unsigned char)(CurrentPosition & 0xFF);
CurrentPosition >>= 8;
CP2 = (unsigned char)(CurrentPosition & 0xFF);
...

回答5:

unsigned char *CP = &CurrentPosition;

Now CPn per your original code is accessed via CP[n].

回答6:

I know this was posted some time ago. But for anyone still reading the thread: Many people take the approach of sequentially shifting the original value. Why not let the compiler do the work for you. Use a union & to allow you to store the values in the same location. Define a union consisting of both a 32 bit long variable (this will be where you save your CurrentPosition) and a structure consisting of 4 char variables. Or just a simple 8 bit integer array. When you write your CurrentPosition to the long variable, it will be stored in the same location accessed when you read the 4 char variables. This method is much less labour intensive and does not allows the compiler to do the work instead of wasting time & resources.