Understanding C code

[ed181]

#define HIGH_BYTE(w) (unsigned char)((unsigned int)w >> 8)

Does anyone understand this line of code? I'm fairly new to C and i can't seem to grasp what this line means.

Thanks for any help in advance

 

[(*steve*)]

This is the definition of a macro which casts w to an unsigned integer, right shifts it by 8 bits, and returns the lower 8 bits as a char.

Essentially it returns the next to lowest significant byte of w.

Note that this is NOT a function, the calculations are done in-line.

char c ;
c = HIGH_BYTE(0x4100);

would be the same as

char c;
c = 'A';

because 0x41 = 65 = 'A'

 

[ed181]

That's great, thanks for the excellent explanation. Now i need to figure out why on earth someone would want to do that

 

[Harald Kapp]

This is useful for w being a 16 bit (or more) variable where you want to access bits 8...15.
An exampel is text that is presented not as a string of 8 bit characters, but as a string of 16 (32, 64...) variables where each variable contains 2 (4, 8, ...) characters.
One will have to look at the source code where the macro is used to find out why it is used (what it does at that location in the code).