C语言中的语法理解：指针

Question

I have medium knowledge working with pointers. Some of the syntax styles baffles me.

Like:

1. *(uint8 *) (a) = (b)
2. typecasting the reference when parsing it in a function. The function is prototyped as func(unsigned char * a); but it is used as func((unsigned short *) &b); .... As far I understand they were casting a expected char type pointer into a short.

Can anyone help me understand what these statements exactly mean in C?

Answer 1

The first one simply casts (a) into a pointer to an unsigned 8-bit integer, then writes the result of evaluating (b) to the resulting address (as an 8-bit number).

The second one isn't valid, it shouldn't compile. Perhaps you mis-remember, it's a bit sketchy-looking.

Answer 2

To understand what definitions mean in C take a look at the right-left rule, it's very simple and explains a lot http://ieng9.ucsd.edu/~cs30x/rt_lt.rule.html . What it basically means is that you start out with the name and move right building up the definition then move left only when you can't move right. This might sound weird but let's take an example with number 1.

1. *(uint8 *) (a) = (b)

   So step 1 what is a? Start with the name of the variable which in this case is a. Then look to see what is defined to the right of that, here it is nothing, so move to the part immediately to the right, which is (uint8 ). Which means that a is a pointer to a uint8. Ok now move right from the uint8 and you get back to a so that means you can't move right anymore, so go right then which gets you *, which is the dereferencing a. So for this line the uint8 stored at the address a is set to the value of b.

2. Ok for the second part I think the func((unsigned short *) &b) is where they begin defining the function body? If that is case then that means for the function body the passed in parameter is used as a pointer to an unsigned short, but if the function is called like this I think the compiler wouldn't be happy because of the type mismatch since the function is expecting an unsigned char * not an unsigned short * .