How can I distinguish a pointer from a dereference in C?

Question

int value =5;
void testPointer( int* pa, int* pb) {

*pa = *pb +5;
*pb = value;
value += 10;
}

How can I distingush both from each other? I dont get it

Answer 1

A unary * always indicates a dereference.

This may feel familiar and intuitive in an expression: *pb + 5 means to get the value pb points to and add five. In contrast, you may find a declaration less intuitive; what does * mean in int *pa?

The way to think of this is that a declaration gives a picture of how something will be used. The declaration int *pb says *pb will be used as an int. In other words, when we get the value pb points to, it is an int. The * in a declaration represents the same thing that happens in an expression: dereferencing the pointer.

Kernighan and Ritchie tell us this in The C Programming Language, 1978, page 90:

The declaration of the pointer px is new.

int *px;

is intended as a mnemonic; it says the combination *px is an int, that is, if px occurs in the context *px, it is equivalent to a variable of the type int. In effect, the syntax of the declaration for a variable mimics the syntax of expressions in which the variable might appear.

As a more involved example, consider int (*p)[];. This tells us that (*p)[] is an int. Since [] is used to access array elements, this means (*p) must be an array of int. And that means p must be a pointer to an array of of int. Just like *, [] does not have a reversed meaning in declarations. It does not mean “is an array of” instead of “access an element of”; it is still an image of how the thing will be used in an expression.

Answer 2

When specifying a type, for example inside a declaration, the * means "pointer". Otherwise, the * means "dereference" or "multiplication" (depending on the context).

For example, when initializing a variable inside a declaration, all * before the = means "pointer", and all * after the = means "dereference" or "multiplication":

int i = 80;
int *p = &i; // In this case, * means "pointer"
int j = *p; // In this case, * means "dereference", so j gets the value 80