Accessing struct member of pointer's address in C

Question

I\'m reading this book, and I found this code snippet in Chapter 14.

struct kobject *cdev_get(struct cdev *p)
{
    struct module *owner = p->owner;
    s         


        

Answer 1

As per p being defined as struct cdev *p, p is very much a "memory address" but that's not all it is - it also has a type attached to it.

Since the expression *ptr is "the object pointed to by ptr", that also has the type attached, so you can logically do (*ptr).member.

And, since ptr->member is identical to (*ptr).member, it too is valid.

Bottom line is, your contention that "pointers [aren't] much more than memory addresses" is correct. But they are a little bit more :-)

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In terms of &ptr->member, you seem to be reading that as (&ptr)->member, which is not correct.

Instead, as per C precedence rules, it is actually &(ptr->member), which means the address of the member of that structure.

These precedence rules are actually specified by the ISO C standard (C11 in this case). From 6.5 Expressions, footnote 85:

The syntax specifies the precedence of operators in the evaluation of an expression, which is the same as the order of the major subclauses of this subclause, highest precedence first.

And, since 6.5.2 Postfix operators (the bit covering ->) comes before 6.5.3 Unary operators (the bit covering &), that means -> evaluates first.