What does the asterisk in C do?

Question

What does the * mean in C? I see it used when declaring a char or FILE variable (char = *test) How does that change how a variable behaves?

I don't understand the downvotes. To someone, who is just learning C this is a more relevant question compared to "Pointers in C: when to use ...", because the latter implies, that you already know about pointers. — Lajos Mészáros, May 30 '20 at 22:32

score 6 · Answer 1 · answered Dec 15 '15 at 13:40

6

It dereferences a pointer:

*ptr = 42; // access the value that ptr points to, and set it to 42

or it declares a pointer:

int* ptr; // the type of ptr is pointer to int

answered Dec 15 '15 at 13:40

emlai

In the same manner, *p=42; means, that you assign 42 to p, which is wrong if p points to nowhere. – Michi Dec 15 '15 at 15:00
2

No, `*p = 42` doesn't assign to `p`, it assigns to `*p` (i.e. the thing that `p` points to). And yes, that's undefined behavior if `p` doesn't point to valid memory. – emlai Dec 15 '15 at 15:03
Was a typo, I wrote from my phone...I meant *p :) – Michi Dec 15 '15 at 15:06

score 5 · Accepted Answer · edited Dec 15 '15 at 13:44

5

This type of * is called "indirection operator", and *test means "get the data from where the pointer test points".

char is reserved for use as a keyword, so char = *test won't compile unless char is defined as a macro.

edited Dec 15 '15 at 13:44

Haris

answered Dec 15 '15 at 13:40

MikeCAT

Precise. Nice Answer. – Koby Douek Dec 15 '15 at 13:41
Thanks! I meant (char *test) :P – Emerald Spire Dec 15 '15 at 13:48
This type of asterisk is for declaring a pointer, not for dereferencing. – MikeCAT Dec 15 '15 at 14:40

2 Answers2