Grisha Trubetskoy

Notes to self.

Linus on Understanding Pointers

| Comments

A while back Linus Torvalds answered questions on Slashdot.

One of the answers was on the subject of understanding of pointers:

At the opposite end of the spectrum, I actually wish more people understood the really core low-level kind of coding. Not big, complex stuff like the lockless name lookup, but simply good use of pointers-to-pointers etc. For example, I’ve seen too many people who delete a singly-linked list entry by keeping track of the “prev” entry, and then to delete the entry, doing something like

if (prev)
prev->next = entry->next;
else
list_head = entry->next;

and whenever I see code like that, I just go “This person doesn’t understand pointers”. And it’s sadly quite common.

People who understand pointers just use a “pointer to the entry pointer”, and initialize that with the address of the list_head. And then as they traverse the list, they can remove the entry without using any conditionals, by just doing a “*pp = entry->next”

There were a few comments, but none explained what he really meant. So here it is.

Imagine you have a linked list defined as:

1
2
3
4
typedef struct list_entry {
    int val;
    struct list_entry *next;
} list_entry;

You need to iterate over it from the begining to end and remove a specific element whose value equals the value of to_remove. The more obvious way to do this would be:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
list_entry *entry = head; /* assuming head exists and is the first entry of the list */
list_entry *prev = NULL;

while (entry) {
    if (entry->val == to_remove)     /* this is the one to remove */
        if (prev)
           prev->next = entry->next; /* remove the entry */
        else
            head = entry->next;      /* special case - first entry */

    /* move on to the next entry */
    prev = entry;
    entry = entry->next;
}

What we are doing above is iterating over the list until entry is NULL, which means we’ve reached the end of the list (line 4). When we come across an entry we want removed (line 5), we assign the value of current next pointer to the previous one, thus eliminating the current element (line 7).

There is a special case above - at the beginning of the iteration there is no previous entry (prev is NULL), and so to remove the first entry in the list you have to modify head itself (line 9).

What Linus was saying is that the above code could be simplified by making the previous element a pointer to a pointer rather than just a pointer. The code then looks like this:

1
2
3
4
5
6
7
8
9
10
list_entry **pp = &head; /* pointer to a pointer */
list_entry *entry = head;

while (entry) {
    if (entry->val == to_remove)
        *pp = entry->next;

    pp = &entry->next;
    entry = entry->next;
}

The above code is very similar to the previous variant, but notice how we no longer need to watch for the special case of the first element of the list, since pp is not NULL at the beginning. Simple and clever.

Also, someone in that thread commented that the reason this is better is because *pp = entry->next is atomic. It is most certainly NOT atomic. The above expression contains two dereference operators (* and ->) and one assignment, and neither of those three things is atomic. This is a common misconception, but alas pretty much nothing in C should ever be assumed to be atomic (including the ++ and -- operators)!

Comments