Linked Lists example - Why are they used?

Question

I was looking at a code block on how to get interface information for Unix / iOS / Mac OS X (IP address, interface names, etc.), and wanted to understand more of why linked lists are used. I'm not a full-time programmer, but I can code and always trying to learn. I do understand basic C/C++ but never had experience or had to use linked lists.

I'm trying to learn OS X and iOS development and was trying to get network interface information and came across this: https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man3/getifaddrs.3.html

If I understand this correctly, it appears a linked list is used to link a bunch of structs together for each interface. Why is a linked list used in this situation? How come the structs aren't just created and stored in an array?

Thanks

Answer 1

Linked list algorithms are very nice when you don't know how many elements are going to be in the list when you get started, or if you may add or remove elements over time. Linked lists are especially powerful if you want to add or remove elements anywhere other than the end of the list. Linked lists are very common in Unix. Probably the best place to research is Wikipedia, which discuss the advantages, disadvantages, and other details. But the primary lesson is that linked lists are very good for dynamic data structures, while arrays tend to be better when things are static.

Network interfaces may feel very static if you think of them as "network cards," but they're used for many other things like VPN connections and can change quite often.

Answer 2

[...] and wanted to understand more of why linked lists are used. I'm not a full-time programmer, but I can code and always trying to learn. I do understand basic C/C++ but never had experience or had to use linked lists.

Linked lists are actually an extremely simple data structure. They come in a few varieties but the overall concept is just to allocate nodes and link them together through indices or pointers, like so:

Why is a linked list used in this situation?

Linked lists have some interesting properties, one of which is noted in the above diagram, like constant-time removals and insertions from/to the middle.

How come the structs aren't just created and stored in an array?

They actually could be. In the diagram above, the nodes can be directly stored in the array. The point of then linking the nodes is to allow things like rapid insertions and removals. An array of elements doesn't offer that flexibility, but if you store an array of nodes which store indices or pointers to next and possibly previous elements, then you can start rearranging the structure and removing things and inserting things to the middle all in constant-time by just playing with the links.

The most efficient uses of linked lists often store the nodes contiguously or partially contiguously (ex: using a free list) and just link them together to allow rapid insertions and removals. You can just store the nodes in a big array, like vector, and then link things up and unlink them through indices. Another interesting property of linked lists is that you can rapidly transfer the elements from the middle of one list to another by just changing a couple of pointers.

They also have a property which makes them very efficient to store contiguously when care is paid to their allocation in that every node is of the same size. As an example, it can be tricky to represent a bunch of variable-sized buckets efficiently if they all use their own array-like container, since each one would want to allocate a different amount of memory. However, if they just store an index/pointer to a list node, they can easily store all the nodes in one giant array for all the buckets.

That said, in C++, linked lists are often misused. In spite of their algorithmic benefits, of lot of that doesn't actually translate to superior performance if the nodes are not allocated in a way that provides spatial locality. Otherwise you can incur a cache miss and possibly some page faults accessing every single node.

Nevertheless, used with care about where the nodes go in memory, they can be tremendously useful. Here is one example usage:

In this case, we might have a particle simulation where every single particle is moving around each frame with collision detection where we partition the screen into grid cells. This allows us to avoid quadratic complexity collision detection, since a particle only needs to check for collision with other particles in the same cell. A real-world version might store 100x100 grid cells (10,000 grid cells).

However, if we used an array-based data structure like std::vector for all 10,000 grid cells, that would be explosive in memory. On top of that, transferring each particle from one cell to another would be a costly linear-time operation. By utilizing a linked list here (and one that just uses integers into an array for the links), we can just change a few indices (pointers) here and there to transfer a particle from one cell to another as it moves, while the memory usage is quite cheap (10,000 grid cells means 10,000 32-bit integers which translates to about 39 kilobytes with 4 bytes of overhead per particle for the link).

Used carefully, linked lists are a tremendously useful structure. However, they can often be misused since a naive implementation which wants to allocate every single node separately against a general-purpose memory allocator tends to incur cache misses galore as the nodes will be very fragmented in memory. The useful of linked lists tends to be a detail forgotten lately, especially in C++, since the std::list implementation, unless used with a custom allocator, is in that naive cache misses-galore category. However, the way they're used in operating systems tends to be very efficient, reaping these algorithmic benefits mentioned above without losing locality of reference.

Answer 3

There are various ways to store data. In c++, the first choice is typically a std::vector, but there are std::list and other containers - the choice will depend on several factors such as how often and where do you want to insert/delete things (vector is great for deleting/adding at the end, but inserting in the middle is bad - linked lists take much less to insert in the middle, but will be less good to iterate over).

However, the API for this function is a classic C (rather than C++), so we have to have a "variable length container", and of course, we could implement something in C that resembles std::vector (a value that holds number of elements and a pointer to the actual elements). I'm not sure why the designers DIDN'T do that in this case, but a linked list has the great advantage that it is near zero cost to extend it with one more element. If you don't know beforehand how many there will be, this is a good benefit. And my guess is that there aren't many enough of these objects worry about performance as such [the caller can always rearrange it into a more suitable form later].

Answer 4

Linked lists are very perfect data structures to store very large amount of data's whose number of element is not known. It is very flexible data structure which expand and contract on run time. It also reduce the extra memory allocation or waste because they use dynamic memories to store data. When we finish to use the data it deletes the data as well as that memory allocation.

Answer 5

I agree with everyone here about the benefits of linked list over array for dynamic data length but i need to add something

if the ifaddrs allocated structures are identical in length ... there is no any advantage of using linked list over array.. and if so i can consider it as a "bad design"

but if not (and may be this is the case ..please notice " The ifaddrs structure contains at least the following entries"... the array will not be the proper representation for variable length structures consider this example

struct ifaddrs
{
     struct ifaddrs   *ifa_next;         /* Pointer to next struct */
     char             *ifa_name;         /* Interface name */
     u_int             ifa_flags;        /* Interface flags */
     struct sockaddr  *ifa_addr;         /* Interface address */
     struct sockaddr  *ifa_netmask;      /* Interface netmask */
     struct sockaddr  *ifa_dstaddr;      /* P2P interface destination */
     void             *ifa_data;         /* Address specific data */
};

struct ifaddrs_ofothertype
{
     struct ifaddrs   ifaddrs;         /* embed the original structure */
     char             balhblah[256];         /* some other variable */
};

the mentioned function can return a list of mixed structure like (ifaddrs_ofothertype* casted to ifaddrs*) and (ifaddrs*) without worrying about structure length for each element

Answer 6

If you want to learn iOS you have to learn pointer and memory allocation knowledge from the very base. Although Objective-C is the next generation programming language of C programming language but has a bit difference in syntax specially in method calling and definition. Before you get into iOS/Mac OSX you should have understand Pointers knowledge, MVC knowledge and also understand the core information of iOS Frameworks then you can be a professional iOS Developer. For that visit RayWenderLich iOS Tutiorials