Linux Kernel Networking and Development

If you want to get a brief information about how to use Linux's linked list data structure, please see the below sample code. You can check-out the code from the github location, build and run on your own Linux development environment. /* * my_object structure. */ struct my_object { int data; struct list_head list; }; /* * list_init() */ static int list_init ( void ) { struct my_object *tmp; /* Temporary object created for each list member */ struct my_object obj_list; /* List constructed with the tmp objects */ struct list_head *pos, *n; /* Position pointers in the list */ int i; printk(KERN_INFO "list init \n " ); /* * Initialize the list object. This object is used only as the * head of the list. It doesn't have any data in it. */ INIT_LIST_HEAD(&obj_list.li...

In this post, I will explain the usage of linux's container_of macro with a sample code. If you know address of a data structure's field, you can get the address of the instance of that data structure by using the container_of macro. This is the simplest explanation of this macro. Let's say you have a function called with a pointer parameter and you know that the pointer is pointing an address of a data structure field. struct my_object { int a; int b; } obj; function(&obj->b); If you want to access the other field "a" in the function, you need to use container_of macro. Let's see the below sample code and it's output to understand it easily. /* * my_object structure. */ struct my_object { int a; int b; int c; }; /* * container_test() * Test function for usage of container_of macro. * * This function is passed a pointer which points a field of * an allocated my_obj...

OpenWrt is an open source project for embedded operating system based on Linux, primarily used on embedded devices to route network traffic. [ 1 ] In this post, I will write about how to get the OpenWrt source, compile it for your Raspberry Pi board type and flash the SD card with the generated image. First of all, you need a Linux build environment installed with the requirements needed to build the OpenWrt. I use Ubuntu 14.04. In the market, there are 4 types of Raspberry Pi boards. Type 1, 2, 3 and Zero. OpenWrt's latest trunk branch supports all of these types. I will go through trunk. Check out the source: git clone git://github.com/openwrt/openwrt.git This will create a directory called "openwrt" in your current directory. Go inside this directory and run make package/symlinks command. msezgin@ubuntu:~/openwrt/make package/symlinks Now, we need to do the configuration before starting the build. msezgin@ubuntu:~/openwrt/make menuconfig ...

In some cases 2 linked list can intersect each other. In the following diagram, you see that 2 NULL terminated singly linked lists are intersected at the node 5. In this implementation, we will find in which node the 2 lists are intersecting each other. The function will return the node address. We will use the helper functions form the previous post which add nodes to the lists, print the lists etc... Function which finds the intersection: /* * link_list_intersect() * Finds the intersection node of 2 lists. */ struct node * link_list_intersect ( struct node *list1, struct node *list2) { struct node *long_list; struct node *short_list; int diff = 0 ; /* * Get the lengths of the lists. */ int len1 = list_lenght_get(list1); int len2 = list_lenght_get(list2); /* * Find which list is long and which list is short * and find the absolute lenght difference. */ if (len1 >= len2) { diff = l...

In this post, you will see the implementation of a singly linked list. It is one of the main data structures in computer programming. A singly linked list consists of nodes which are bound to each other with their "next" pointers. A simple node is represented at least 2 fields. A "next" node pointer and a "data" field. /*   * node structure.   */ struct node { struct node *next; int data; }; Some of the basic methods of a linked list and node data structures are implemented below. Create a Node Method: /*   * node_create()   * Creates a node.   */ struct node *node_create( int data) {     struct node *n = malloc( sizeof ( struct node));     if (!n) {         printf( "Unable to allocate memory for node\n" );         return NULL ;     }     n->data = data;     n->next = NULL ; ...