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14. Static variables

14.1 Overview

Linux is written in ''C'' language, and as every application has:

  1. Local variables
  2. Module variables (inside the source file and relative only to that module)
  3. Global/Static variables present in only 1 copy (the same for all modules)

When a Static variable is modified by a module, all other modules will see the new value.

Static variables under Linux are very important, cause they are the only kind to add new support to kernel: they typically are pointers to the head of a list of registered elements, which can be: 

                           _______      _______      _______
Global variable  -------> |Item(1)| -> |Item(2)| -> |Item(3)|  ..
                          |_______|    |_______|    |_______|

14.2 Main variables

Current

                           ________________
Current ----------------> | Actual process |
                          |________________|

Current points to ''task_struct'' structure, which contains all data about a process like:

Current is not a real variable, it is 

static inline struct task_struct * get_current(void) { 
   struct task_struct *current; 
   __asm__("andl %%esp,%0; ":"=r" (current) : "0" (~8191UL)); 
   return current; 
}
#define current get_current()

Above lines just takes value of ''esp'' register (stack pointer) and get it available like a variable, from which we can point to our task_struct structure.

From ''current'' element we can access directly to any other process (ready, stopped or in any other state) kernel data structure, for example changing STATE (like a I/O driver does), PID, presence in ready list or blocked list, etc.

Registered filesystems

                       ______      _______      ______
file_systems  ------> | ext2 | -> | msdos | -> | ntfs |
 [fs/super.c]         |______|    |_______|    |______|

When you use command like ''modprobe some_fs'' you will add a new entry to file systems list, while removing it (by using ''rmmod'') will delete it.

Mounted filesystems

                        ______      _______      ______
mount_hash_table  ---->|   /  | -> | /usr  | -> | /var |
[fs/namespace.c]       |______|    |_______|    |______|

When you use ''mount'' command to add a fs, the new entry will be inserted in the list, while an ''umount'' command will delete the entry.

Registered Network Packet Type

                        ______      _______      ______ 
     ptype_all  ------>|  ip  | -> |  x25  | -> | ipv6 |
[net/core/dev.c]       |______|    |_______|    |______|

For example, if you add support for IPv6 (loading relative module) a new entry will be added in the list.

Registered Network Internet Protocol

                          ______      _______      _______ 
inet_protocol_base ----->| icmp | -> |  tcp  | -> |  udp  |
[net/ipv4/protocol.c]    |______|    |_______|    |_______|

Also others packet type have many internal protocols in each list (like IPv6). 

                          ______      _______      _______ 
inet6_protos ----------->|icmpv6| -> | tcpv6 | -> | udpv6 |
[net/ipv6/protocol.c]    |______|    |_______|    |_______|

Registered Network Device

                          ______      _______      _______ 
dev_base --------------->|  lo  | -> |  eth0 | -> |  ppp0 |
[drivers/core/Space.c]   |______|    |_______|    |_______|

Registered Char Device

                          ______      _______      ________ 
chrdevs ---------------->|  lp  | -> | keyb  | -> | serial |
[fs/devices.c]           |______|    |_______|    |________|

''chrdevs'' is not a pointer to a real list, but it is a standard vector.

Registered Block Device

                          ______      ______      ________ 
bdev_hashtable --------->|  fd  | -> |  hd  | -> |  scsi  |
[fs/block_dev.c]         |______|    |______|    |________|

''bdev_hashtable'' is an hash vector.

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