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In recent years, that is in past 5 years Linux has seen significant growth as a server operating system and has been successfully deployed as an enterprise for Web, file and print servicing. With the advent of Kernel Version 2.4, Linux has seen a tremendous boost in scalability and robustness which further makes it feasible to deploy even more demanding enterprise applications such as high end database, business intelligence software ,application servers, etc. As a result, whole enterprise business suites and middleware such as SAP, Websphere, Oracle, etc., are now available on Linux.

For these enterprise applications to run efficiently on Linux, or on any other operating system, the OS must provide the proper abstractions and services. Usually these enterprise applications and applications suites or software are increasingly built as multi process / multithreaded applications. These application suites are often a collection of multiple independent subsystems. Despite functional variations between these applications often they require to communicate with each other and also sometimes they need to share a common state. Examples of this are database systems, which typically maintain shared I/O buffers in user space.

Access to such shared state must be properly synchronized. Allowing multiple processes to access the same resources in a time sliced manner or potentially consecutively in the case of multiprocessor systems can cause many problems. This is due to the need to maintain data consistency, maintain true temporal dependencies and to ensure that each thread will properly release the resource as required when it has completed its action. Synchronization can be established through locks. There are mainly two types of locks: - Exclusive locks and shared locks. Exclusive locks are those which allows only a single user to access the protected entity, while shared locks are those which implements the multiple reader - single writer semantics. Synchronization implies a shared state, indicating that a particular resource is available or busy, and a means to wait for its availability. The latter one can either be accomplished through busy-waiting or through an explicit / implicit call to the scheduler.


As different processes interact with each other they may often need to access and modify shared section of code, memory locations and data. The section of code belonging to a process or thread which manipulates a variable which is also being manipulated by another process or thread is commonly called critical section. Proper synchronization problems usually serialize the access over critical section. Processes operate within their own virtual address space and are protected by the operating system from interference by other processes. By default a user process cannot communicate with another process unless it makes use of secure, kernel managed mechanisms. There are many times when processes will need to share common resources or synchronize their actions. One possibility is to use threads, which by definition can share memory within a process.

This option is not always possible (or wise) due to the many disadvantages which can be experienced with threads. Methods of passing messages or data between processes are therefore required. In traditional UNIX systems the basic mechanisms for synchronization were System V IPC (inter process communication) such as semaphores, msgqueues, sockets and the file locking mechanisms such as flock() and fcntl() functions.

Message queues (msgqueues) consist of a linked list within the kernel's addressing space. Messages are added to the queue sequentially and may be retrieved from the queue in several different ways. Semaphores are counters used to control access to shared resources by multiple processes.

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