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Concept
FIFO (computing and electronics)
Summary
In computing and in systems theory, first in, first out (the first in is the first out), acronymized as FIFO, is a method for organizing the manipulation of a data structure (often, specifically a data buffer) where the oldest (first) entry, or "head" of the queue, is processed first.
Such processing is analogous to servicing people in a queue area on a first-come, first-served (FCFS) basis, i.e. in the same sequence in which they arrive at the queue's tail.
FCFS is also the jargon term for the FIFO operating system scheduling algorithm, which gives every process central processing unit (CPU) time in the order in which it is demanded. FIFO's opposite is LIFO, last-in-first-out, where the youngest entry or "top of the stack" is processed first. A priority queue is neither FIFO or LIFO but may adopt similar behaviour temporarily or by default. Queueing theory encompasses these methods for processing data structures, as well as interactions between strict-FIFO queues.
Depending on the application, a FIFO could be implemented as a hardware shift register, or using different memory structures, typically a circular buffer or a kind of list. For information on the abstract data structure, see Queue (data structure). Most software implementations of a FIFO queue are not thread safe and require a locking mechanism to verify the data structure chain is being manipulated by only one thread at a time.
The following code shows a linked list FIFO C++ language implementation. In practice, a number of list implementations exist, including popular Unix systems C sys/queue.h macros or the C++ standard library std::list template, avoiding the need for implementing the data structure from scratch.
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