Where are queues used?

Wherever things must be handled in arrival order: waiting lines, shared resources such as a printer, and scheduling in multiprogramming and multithreading; indirectly as helper structures in other algorithms and data structures.

Direct applications — all of them "first come, first served":

• Waiting lines: counters, customs, a ski lift.
• Shared resources: print jobs from many users go into one queue, and the printer handles them in order.
• Multiprogramming and multithreading: processes or tasks that are ready to run wait in a queue for the CPU; messages between threads are passed through queues.

Indirect applications: a queue is a building block inside other algorithms (breadth-first search visits nodes level by level by keeping the frontier in a queue) and inside other data structures, for example buffers between a producer and a consumer that work at different speeds.

The common thread is fairness: a queue guarantees that nothing that arrived earlier is overtaken by something that arrived later.

Go deeper:

• Circular buffer — the fixed-size array queue used for buffering between producers and consumers.