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A concurrent programming language lets a program express multiple independent activities and how they coordinate or communicate. Those activities can make progress during overlapping periods: they may take turns on one processor, or run at the same time on multiple processors. Concurrency does not by itself mean simultaneous execution or guarantee faster performance.
What does concurrent mean in programming?
Concurrency is a way to structure a program around activities that can make progress independently. A program can start or represent those activities and define how they interact using facilities in the language or its runtime model.
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The activities do not have to execute at precisely the same moment. A processor can switch among them, allowing each to advance over the same span of time. Erlang’s official documentation describes this time-sharing model as a way to give the appearance of simultaneous activity on a single processor: Erlang/OTP: Concurrent Programming.
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Concurrency is overlapping progress; parallelism is simultaneous execution. Concurrent activities may run in parallel when multiple processors are available, but they may instead take turns on one processor. A program can therefore be concurrent without being parallel.
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Nor does adding concurrency automatically make a program faster. The Go FAQ puts the qualification this way: “concurrency only enables parallelism when the underlying problem is intrinsically parallel.” Whether parallel execution helps depends on whether the work can usefully be divided into activities that proceed independently. See the Go FAQ on concurrency.
What makes a language concurrent?
There is no single checklist here that every language must satisfy. A practical test is whether the language’s facilities or defined runtime model let a program express independent activities and the coordination between them as part of the program. That support may appear in different forms, and the language’s documentation may describe it at the language or runtime level.
Useful questions for understanding a language’s model include:
- How does a program create or start independent activities?
- Do activities communicate through shared memory, messages, channels, or another mechanism?
- How does the system define synchronization and the visibility of shared data?
- How are activities mapped onto available processors?
These questions help describe and compare designs; they are not a universal classification test.
How do languages express concurrency?
Go: goroutines and channels
Go’s specification says the language supports concurrent programming. A go statement starts a goroutine, and channels provide communication between concurrently executing functions. The specification defines these facilities in its sections on go statements and channel types.
Go’s Effective Go guide describes the channel-oriented approach with the slogan: “Do not communicate by sharing memory; instead, share memory by communicating.” This is guidance for using Go’s communication model, not a rule that Go programs cannot synchronize access to shared data. The guide also relates the approach to communicating sequential processes (CSP): Effective Go: Concurrency.
Erlang: processes
Erlang/OTP’s documentation describes processes as threads of execution and identifies concurrent and distributed programming as central capabilities of Erlang. Its explanation of time-sharing also illustrates why multiple processes can make progress concurrently even on a single processor: Erlang/OTP: Concurrent Programming.
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The Java SE 26 Language Specification describes Java as a “general-purpose, concurrent, class-based, object-oriented language.” This is an explicit classification in the language specification: Java Language Specification, Java SE 26, Chapter 1.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does concurrency make a program safe or faster?
No. Concurrency support gives a program ways to express independent activities and coordinate them; it does not ensure that the chosen coordination is correct or that the work will run faster. The behavior depends on how activities communicate, synchronize, and use shared data, as well as whether the work can benefit from parallel execution.
For Go, the advice to communicate through channels is a recommended approach, not a guarantee that every concurrent program is free of synchronization problems. More broadly, a language’s concurrency features describe what programs can express; they do not promise a performance gain for every program.
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