Call_once
call_once is a threading operation that coordinates callers so a specified callback is successfully executed at most once for a given once-control object.
Modern C, Third Edition
call_once is a C threads-library function that invokes a callback at most once for a specified once-control object, coordinating concurrent callers.
- How it is used
- Coordinates dynamic initialization that must become safe before concurrent threads access shared data.
- How deep it goes
- Explains the API, a shared-stream example, waiting behavior, and release-acquire visibility, but not callback failure handling.
- Examples
- Uses a lazily opened error log whose filename, stream, and buffering are initialized by one callback.
- What you can do after
- Apply call_once to publish a correctly initialized shared object across concurrent callers.
C++ Concurrency in Action, Second Edition
`std::call_once` is a C++ standard library function template that ensures a callable is executed exactly once for a given `std::once_flag`.
- How it is used
- Protects shared data during one-time initialization so callers see initialized state after call_once returns.
- How deep it goes
- Explains flag association, callable forms, synchronization, and resource and member initialization; defers detailed memory-model treatment.
- Examples
- Uses a shared resource pointer and a class connection initialized before send or receive operations.
- What you can do after
- Apply it via std::once_flag and std::call_once to initialize shared resources or class members safely.
Read how C++ Concurrency in Action, Second Edition explains call_once →
Modern C
call_once is a C threading function that invokes a specified function exactly once for a given once_flag, synchronizing concurrent callers until that invocation completes.
- How it is used
- Uses call_once as a last resort for dynamically initializing shared data before concurrent access.
- How deep it goes
- Explains the interface, callback selection, once_flag state, and waiting behavior, but does not develop implementation internals.
- Examples
- Demonstrates initializing a shared FILE stream with fopen, setvbuf, and an atexit cleanup callback.
- What you can do after
- Operate/configure a once_flag and callback so concurrent callers trigger shared initialization exactly once.
Related concepts
- std::once_flaguses · A once_flag is a synchronization object that coordinates threads to ensure a one-time function invocation occurs only once.
- callbackuses · A callback is a function or other callable supplied to software for invocation later in response to a specified event, condition, or point in execution.
- atexitcontrasts with · atexit is a C standard-library function that registers a function to execute during normal program termination.
- condition variablealternative to · A condition variable is a synchronization object that allows threads to wait for a shared-state condition and be notified when it may have changed.
- release operationimplements · An atomic operation with release ordering that synchronizes with a corresponding acquire operation, making preceding effects visible.
- initializationuses · Initialization is the process of establishing initial values or a starting state for a program entity, component, resource, or computation before subsequent operation.
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