libcamera: Reentrancy and Thread-Safety

libcamera v0.7.0 Supporting cameras in Linux since 2019

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Reentrancy and Thread-Safety

Through the documentation, several terms are used to define how classes and their member functions can be used from multiple threads.

- A reentrant function may be called simultaneously from multiple threads if and only if each invocation uses a different instance of the class. This is the default for all member functions not explictly marked otherwise.
- A thread-safe function may be called simultaneously from multiple threads on the same instance of a class. A thread-safe function is thus reentrant. Thread-safe functions may also be called simultaneously with any other reentrant function of the same class on the same instance.

- A thread-bound function may be called only from the thread that the class instances lives in (see section Threads and Objects). For instances of classes that do not derive from the Object class, this is the thread in which the instance was created. A thread-bound function is not thread-safe, and may or may not be reentrant.

Neither reentrancy nor thread-safety, in this context, mean that a function may be called simultaneously from the same thread, for instance from a callback invoked by the function. This may deadlock and isn't allowed unless separately documented.

A class is defined as reentrant, thread-safe or thread-bound if all its member functions are reentrant, thread-safe or thread-bound respectively. Some member functions may additionally be documented as having additional thread-related attributes.

Most classes are reentrant but not thread-safe, as making them fully thread-safe would incur locking costs considered prohibitive for the expected use cases.