pfnUserCallback

pfnUserCallback is a naming convention used in C and C++ libraries to describe a pointer to a user-implemented callback function. The term denotes a function pointer type that the library expects the application to supply so the library can notify the application about events, data availability, or other asynchronous activity.

The actual signature of a pfnUserCallback is defined by the API and can vary between libraries. A common pattern is a typedef for a function pointer that accepts an event identifier, a user-provided context pointer, and optionally additional information about the event. For example, a library might define typedef void (*pfnUserCallback)(int eventCode, void* userData, const void* eventInfo); The key point is that the library specifies the exact parameters and calling convention, and the application must provide a function matching that signature when registering the callback.

Usage typically involves registering the callback with the library, along with a userData pointer that the library will pass back on each invocation. This userData allows the callback to access application state without requiring global variables. When an event occurs, the library invokes the registered function, passing the event details and the user data as defined by the API.

Important considerations include thread safety and lifetime management. Callbacks may be invoked from different threads and possibly while the main thread is performing other work, so reentrancy and synchronization requirements should be observed. The application should ensure that the callback remains valid for as long as the library may call it and that any resources accessed from the callback are properly synchronized.

In practice, pfnUserCallback enables asynchronous and event-driven interactions between a library and its users, providing a flexible mechanism for integrating library functionality into an application.