SetEvent

SetEvent is a function in the Windows API used to signal an event object, a synchronization primitive that coordinates threads within a process or across processes. The event object is typically created by CreateEvent or opened with OpenEvent and identified by a handle.

The SetEvent function takes a single parameter, a handle to an event object, and marks the event as signaled. It returns a nonzero value on success and zero on failure; in case of failure, callers can obtain extended error information with GetLastError. The exact effect of signaling depends on the event’s reset mode, defined when the event was created.

There are two reset modes: manual-reset and auto-reset. For a manual-reset event, signaling with SetEvent releases all waiting threads and the event remains signaled until ResetEvent is called. For an auto-reset event, signaling releases a single waiting thread (if any) and the event then automatically resets to the nonsignaled state. If no threads are waiting when SetEvent is called on an auto-reset event, the event remains signaled until a thread waits on it, after which it resets automatically.

SetEvent is commonly used in producer-consumer and synchronization scenarios where one thread signals that a condition has occurred, allowing waiting threads to proceed via WaitForSingleObject or WaitForMultipleObjects. Because events can be unnamed or named, they can also synchronize across different processes.

See also: CreateEvent, ResetEvent, OpenEvent, WaitForSingleObject, WaitForMultipleObjects.