prepare Called before all the file descriptors are polled. If the source can determine that it is ready here (without waiting for the results of the poll() call) it should return TRUE. It can also return a timeout_ value which should be the maximum timeout (in milliseconds) which should be passed to the poll() call. The actual timeout used will be -1 if all sources returned -1, or it will be the minimum of all the timeout_ values returned which were >= 0. check Called after all the file descriptors are polled. The source should return TRUE if it is ready to be dispatched. Note that some time may have passed since the previous prepare function was called, so the source should be checked again here. dispatch Called to dispatch the event source, after it has returned TRUE in either its prepare or its check function. The dispatch function is passed in a callback function and data. The callback function may be NULL if the source was never connected to a callback using g_source_set_callback(). The dispatch function should call the callback function with user_data and whatever additional parameters are needed for this type of event source. finalize Called when the source is finalized. For idle sources, the prepare and check functions always return TRUE to indicate that the source is always ready to be processed. The prepare function also returns a timeout value of 0 to ensure that the poll() call doesn't block (since that would be time wasted which could have been spent running the idle function). For timeout sources, the prepare and check functions both return TRUE if the timeout interval has expired. The prepare function also returns a timeout value to ensure that the poll() call doesn't block too long and miss the next timeout. For file descriptor sources, the prepare function typically returns FALSE, since it must wait until poll() has been called before it knows whether any events need to be processed. It sets the returned timeout to -1 to indicate that it doesn't mind how long the poll() call blocks. In the check function, it tests the results of the poll() call to see if the required condition has been met, and returns TRUE if so.