Audio callback
The function the audio driver calls whenever the next buffer needs filling — and which must never be late.
Almost every audio API, from ASIO and Core Audio to JACK and Web Audio, uses the same model: the program registers a function, and the driver calls it from a high-priority thread, perhaps a thousand times a second, with an empty buffer that must be full before it returns. At 64 samples and 48 kHz that leaves about 1.3 milliseconds to work with.
Hence the rules Ross Bencina summed up in 2011 under the heading time waits for nothing: no memory allocation, no locks, no file or network I/O, no system calls that might block. A malloc is usually fast but not always, and "not always" is audible as a click. Talking to the rest of the program therefore goes through lock-free queues. Garbage-collected languages struggle here, for the same reason.