Use this function to lock out the audio callback function for a specified device.
void SDL_LockAudioDevice(SDL_AudioDeviceID dev);
the ID of the device to be locked
The lock manipulated by these functions protects the audio callback function specified in SDL_OpenAudioDevice(). During a SDL_LockAudioDevice()/SDL_UnlockAudioDevice() pair, you can be guaranteed that the callback function for that device is not running, even if the device is not paused. While a device is locked, any other unpaused, unlocked devices may still run their callbacks.
Calling this function from inside your audio callback is unnecessary. SDL obtains this lock before calling your function, and releases it when the function returns.
You should not hold the lock longer than absolutely necessary. If you hold it too long, you'll experience dropouts in your audio playback. Ideally, your application locks the device, sets a few variables and unlocks again. Do not do heavy work while holding the lock for a device.
It is safe to lock the audio device multiple times, as long as you unlock it an equivalent number of times. The callback will not run until the device has been unlocked completely in this way. If your application fails to unlock the device appropriately, your callback will never run, you might hear repeating bursts of audio, and SDL_CloseAudioDevice() will probably deadlock.
Internally, the audio device lock is a mutex; if you lock from two threads at once, not only will you block the audio callback, you'll block the other thread.
This function is available since SDL 2.0.0.