(This is the documentation for SDL3, which is the current stable version. SDL2 was the previous version!)

SDL_AudioPostmixCallback

A callback that fires when data is about to be fed to an audio device.

Header File

Syntax

typedef void (SDLCALL *SDL_AudioPostmixCallback)(void *userdata, const SDL_AudioSpec *spec, float *buffer, int buflen);

Function Parameters

userdata	a pointer provided by the app through SDL_SetAudioPostmixCallback, for its own use.
spec	the current format of audio that is to be submitted to the audio device.
buffer	the buffer of audio samples to be submitted. The callback can inspect and/or modify this data.
buflen	the size of `buffer` in bytes.

Remarks

This is useful for accessing the final mix, perhaps for writing a visualizer or applying a final effect to the audio data before playback.

This callback should run as quickly as possible and not block for any significant time, as this callback delays submission of data to the audio device, which can cause audio playback problems.

The postmix callback must be able to handle any audio data format specified in spec, which can change between callbacks if the audio device changed. However, this only covers frequency and channel count; data is always provided here in SDL_AUDIO_F32 format.

The postmix callback runs after logical device gain and audiostream gain have been applied, which is to say you can make the output data louder at this point than the gain settings would suggest.

Thread Safety

This will run from a background thread owned by SDL. The application is responsible for locking resources the callback touches that need to be protected.

Version

This datatype is available since SDL 3.1.3.