SDL_SetAudioPostmixCallback(3)

💖 SDL_SetAudioPostmixCallback 💖

okay bestie, so this one lives in the SDL3 FUNCTIONS manual (the page is literally called SDL_SetAudioPostmixCallback(3), iykyk), and it's all about sneaking a lil peek at your audio right before it hits the speakers. let's get into it ✨

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🌸 NAME 🌸

SDL_SetAudioPostmixCallback — set a callback that fires when data is about to be fed to an audio device. that's it, that's the whole vibe 🎀

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🦋 SYNOPSIS 🦋

#include <SDL3/SDL_audio.h>

bool SDL_SetAudioPostmixCallback(SDL_AudioDeviceID devid, SDL_AudioPostmixCallback callback, void *userdata);

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✨ DESCRIPTION ✨

so like, this is useful for getting your hands on the final mix — think writing a visualizer, or applying one last little effect to the audio data before it goes out to play. main character moment for your audio pipeline 💅

the buffer you get is the final mix of all bound audio streams on an opened device. this callback will fire regularly for any device that is both opened and unpaused. and here's the tea: if there's no new data to mix — either because no streams are bound to the device, or all the streams are empty — this callback still fires, with the entire buffer set to silence. so no, you don't get to ghost it 🙈

also bestie, this callback is allowed to make changes to the data. the contents of the buffer after this call is what ultimately gets passed along to the hardware. that's real power, use it responsibly 🔥

the callback is always provided the data in float format (values from -1.0f to 1.0f), BUT the number of channels or sample rate may be different than the format the app requested when opening the device. why? because SDL might have had to manage a conversion behind the scenes, or the playback might have jumped to new physical hardware when a system default changed, etc. these details may change between calls. accordingly, the size of the buffer might change between calls as well. it's giving chaos, but it's fine 🌈

this callback can run at any time, and from any thread. so if you need to serialize access to your app's data, you should provide and use a mutex or other synchronization device. protect your data, protect your peace 🫶

all of this to say: there are specific needs this callback can fulfill, but it is not the simplest interface. apps should generally provide audio in their preferred format through an SDL_AudioStream and let SDL handle the difference. lowkey that's the move 😌

⚠️ this function is extremely time-sensitive. the callback should do the least amount of work possible and return as quickly as it can. the longer the callback runs, the higher the risk of audio dropouts or other problems. no cap, keep it snappy 💀

this function will block until the audio device is in between iterations, so any existing callback that might be running will finish before this function sets the new callback and returns. so polite of it honestly ✋

setting a NULL callback function disables any previously-set callback. yeet that callback into the void 🧚

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🎀 FUNCTION PARAMETERS 🎀

• devid — the ID of an opened audio device.
• callback — a callback function to be called. Can be NULL.
• userdata — app-controlled pointer passed to callback. Can be NULL.

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💜 RETURN VALUE 💜

returns true on success or false on failure — and bestie, call SDL_GetError() for more information. always check the receipts 👑

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⭐ THREAD SAFETY ⭐

it is safe to call this function from any thread. slay 💅

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🌸 AVAILABILITY 🌸

this function is available since SDL 3.2.0. so if you're on anything older... girl. upgrade 🍓

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that's the whole page babes 💖 it came from the Simple Directmedia Layer crew, out here shipping SDL 3.4.18 — and yeah, the footer date on this one is baked right into that version stamp, so consider it noted, periodt. go forth and postmix 👑