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# Audio Function Reference
Introduction | Function List | Function Reference | Examples
`extern int SDL_OpenAudio(SDL_AudioSpec *desired, SDL_AudioSpec *obtained);` This function opens the audio device with the desired parameters, and returns `0` if successful, placing the actual hardware parameters in the structure pointed to by '`obtained`'. If '`obtained`' is `NULL`, the audio data passed to the callback function will be guaranteed to be in the requested format, and will be automatically converted to the hardware audio format if necessary. This function returns `-1` if it failed to open the audio device, or couldn't set up the audio thread. When filling in the desired audio spec structure, '`desired->freq`' should be the desired audio frequency in samples-per-second. '`desired->format`' should be the desired audio format. '`desired->samples`' is the desired size of the audio buffer, in samples. This number should be a power of two, and may be adjusted by the audio driver to a value more suitable for the hardware. Good values seem to range between 512 and 8096 inclusive, depending on the application and CPU speed. Smaller values yield faster response time, but can lead to underflow if the application is doing heavy processing and cannot fill the audio buffer in time. A stereo sample consists of both right and left channels in LR ordering. Note that the number of samples is directly related to time by the following formula: ms = (samples*1000)/freq '`desired->size`' is the size in bytes of the audio buffer, and is calculated by `SDL_OpenAudio()`. '`desired->silence`' is the value used to set the buffer to silence, and is calculated by `SDL_OpenAudio()`. `desired->callback`' should be set to a function that will be called when the audio device is ready for more data. It is passed a pointer to the audio buffer, and the length in bytes of the audio buffer. This function usually runs in a separate thread, and so you should protect data structures that it accesses by calling `SDL_LockAudio()` and `SDL_UnlockAudio()` in your code. `desired->userdata`' is passed as the first parameter to your callback function. The audio device starts out playing silence when it's opened, and should be enabled for playing by calling `SDL_PauseAudio(0)` when you are ready for your audio callback function to be called. Since the audio driver may modify the requested size of the audio buffer, you should allocate any local mixing buffers after you open the audio device. `extern void SDL_PauseAudio(int pause_on);` This function pauses and unpauses the audio callback processing. It should be called with a parameter of 0 after opening the audio device to start playing sound. This is so you can safely initialize data for your callback function after opening the audio device. Silence will be written to the audio device during the pause. `extern SDL_AudioSpec *SDL_LoadWAV(const char *file, SDL_AudioSpec *spec, Uint8 **audio_buf, Uint32 *audio_len);` This function loads a WAVE file into memory. If this function succeeds, it returns the given `SDL_AudioSpec`, filled with the audio data format of the wave data, and sets '`audio_buf`' to a malloc()'d buffer containing the audio data, and sets '`audio_len`' to the length of that audio buffer, in bytes. You need to free the audio buffer with `SDL_FreeWAV()` when you are done with it. This function returns `NULL` and sets the SDL error message if the wave file cannot be opened, uses an unknown data format, or is corrupt. Currently Currently raw and MS-ADPCM WAVE files are supported. `extern void SDL_FreeWAV(Uint8 *audio_buf)` This function frees data previously allocated with `SDL_LoadWAV()` `extern int SDL_BuildAudioCVT(SDL_AudioCVT *cvt, Uint16 src_format, Uint8 src_channels, int src_rate, Uint16 dst_format, Uint8 dst_channels, int dst_rate);` This function takes a source format and rate and a destination format and rate, and initializes the '`cvt`' structure with information needed by `SDL_ConvertAudio()` to convert a buffer of audio data from one format to the other. This function returns `0`, or `-1` if there was an error. `extern int SDL_ConvertAudio(SDL_AudioCVT *cvt);` Once you have initialized the '`cvt`' structure using `SDL_BuildAudioCVT()`, created an audio buffer '`cvt->buf`', and filled it with '`cvt->len`' bytes of audio data in the source format, this function will convert it in-place to the desired format. The data conversion may expand the size of the audio data, so the buffer `cvt->buf` should be allocated after the '`cvt`' structure is initialized by `SDL_BuildAudioCVT()`, and should be `cvt->len*cvt->len_mult` bytes long. `extern void SDL_MixAudio(Uint8 *dst, Uint8 *src, Uint32 len, int volume);` This takes two audio buffers of the playing audio format and mixes them, performing addition, volume adjustment, and overflow clipping. The volume ranges from 0 - 128, and should be set to SDL_MIX_MAXVOLUME for full audio volume. Note this does not change hardware volume. This is provided for convenience -- you can mix your own audio data. `extern void SDL_LockAudio(void); extern void SDL_UnlockAudio(void);` The lock manipulated by these functions protects the callback function. During a LockAudio/UnlockAudio pair, you can be guaranteed that the callback function is not running. Do not call these from the callback function or you will cause deadlock. `extern void SDL_CloseAudio(void);` This function shuts down audio processing and closes the audio device. ### Structure and format flag definitions:
    /* The calculated values in this structure are calculated by SDL_OpenAudio() */
    typedef struct {
            int freq;               /* DSP frequency -- samples per second */
            Uint16 format;          /* Audio data format */
            Uint8  channels;        /* Number of channels: 1 mono, 2 stereo */
            Uint8  silence;         /* Audio buffer silence value (calculated) */
            Uint16 samples;         /* Audio buffer size in samples */
            Uint32 size;            /* Audio buffer size in bytes (calculated) */
            /* This function is called when the audio device needs more data.
               'stream' is a pointer to the audio data buffer
               'len' is the length of that buffer in bytes.
               Once the callback returns, the buffer will no longer be valid.
               Stereo samples are stored in a LRLRLR ordering.
            */
            void (*callback)(void *userdata, Uint8 *stream, int len);
            void  *userdata;
    } SDL_AudioSpec;

    /* Audio format flags (defaults to LSB byte order) */
    #define AUDIO_U8        0x0008  /* Unsigned 8-bit samples */
    #define AUDIO_S8        0x8008  /* Signed 8-bit samples (unsupported) */
    #define AUDIO_U16LSB    0x0010  /* Unigned 16-bit samples (unsupported) */
    #define AUDIO_S16LSB    0x8010  /* Signed 16-bit samples */
    #define AUDIO_U16MSB    0x1010  /* As above, but big-endian byte order */
    #define AUDIO_S16MSB    0x9010  /* As above, but big-endian byte order */
    #define AUDIO_U16       AUDIO_U16LSB
    #define AUDIO_S16       AUDIO_S16LSB

    /* A structure to hold a set of audio conversion filters and buffers */
    typedef struct SDL_AudioCVT {
            int needed;                     /* Set to 1 if conversion possible */
            Uint16 src_format;              /* Source audio format */
            Uint16 dst_format;              /* Target audio format */
            double rate_incr;               /* Rate conversion increment */
            Uint8 *buf;                     /* Buffer to hold entire audio data */
            int    len;                     /* Length of original audio buffer */
            int    len_cvt;                 /* Length of converted audio buffer */
            int    len_mult;                /* buffer must be len*len_mult big */
            double len_ratio;       /* Given len, final size is len*len_ratio */
            void (*filters[10])(struct SDL_AudioCVT *cvt, Uint16 format);
            int filter_index;               /* Current audio conversion function */
    } SDL_AudioCVT;