shairport-sync/audio.h

#ifndef _AUDIO_H
#define _AUDIO_H

#include <libconfig.h>
#include <stdint.h>

// clang-format off
// Play samples provided may be from the source, in which case they will be timed
// or they may be generated by Shairport Sync, in which case they will not be timed.

// Typically these would be samples of silence, which may be dithered, sent during the lead-in to 
// the start of the material, or inserted instead of a missing frame, or after a flush.
// clang-format on

typedef enum {
  play_samples_are_untimed = 0, // typically the samples are (possibly dithered) silence
  play_samples_are_timed,       // timed and numbered.
} play_samples_type;

typedef struct {
  unsigned int channels;
  unsigned int rate;
  unsigned int format; // this contains an sps_format_t enum but "common.h" is not imported here.
  char short_description[32]; // e.g. S32/44100/8.
  char channel_map[128];
} output_configuration_t;

typedef struct {
  int32_t minimum_volume_dB;
  int32_t maximum_volume_dB;
} volume_range_t;

typedef struct {
  volume_range_t *volume_range;
} output_parameters_t;

// backend interface
typedef struct {
  // may be NULL if no implemented
  void (*help)(void);
  char *name;
  // start of program
  int (*init)(int argc, char **argv);
  // at end of program
  void (*deinit)(void);

  int (*prepare)(void); // send when a play session is about to begin

  // Get an encoded configuration for the given channels, rate and depth
  // The response may have a different rate, format and channel count!
  // See FROM_ENCODED_FORMAL macros in common.h for the encoding scheme
  // An encoded configuration will always be positive.
  // 0 if not successful
  // may be NULL if not implemented
  // sps_format is not typed because the definition is in common.h, which can't be included here,
  // sigh.
  int32_t (*get_configuration)(unsigned int channels, unsigned int rate, unsigned int sps_format);

  // Set the output configuration
  // Returns 0 and a channel map (if available) if successful
  // may be NULL if not implemented.
  // Set channel_map to NULL if you don't want it.
  // Otherwise, a space-separated channel map string will be returned
  // and you are responsible for freeing it.
  // If there isn't a channel map a NULL will be returned.
  int (*configure)(int32_t encoded_output_format, char **channel_map);

  void (*start)(int sample_rate, int sample_format);

  // block of samples
  int (*play)(void *buf, int samples, int sample_type, uint32_t timestamp, uint64_t playtime);
  void (*stop)(void);

  // may be NULL if no implemented
  int (*is_running)(
      void); // if implemented, will return 0 if everything is okay, non-zero otherwise

  // may be NULL if no implemented
  void (*flush)(void);

  // returns the delay before the next frame to be sent to the device would actually be audible.
  // almost certainly wrong if the buffer is empty, so play silent buffers to make it busy.
  // will change dynamically, so keep watching it.
  // returns a negative error code if there's a problem
  int (*delay)(long *the_delay); // snd_pcm_sframes_t is a long
  int (*stats)(uint64_t *raw_measurement_time, uint64_t *corrected_measurement_time,
               uint64_t *delay,
               uint64_t *frames_sent_to_dac); // use this to get the true rate of the DAC

  // may be NULL, in which case soft volume is applied
  void (*volume)(double vol);

  // may be NULL, in which case defaults are used
  output_parameters_t *(*parameters)();

  // may be NULL, in which case software muting is used.
  // also, will return a 1 if it is actually using the mute facility, 0 otherwise
  int (*mute)(int do_mute);

} audio_output;

// this looks for a suitable configuration in the right order and checks possible
// configurations using the check_configuration method passed in.
int32_t search_for_suitable_configuration(
    unsigned int channels, unsigned int rate, unsigned int format,
    int (*check_configuration)(unsigned int channels, unsigned int rate, unsigned int format));

audio_output *audio_get_output(const char *name);
void audio_ls_outputs(void);
void parse_audio_options(const char *named_stanza, uint32_t default_format_set,
                         uint32_t default_rate_set,
                         uint32_t default_channel_set); // look in "general" and in the named stanza
uint32_t get_format_settings(const char *stanza_name, const char *setting_name);
uint32_t get_rate_settings(const char *stanza_name, const char *setting_name);
uint32_t get_channel_settings(const char *stanza_name, const char *setting_name);

#endif //_AUDIO_H