Idea for FluidSynth internal effects API

fluid_freeverb.c

#include "fluid_fx"
#include "fluid_freverb.h"

fluid_fx_handle_t *freeverb_instantiate(fluid_fx_descriptor_t *desc, fluid_real_t sample_rate)
{
    // create freeverb instance
}

void freeverb_cleanup(fluid_fx_handle_t *fx_handle)
{
  // cleanup freeverb instance
}

void freeverb_run(fluid_fx_handle_t *fx_handle,
        fluid_real_t *left_in, fluid_real_t *right_in,
        fluid_real_t *left_out, fluid_real_t *right_out,
        int block_count, int block_size, int mix_output)
{
  // run freeverb
}

void freeverb_reset(fluid_fx_handle_t *fx_handle)
{
  // reset freverb
}

void freeverb_set_sample_rate(fluid_fx_handle_t *fx_handle, fluid_real_t sample_rate)
{
  // update sample rate
}

fluid_fx_descriptor_t fluid_reverb_fx_freeverb = {
    .name = "freeverb",
    .description = "The default reverb implementation of FluidSynth",
    .type = FLUID_FX_REVERB,
    .instantiate = freeverb_instantiate,
    .cleanup = freeverb_cleanup,
    .run = freeverb_run,
    .reset = freeverb_reset,
    .set_sample_rate = freeverb_set_sample_rate,

    .set_param = fluid_fx_default_set_param,
    .get_param = fluid_fx_default_get_param,
    .has_param = fluid_fx_default_has_param,

    .params = {
        {
            .name = "roomsize",
            .value = 0.0f,
        },
        {
            .name = "damping",
            .value = 0.0f,
        },
        {
            .name = "width",
            .value = 0.0f,
        },
        {
            .name = "level",
            .value = 0.0f,
        }
    },
};

fluid_fx.c

#include "fluid_fx.h"

void fluid_fx_default_set_param(fluid_fx_handle_t *fx_handle, const char *name, fluid_real_t value)
{
    // default implementation to set a named parameter
}

fluid_real_t fluid_fx_default_get_param(fluid_fx_handle_t *fx_handle, const char *name)
{
    // default implementation to get a named parameter
}

int fluid_fx_default_has_param(fluid_fx_handle_t *fx_handle, const char *name)
{
    // default implementation to check if a named parameter exists
}

fluid_fx.h

typedef enum _fluid_fx_type_t
{
    FLUID_FX_REVERB = 0,
    FLUID_FX_CHORUS

} fluid_fx_type_t;

typedef struct _fluid_fx_param_t
{
    const char *name;
    fluid_real_t value;
    const char *description;

} fluid_fx_param_t;

typedef _fluid_fx_handle_t
{
    /* Descriptor of the effect (see below) */
    fluid_fx_descriptor_t *descriptor;
  
    /* Private data used for the effect */
    void *private;
    
} fluid_fx_handle_t;

typedef struct _fluid_fx_descriptor_t
{
    /* The name of the effect, used to select it from the command line via API */
    char *name;

    /* Description of the effect */
    char *description;

    /* The type of the effect, could also determine in which channel to write the output
     *to keep the fixed reverb = 0, chorus = 1 mapping */
    fluid_fx_type_t type;

    /* Creates a new instance of the effect */
    fluid_fx_handle_t *(*instantiate)(struct _fluid_fx_descriptor_t *desc, fluid_real_t sample_rate);

    /* Delete an effect instance */
    void (*cleanup)(fluid_fx_handle_t *fx_handle);


    /* Process one or more blocks of audio and either mix into or overwrite the output buffers */
    void (*run)(fluid_fx_handle_t *fx_handle,
                fluid_real_t *left_in, fluid_real_t *right_in,
                fluid_real_t *left_out, fluid_real_t *right_out,
                int block_count, int block_size,
                int mix_output);

    /* Reset the effects, clearing all internal state and buffers */
    void (*reset)(fluid_fx_handle_t *fx_handle);

    /* Update the sample rate */
    void (*set_sample_rate)(fluid_fx_handle_t *fx_handle, fluid_real_t sample_rate);

    /* Set a named parameter */
    void (*set_param)(fluid_fx_handle_t *fx_handle, const char *name, fluid_real_t value);

    /* Get the value of a named parameter */
    fluid_real_t (*get_param)(fluid_fx_handle_t *fx_handle, const char *name);

    /* Check if the named parameter is available */
    int (*has_param)(fluid_fx_handle_t *fx_handle, const char *name);    
    
      /* Not part of the public API, should be accessed via *_param functions on the handle */
    fluid_fx_param_t params[];
    
} fluid_fx_descriptor_t;


void fluid_fx_default_set_param(fluid_fx_handle_t *fx_handle, const char *name, fluid_real_t value);
fluid_real_t fluid_fx_default_get_param(fluid_fx_handle_t *fx_handle, const char *name);
int fluid_fx_default_has_param(fluid_fx_handle_t *fx_handle, const char *name);