diff --git a/src/sfizz.h b/src/sfizz.h
index 5deaea5f..343d73a0 100644
--- a/src/sfizz.h
+++ b/src/sfizz.h
@@ -5,9 +5,28 @@
 // If not, contact the sfizz maintainers at https://github.com/sfztools/sfizz
 
 /**
-  @file
-  @brief sfizz public C API.
-*/
+ * @file
+ * @brief sfizz public C API.
+ *
+ * sfizz is a synthesizer for SFZ instruments.
+ *
+ * The synthesizer must be operated under indicated constraints in order to
+ * guarantee thread-safety.
+ *
+ * At any given time, no more than 2 tasks must interact in parallel with this
+ * library:
+ * - a processing tasks @b RT for audio and MIDI, which can be real-time
+ * - a Control tasks @b CT
+ *
+ * The tasks RT and CT can be assumed by different threads over the lifetime, as
+ * long as the switch is adequately synchronized. If real-time processing is not
+ * required, it's acceptable for the 2 tasks can be assumed by a single thread.
+ *
+ * Where one or more following items are indicated on a function, the constraints apply.
+ * - @b RT: the function must be invoked from the Real-time thread
+ * - @b CT: the function must be invoked from the Control thread
+ * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
+ */
 
 #pragma once
 #include "sfizz_message.h"
@@ -84,6 +103,10 @@ SFIZZ_EXPORTED_API void sfizz_free(sfizz_synth_t* synth);
  *
  * @return @true when file loading went OK,
  *         @false if some error occured while loading.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
+ * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
  */
 SFIZZ_EXPORTED_API bool sfizz_load_file(sfizz_synth_t* synth, const char* path);
 
@@ -101,6 +124,10 @@ SFIZZ_EXPORTED_API bool sfizz_load_file(sfizz_synth_t* synth, const char* path);
  *
  * @return @true when file loading went OK,
  *         @false if some error occured while loading.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
+ * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
  */
 SFIZZ_EXPORTED_API bool sfizz_load_string(sfizz_synth_t* synth, const char* path, const char* text);
 
@@ -113,6 +140,10 @@ SFIZZ_EXPORTED_API bool sfizz_load_string(sfizz_synth_t* synth, const char* path
  *
  * @return @true when tuning scale loaded OK,
  *         @false if some error occurred.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
+ * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
  */
 SFIZZ_EXPORTED_API bool sfizz_load_scala_file(sfizz_synth_t* synth, const char* path);
 
@@ -125,6 +156,10 @@ SFIZZ_EXPORTED_API bool sfizz_load_scala_file(sfizz_synth_t* synth, const char*
  *
  * @return @true when tuning scale loaded OK,
  *         @false if some error occurred.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
+ * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
  */
 SFIZZ_EXPORTED_API bool sfizz_load_scala_string(sfizz_synth_t* synth, const char* text);
 
@@ -134,6 +169,9 @@ SFIZZ_EXPORTED_API bool sfizz_load_scala_string(sfizz_synth_t* synth, const char
  *
  * @param synth     The synth.
  * @param root_key  The MIDI number of the Scala root key (default 60 for C4).
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_set_scala_root_key(sfizz_synth_t* synth, int root_key);
 
@@ -153,6 +191,9 @@ SFIZZ_EXPORTED_API int sfizz_get_scala_root_key(sfizz_synth_t* synth);
  *
  * @param synth      The synth.
  * @param frequency  The frequency which indicates where standard tuning A4 is (default 440 Hz).
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_set_tuning_frequency(sfizz_synth_t* synth, float frequency);
 
@@ -174,6 +215,9 @@ SFIZZ_EXPORTED_API float sfizz_get_tuning_frequency(sfizz_synth_t* synth);
  *
  * @param synth  The synth.
  * @param ratio  The parameter in domain 0-1.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_load_stretch_tuning_by_ratio(sfizz_synth_t* synth, float ratio);
 
@@ -249,6 +293,10 @@ SFIZZ_EXPORTED_API int sfizz_get_num_active_voices(sfizz_synth_t* synth);
  *
  * @param synth              The synth.
  * @param samples_per_block  The number of samples per block.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
+ * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
  */
 SFIZZ_EXPORTED_API void sfizz_set_samples_per_block(sfizz_synth_t* synth, int samples_per_block);
 
@@ -260,6 +308,10 @@ SFIZZ_EXPORTED_API void sfizz_set_samples_per_block(sfizz_synth_t* synth, int sa
  *
  * @param synth        The synth
  * @param sample_rate  The sample rate.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
+ * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
  */
 SFIZZ_EXPORTED_API void sfizz_set_sample_rate(sfizz_synth_t* synth, float sample_rate);
 
@@ -274,6 +326,9 @@ SFIZZ_EXPORTED_API void sfizz_set_sample_rate(sfizz_synth_t* synth, float sample
  * @param delay        The delay of the event in the block, in samples.
  * @param note_number  The MIDI note number.
  * @param velocity     The MIDI velocity.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_send_note_on(sfizz_synth_t* synth, int delay, int note_number, char velocity);
 
@@ -290,6 +345,9 @@ SFIZZ_EXPORTED_API void sfizz_send_note_on(sfizz_synth_t* synth, int delay, int
  * @param delay        The delay of the event in the block, in samples.
  * @param note_number  The MIDI note number.
  * @param velocity     The MIDI velocity.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_send_note_off(sfizz_synth_t* synth, int delay, int note_number, char velocity);
 
@@ -304,6 +362,9 @@ SFIZZ_EXPORTED_API void sfizz_send_note_off(sfizz_synth_t* synth, int delay, int
  * @param delay      The delay of the event in the block, in samples.
  * @param cc_number  The MIDI CC number.
  * @param cc_value   The MIDI CC value.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_send_cc(sfizz_synth_t* synth, int delay, int cc_number, char cc_value);
 
@@ -318,6 +379,9 @@ SFIZZ_EXPORTED_API void sfizz_send_cc(sfizz_synth_t* synth, int delay, int cc_nu
  * @param delay       The delay of the event in the block, in samples.
  * @param cc_number   The MIDI CC number.
  * @param norm_value  The normalized CC value, in domain 0 to 1.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_send_hdcc(sfizz_synth_t* synth, int delay, int cc_number, float norm_value);
 
@@ -335,6 +399,9 @@ SFIZZ_EXPORTED_API void sfizz_send_hdcc(sfizz_synth_t* synth, int delay, int cc_
  * @param delay       The delay of the event in the block, in samples.
  * @param cc_number   The MIDI CC number.
  * @param norm_value  The normalized CC value, in domain 0 to 1.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_automate_hdcc(sfizz_synth_t* synth, int delay, int cc_number, float norm_value);
 
@@ -348,6 +415,9 @@ SFIZZ_EXPORTED_API void sfizz_automate_hdcc(sfizz_synth_t* synth, int delay, int
  * @param synth  The synth.
  * @param delay  The delay.
  * @param pitch  The pitch.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_send_pitch_wheel(sfizz_synth_t* synth, int delay, int pitch);
 
@@ -357,8 +427,11 @@ SFIZZ_EXPORTED_API void sfizz_send_pitch_wheel(sfizz_synth_t* synth, int delay,
  *
  * @param synth      The synth.
  * @param delay      The delay at which the event occurs; this should be lower
-     *               than the size of the block in the next call to renderBlock().
+ *                   than the size of the block in the next call to renderBlock().
  * @param aftertouch The aftertouch value.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_send_aftertouch(sfizz_synth_t* synth, int delay, char aftertouch);
 
@@ -369,6 +442,9 @@ SFIZZ_EXPORTED_API void sfizz_send_aftertouch(sfizz_synth_t* synth, int delay, c
  * @param synth             The synth.
  * @param delay             The delay.
  * @param seconds_per_beat  The seconds per beat.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_send_tempo(sfizz_synth_t* synth, int delay, float seconds_per_beat);
 
@@ -380,6 +456,9 @@ SFIZZ_EXPORTED_API void sfizz_send_tempo(sfizz_synth_t* synth, int delay, float
  * @param delay          The delay.
  * @param beats_per_bar  The number of beats per bar, or time signature numerator.
  * @param beat_unit      The note corresponding to one beat, or time signature denominator.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_send_time_signature(sfizz_synth_t* synth, int delay, int beats_per_bar, int beat_unit);
 
@@ -391,6 +470,9 @@ SFIZZ_EXPORTED_API void sfizz_send_time_signature(sfizz_synth_t* synth, int dela
  * @param delay     The delay.
  * @param bar       The current bar.
  * @param bar_beat  The fractional position of the current beat within the bar.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_send_time_position(sfizz_synth_t* synth, int delay, int bar, double bar_beat);
 
@@ -401,6 +483,9 @@ SFIZZ_EXPORTED_API void sfizz_send_time_position(sfizz_synth_t* synth, int delay
  * @param synth           The synth.
  * @param delay           The delay.
  * @param playback_state  The playback state, 1 if playing, 0 if stopped.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_send_playback_state(sfizz_synth_t* synth, int delay, int playback_state);
 
@@ -419,6 +504,9 @@ SFIZZ_EXPORTED_API void sfizz_send_playback_state(sfizz_synth_t* synth, int dela
  * @param num_channels  Should be equal to 2 for the time being.
  * @param num_frames    Number of frames to fill. This should be less than
  *                      or equal to the expected samples_per_block.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_render_block(sfizz_synth_t* synth, float** channels, int num_channels, int num_frames);
 
@@ -443,6 +531,10 @@ SFIZZ_EXPORTED_API unsigned int sfizz_get_preload_size(sfizz_synth_t* synth);
  *
  * @param      synth         The synth.
  * @param[in]  preload_size  The preload size.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
+ * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
  */
 SFIZZ_EXPORTED_API void sfizz_set_preload_size(sfizz_synth_t* synth, unsigned int preload_size);
 
@@ -471,16 +563,16 @@ SFIZZ_EXPORTED_API sfizz_oversampling_factor_t sfizz_get_oversampling_factor(sfi
  * the loading speed. You can tweak the size of the preloaded data to compensate
  * for the memory increase, but the full loading will need to take place anyway.
  *
- * This function takes a lock and disables the callback; prefer calling it out
- * of the RT thread. It can also take a long time to return.
- * If the new oversampling factor is the same as the current one, it will
- * release the lock immediately and exit.
  * @since 0.2.0
  *
  * @param      synth         The synth.
  * @param[in]  oversampling  The oversampling factor.
  *
  * @return @true if the oversampling factor was correct, @false otherwise.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
+ * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
  */
 SFIZZ_EXPORTED_API bool sfizz_set_oversampling_factor(sfizz_synth_t* synth, sfizz_oversampling_factor_t oversampling);
 
@@ -512,6 +604,9 @@ SFIZZ_EXPORTED_API int sfizz_get_sample_quality(sfizz_synth_t* synth, sfizz_proc
  * @param      synth    The synth.
  * @param[in]  mode     The processing mode.
  * @param[in]  quality  The desired sample quality, in the range 1 to 10.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_set_sample_quality(sfizz_synth_t* synth, sfizz_process_mode_t mode, int quality);
 
@@ -521,6 +616,9 @@ SFIZZ_EXPORTED_API void sfizz_set_sample_quality(sfizz_synth_t* synth, sfizz_pro
  *
  * @param synth   The synth.
  * @param volume  The new volume.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_set_volume(sfizz_synth_t* synth, float volume);
 
@@ -535,14 +633,14 @@ SFIZZ_EXPORTED_API float sfizz_get_volume(sfizz_synth_t* synth);
 /**
  * @brief Set the number of voices used by the synth.
  *
- * This function takes a lock and disables the callback; prefer calling
- * it out of the RT thread. It can also take a long time to return.
- * If the new number of voices is the same as the current one, it will
- * release the lock immediately and exit.
  * @since 0.2.0
  *
  * @param synth       The synth.
  * @param num_voices  The number of voices.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
+ * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
  */
 SFIZZ_EXPORTED_API void sfizz_set_num_voices(sfizz_synth_t* synth, int num_voices);
 
@@ -578,6 +676,9 @@ SFIZZ_EXPORTED_API int sfizz_get_num_bytes(sfizz_synth_t* synth);
  * @since 0.2.0
  *
  * @param synth  The synth.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_enable_freewheeling(sfizz_synth_t* synth);
 
@@ -586,6 +687,9 @@ SFIZZ_EXPORTED_API void sfizz_enable_freewheeling(sfizz_synth_t* synth);
  * @since 0.2.0
  *
  * @param synth  The synth.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_disable_freewheeling(sfizz_synth_t* synth);
 
@@ -609,7 +713,10 @@ SFIZZ_EXPORTED_API char* sfizz_get_unknown_opcodes(sfizz_synth_t* synth);
  * @param synth  The synth.
  *
  * @return @true if any included files (including the root file)
-           have been modified since the sfz file was loaded, @false otherwise.
+ *         have been modified since the sfz file was loaded, @false otherwise.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
  */
 SFIZZ_EXPORTED_API bool sfizz_should_reload_file(sfizz_synth_t* synth);
 
@@ -622,6 +729,9 @@ SFIZZ_EXPORTED_API bool sfizz_should_reload_file(sfizz_synth_t* synth);
  * @param synth  The synth.
  *
  * @return @true if the scala file has been modified since loading.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
  */
 SFIZZ_EXPORTED_API bool sfizz_should_reload_scala(sfizz_synth_t* synth);
 
@@ -632,6 +742,9 @@ SFIZZ_EXPORTED_API bool sfizz_should_reload_scala(sfizz_synth_t* synth);
  * @note This can produce many outputs so use with caution.
  *
  * @param synth  The synth.
+ *
+ * @par Thread-safety constraints
+ * - TBD ?
  */
 SFIZZ_EXPORTED_API void sfizz_enable_logging(sfizz_synth_t* synth);
 
@@ -640,6 +753,9 @@ SFIZZ_EXPORTED_API void sfizz_enable_logging(sfizz_synth_t* synth);
  * @since 0.3.0
  *
  * @param synth  The synth.
+ *
+ * @par Thread-safety constraints
+ * - TBD ?
  */
 SFIZZ_EXPORTED_API void sfizz_disable_logging(sfizz_synth_t* synth);
 
@@ -651,6 +767,9 @@ SFIZZ_EXPORTED_API void sfizz_disable_logging(sfizz_synth_t* synth);
  *
  * @param synth  The synth.
  * @param prefix The prefix.
+ *
+ * @par Thread-safety constraints
+ * - TBD ?
  */
 SFIZZ_EXPORTED_API void sfizz_set_logging_prefix(sfizz_synth_t* synth, const char* prefix);
 
@@ -659,6 +778,9 @@ SFIZZ_EXPORTED_API void sfizz_set_logging_prefix(sfizz_synth_t* synth, const cha
  * @since 0.3.2
  *
  * @param synth  The synth.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_all_sound_off(sfizz_synth_t* synth);
 
@@ -672,6 +794,9 @@ SFIZZ_EXPORTED_API void sfizz_all_sound_off(sfizz_synth_t* synth);
  * @param synth  The synth.
  * @param id     The definition variable name.
  * @param value  The definition value.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
  */
 SFIZZ_EXPORTED_API void sfizz_add_external_definitions(sfizz_synth_t* synth, const char* id, const char* value);
 
@@ -680,6 +805,9 @@ SFIZZ_EXPORTED_API void sfizz_add_external_definitions(sfizz_synth_t* synth, con
  * @since 0.4.0
  *
  * @param synth  The synth.
+ *
+ * @par Thread-safety constraints
+ * - @b CT: the function must be invoked from the Control thread
  */
 SFIZZ_EXPORTED_API void sfizz_clear_external_definitions(sfizz_synth_t* synth);
 
@@ -805,6 +933,9 @@ SFIZZ_EXPORTED_API void sfizz_set_receive_callback(sfizz_client_t* client, sfizz
  * @param path         The OSC address pattern.
  * @param sig          The OSC type tag string.
  * @param args         The OSC arguments, whose number and format is determined the type tag string.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_send_message(sfizz_synth_t* synth, sfizz_client_t* client, int delay, const char* path, const char* sig, const sfizz_arg_t* args);
 
@@ -815,6 +946,9 @@ SFIZZ_EXPORTED_API void sfizz_send_message(sfizz_synth_t* synth, sfizz_client_t*
  * @param synth        The synth.
  * @param broadcast    The pointer to the receiving function.
  * @param data         The opaque data pointer which is passed to the receiver.
+ *
+ * @par Thread-safety constraints
+ * - @b RT: the function must be invoked from the Real-time thread
  */
 SFIZZ_EXPORTED_API void sfizz_set_broadcast_callback(sfizz_synth_t* synth, sfizz_receive_t* broadcast, void* data);
 
diff --git a/src/sfizz.hpp b/src/sfizz.hpp
index e6b34daf..dfef34f1 100644
--- a/src/sfizz.hpp
+++ b/src/sfizz.hpp
@@ -31,8 +31,25 @@ namespace sfz
 class Synth;
 class Client;
 /**
- * @brief Main class.
- */
+ * @brief Synthesizer for SFZ instruments
+*
+* The synthesizer must be operated under indicated constraints in order to
+* guarantee thread-safety.
+*
+* At any given time, no more than 2 tasks must interact in parallel with this
+* library:
+* - a processing tasks @b RT for audio and MIDI, which can be real-time
+* - a Control tasks @b CT
+*
+* The tasks RT and CT can be assumed by different threads over the lifetime, as
+* long as the switch is adequately synchronized. If real-time processing is not
+* required, it's acceptable for the 2 tasks can be assumed by a single thread.
+*
+* Where one or more following items are indicated on a function, the constraints apply.
+* - @b RT: the function must be invoked from the Real-time thread
+* - @b CT: the function must be invoked from the Control thread
+* - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
+*/
 class SFIZZ_EXPORTED_API Sfizz
 {
 public:
@@ -57,15 +74,16 @@ public:
     /**
      * @brief Empties the current regions and load a new SFZ file into the synth.
      *
-     * This function will disable all callbacks so it is safe to call from a
-     * UI thread for example, although it may generate a click. However it is
-     * not reentrant, so you should not call it from concurrent threads.
      * @since 0.2.0
      *
      * @param path The path to the file to load, as string.
      *
      * @return @false if the file was not found or no regions were loaded,
      *         @true otherwise.
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
+     * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
      */
     bool loadSfzFile(const std::string& path);
 
@@ -76,6 +94,7 @@ public:
      * This accepts a virtual path name for the imaginary sfz file, which is not
      * required to exist on disk. The purpose of the virtual path is to locate
      * samples with relative paths.
+     *
      * @since 0.4.0
      *
      * @param path The virtual path of the SFZ file, as string.
@@ -83,36 +102,54 @@ public:
      *
      * @return @false if no regions were loaded,
      *         @true otherwise.
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
+     * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
      */
     bool loadSfzString(const std::string& path, const std::string& text);
 
     /**
      * @brief Sets the tuning from a Scala file loaded from the file system.
+     *
      * @since 0.4.0
      *
      * @param path The path to the file in Scala format.
      *
      * @return @true when tuning scale loaded OK,
      *         @false if some error occurred.
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
+     * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
      */
     bool loadScalaFile(const std::string& path);
 
     /**
      * @brief Sets the tuning from a Scala file loaded from memory.
+     *
      * @since 0.4.0
      *
      * @param text The contents of the file in Scala format.
      *
      * @return @true when tuning scale loaded OK,
      *         @false if some error occurred.
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
+     * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
      */
     bool loadScalaString(const std::string& text);
 
     /**
      * @brief Sets the scala root key.
+     *
      * @since 0.4.0
      *
      * @param rootKey The MIDI number of the Scala root key (default 60 for C4).
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void setScalaRootKey(int rootKey);
 
@@ -126,9 +163,13 @@ public:
 
     /**
      * @brief Sets the reference tuning frequency.
+     *
      * @since 0.4.0
      *
      * @param frequency The frequency which indicates where standard tuning A4 is (default 440 Hz).
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void setTuningFrequency(float frequency);
 
@@ -144,9 +185,13 @@ public:
      * @brief Configure stretch tuning using a predefined parametric Railsback curve.
      *
      * A ratio 1/2 is supposed to match the average piano; 0 disables (the default).
+     *
      * @since 0.4.0
      *
      * @param ratio The parameter in domain 0-1.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void loadStretchTuningByRatio(float ratio);
 
@@ -191,9 +236,14 @@ public:
      *
      * The actual size can be lower in each callback but should not be larger
      * than this value.
+     *
      * @since 0.2.0
      *
      * @param samplesPerBlock The number of samples per block.
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
+     * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
      */
     void setSamplesPerBlock(int samplesPerBlock) noexcept;
 
@@ -201,9 +251,14 @@ public:
      * @brief Set the sample rate.
      *
      * If you do not call it it is initialized to `sfz::config::defaultSampleRate`.
+     *
      * @since 0.2.0
      *
      * @param sampleRate The sample rate.
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
+     * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
      */
     void setSampleRate(float sampleRate) noexcept;
 
@@ -229,10 +284,14 @@ public:
      * does not use the opcode `sample_quality`. The engine uses distinct
      * default quality settings for live mode and freewheeling mode,
      * which both can be accessed by the means of this function.
+     *
      * @since 0.4.0
      *
      * @param[in] mode    The processing mode.
      * @param[in] quality The desired sample quality, in the range 1 to 10.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void setSampleQuality(ProcessMode mode, int quality);
 
@@ -246,53 +305,73 @@ public:
      * @brief Set the value for the volume.
      *
      * This value will be clamped within `sfz::default::volumeRange`.
+     *
      * @since 0.2.0
      *
      * @param volume The new volume.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void setVolume(float volume) noexcept;
 
     /**
      * @brief Send a note on event to the synth.
+     *
      * @since 0.2.0
      *
      * @param delay the delay at which the event occurs; this should be lower
      *              than the size of the block in the next call to renderBlock().
      * @param noteNumber the midi note number.
      * @param velocity the midi note velocity.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void noteOn(int delay, int noteNumber, uint8_t velocity) noexcept;
 
     /**
      * @brief Send a note off event to the synth.
+     *
      * @since 0.2.0
      *
      * @param delay the delay at which the event occurs; this should be lower
      *              than the size of the block in the next call to renderBlock().
      * @param noteNumber the midi note number.
      * @param velocity the midi note velocity.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void noteOff(int delay, int noteNumber, uint8_t velocity) noexcept;
 
     /**
      * @brief Send a CC event to the synth
+     *
      * @since 0.2.0
      *
      * @param delay the delay at which the event occurs; this should be lower
      *              than the size of the block in the next call to renderBlock().
      * @param ccNumber the cc number.
      * @param ccValue the cc value.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void cc(int delay, int ccNumber, uint8_t ccValue) noexcept;
 
     /**
      * @brief Send a high precision CC event to the synth
+     *
      * @since 0.4.0
      *
      * @param delay the delay at which the event occurs; this should be lower
      *              than the size of the block in the next call to renderBlock().
      * @param ccNumber the cc number.
      * @param normValue the normalized cc value, in domain 0 to 1.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void hdcc(int delay, int ccNumber, float normValue) noexcept;
 
@@ -308,65 +387,92 @@ public:
      *              than the size of the block in the next call to renderBlock().
      * @param ccNumber the cc number.
      * @param normValue the normalized cc value, in domain 0 to 1.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void automateHdcc(int delay, int ccNumber, float normValue) noexcept;
 
     /**
      * @brief Send a pitch bend event to the synth
+     *
      * @since 0.2.0
      *
      * @param delay the delay at which the event occurs; this should be lower
      *              than the size of the block in the next call to renderBlock().
      * @param pitch the pitch value centered between -8192 and 8192.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void pitchWheel(int delay, int pitch) noexcept;
 
     /**
      * @brief Send a aftertouch event to the synth. (CURRENTLY UNIMPLEMENTED)
+     *
      * @since 0.2.0
      *
      * @param delay the delay at which the event occurs; this should be lower
      *              than the size of the block in the next call to renderBlock().
      * @param aftertouch the aftertouch value.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void aftertouch(int delay, uint8_t aftertouch) noexcept;
 
     /**
      * @brief Send a tempo event to the synth.
+     *
      * @since 0.2.0
      *
      * @param delay the delay at which the event occurs; this should be lower
      *              than the size of the block in the next call to renderBlock().
      * @param secondsPerBeat the new period of the beat.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void tempo(int delay, float secondsPerBeat) noexcept;
 
     /**
      * @brief Send the time signature.
+     *
      * @since 0.5.0
      *
      * @param delay       The delay.
      * @param beatsPerBar The number of beats per bar, or time signature numerator.
      * @param beatUnit    The note corresponding to one beat, or time signature denominator.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void timeSignature(int delay, int beatsPerBar, int beatUnit);
 
     /**
      * @brief Send the time position.
+     *
      * @since 0.5.0
      *
      * @param delay   The delay.
      * @param bar     The current bar.
      * @param barBeat The fractional position of the current beat within the bar.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void timePosition(int delay, int bar, double barBeat);
 
     /**
      * @brief Send the playback state.
+     *
      * @since 0.5.0
      *
      * @param delay         The delay.
      * @param playbackState The playback state, 1 if playing, 0 if stopped.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void playbackState(int delay, int playbackState);
 
@@ -375,11 +481,15 @@ public:
      *
      * This call will reset the synth in its waiting state for the next batch
      * of events. The buffers must be float[numSamples][numOutputs * 2].
+     *
      * @since 0.2.0
      *
      * @param buffers the buffers to write the next block into.
      * @param numFrames the number of stereo frames in the block.
      * @param numOutputs the number of stereo outputs.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void renderBlock(float** buffers, size_t numFrames, int numOutputs = 1) noexcept;
 
@@ -398,13 +508,13 @@ public:
     /**
      * @brief Change the number of voices (the polyphony).
      *
-     * This function takes a lock and disables the callback; prefer calling
-     * it out of the RT thread. It can also take a long time to return.
-     * If the new number of voices is the same as the current one, it will
-     * release the lock immediately and exit.
      * @since 0.2.0
      *
      * @param numVoices The number of voices.
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
+     * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
      */
     void setNumVoices(int numVoices) noexcept;
 
@@ -424,15 +534,15 @@ public:
      * to compensate for the memory increase, but the full loading will
      * need to take place anyway.
      *
-     * This function takes a lock and disables the callback; prefer calling
-     * it out of the RT thread. It can also take a long time to return.
-     * If the new oversampling factor is the same as the current one, it will
-     * release the lock immediately and exit.
      * @since 0.2.0
      *
      * @param factor The oversampling factor.
      *
      * @return @true if the factor did indeed change, @false otherwise.
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
+     * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
      */
     bool setOversamplingFactor(int factor) noexcept;
 
@@ -445,13 +555,13 @@ public:
     /**
      * @brief Set the preloaded file size.
      *
-     * This function takes a lock and disables the callback; prefer calling
-     * it out of the RT thread. It can also take a long time to return.
-     * If the new preload size is the same as the current one, it will
-     * release the lock immediately and exit.
      * @since 0.2.0
      *
      * @param preloadSize  The preload size.
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
+     * - @b OFF: the function cannot be invoked while a thread is calling @b RT functions
      */
     void setPreloadSize(uint32_t preloadSize) noexcept;
 
@@ -478,7 +588,11 @@ public:
      *
      * This will wait for background loaded files to finish loading
      * before each render callback to ensure that there will be no dropouts.
+     *
      * @since 0.2.0
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void enableFreeWheeling() noexcept;
 
@@ -487,7 +601,11 @@ public:
      *
      * You should disable freewheeling before live use of the plugin
      * otherwise the audio thread will lock.
+     *
      * @since 0.2.0
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void disableFreeWheeling() noexcept;
 
@@ -495,10 +613,14 @@ public:
      * @brief Check if the SFZ should be reloaded.
      *
      * Depending on the platform this can create file descriptors.
+     *
      * @since 0.2.0
      *
      * @return @true if any included files (including the root file) have
      *         been modified since the sfz file was loaded, @false otherwise.
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
      */
     bool shouldReloadFile();
 
@@ -506,9 +628,13 @@ public:
      * @brief Check if the tuning (scala) file should be reloaded.
      *
      * Depending on the platform this can create file descriptors.
+     *
      * @since 0.4.0
      *
      * @return @true if a scala file has been loaded and has changed, @false otherwise.
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
      */
     bool shouldReloadScala();
 
@@ -519,41 +645,61 @@ public:
      * @note This can produce many outputs so use with caution.
      *
      * @param prefix the file prefix to use for logging.
+     *
+     * @par Thread-safety constraints
+     * - TBD ?
      */
     void enableLogging() noexcept;
 
     /**
      * @brief Enable logging of timings to sidecar CSV files.
+     *
      * @since 0.3.2
      *
      * @note This can produce many outputs so use with caution.
      *
      * @param prefix the file prefix to use for logging.
+     *
+     * @par Thread-safety constraints
+     * - TBD ?
      */
     void enableLogging(const std::string& prefix) noexcept;
 
     /**
      * @brief Set the logging prefix.
+     *
      * @since 0.3.2
      *
      * @param prefix
+     *
+     * @par Thread-safety constraints
+     * - TBD ?
      */
     void setLoggingPrefix(const std::string& prefix) noexcept;
 
     /**
      * @brief Disable logging of timings to sidecar CSV files.
+     *
      * @since 0.3.0
+     *
+     * @par Thread-safety constraints
+     * - TBD ?
      */
     void disableLogging() noexcept;
 
     /**
      * @brief Shuts down the current processing, clear buffers and reset the voices.
+     *
      * @since 0.3.2
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void allSoundOff() noexcept;
 
     /**
      * @brief Add external definitions prior to loading.
+     *
      * @since 0.4.0
      *
      * @note These do not get reset by loading or resetting the synth.
@@ -561,12 +707,19 @@ public:
      *
      * @param id    The definition variable name.
      * @param value The definition value.
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
      */
     void addExternalDefinition(const std::string& id, const std::string& value);
 
     /**
      * @brief Clears external definitions for the next file loading.
+     *
      * @since 0.4.0
+     *
+     * @par Thread-safety constraints
+     * - @b CT: the function must be invoked from the Control thread
      */
     void clearExternalDefinitions();
 
@@ -624,6 +777,7 @@ public:
 
     /**
      * @brief Send a message to the synth engine
+     *
      * @since 0.6.0
      *
      * @param client       The client sending the message.
@@ -631,15 +785,22 @@ public:
      * @param path         The OSC address pattern.
      * @param sig          The OSC type tag string.
      * @param args         The OSC arguments, whose number and format is determined the type tag string.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void sendMessage(Client& client, int delay, const char* path, const char* sig, const sfizz_arg_t* args);
 
     /**
      * @brief Set the function which receives broadcast messages from the synth engine.
+     *
      * @since 0.6.0
      *
      * @param broadcast    The pointer to the receiving function.
      * @param data         The opaque data pointer which is passed to the receiver.
+     *
+     * @par Thread-safety constraints
+     * - @b RT: the function must be invoked from the Real-time thread
      */
     void setBroadcastCallback(sfizz_receive_t* broadcast, void* data);