diff --git a/src/ffi.rs b/src/ffi.rs
index a8dd212..077517c 100644
--- a/src/ffi.rs
+++ b/src/ffi.rs
@@ -81,7 +81,7 @@ pub unsafe extern "C" fn tts_free(tts: *mut TTS) {
 /// Returns the features supported by this TTS engine.
 /// `tts` must be a valid pointer to a TTS object.
 #[no_mangle]
-pub unsafe extern "C" fn tts_supported_features(tts: *mut TTS) -> Features {
+pub unsafe extern "C" fn tts_supported_features(tts: *const TTS) -> Features {
     tts.as_ref().unwrap().supported_features()
 }
 
@@ -97,7 +97,7 @@ pub unsafe extern "C" fn tts_speak(
     utterance: *mut *mut UtteranceId,
 ) -> bool {
     if tts.is_null() {
-        return true;
+        return false;
     }
     let text = CStr::from_ptr(text).to_string_lossy().into_owned();
     match tts.as_mut().unwrap().speak(text, interrupt) {
@@ -126,6 +126,7 @@ pub unsafe extern "C" fn tts_free_utterance(utterance: *mut UtteranceId) {
     }
     Box::from_raw(utterance);
 }
+
 /// Stops current speech.
 /// Returns true on success, false on error or if `tts` is NULL.
 #[no_mangle]
@@ -145,21 +146,21 @@ pub unsafe extern "C" fn tts_stop(tts: *mut TTS) -> bool {
 /// Returns the minimum rate for this speech synthesizer.
 /// `tts` must be a valid pointer to a TTS object.
 #[no_mangle]
-pub unsafe extern "C" fn tts_min_rate(tts: *mut TTS) -> c_float {
+pub unsafe extern "C" fn tts_min_rate(tts: *const TTS) -> c_float {
     tts.as_ref().unwrap().min_rate()
 }
 
 /// Returns the maximum rate for this speech synthesizer.
 /// `tts` must be a valid pointer to a TTS object.
 #[no_mangle]
-pub unsafe extern "C" fn tts_max_rate(tts: *mut TTS) -> c_float {
+pub unsafe extern "C" fn tts_max_rate(tts: *const TTS) -> c_float {
     tts.as_ref().unwrap().max_rate()
 }
 
 /// Returns the normal rate for this speech synthesizer.
 /// `tts` must be a valid pointer to a TTS object.
 #[no_mangle]
-pub unsafe extern "C" fn tts_normal_rate(tts: *mut TTS) -> c_float {
+pub unsafe extern "C" fn tts_normal_rate(tts: *const TTS) -> c_float {
     tts.as_ref().unwrap().normal_rate()
 }
 
@@ -168,7 +169,7 @@ pub unsafe extern "C" fn tts_normal_rate(tts: *mut TTS) -> c_float {
 /// or if `tts` is NULL.
 /// Does nothing if `rate` is NULL.
 #[no_mangle]
-pub unsafe extern "C" fn tts_get_rate(tts: *mut TTS, rate: *mut c_float) -> bool {
+pub unsafe extern "C" fn tts_get_rate(tts: *const TTS, rate: *mut c_float) -> bool {
     if tts.is_null() {
         return false;
     }
@@ -206,21 +207,21 @@ pub unsafe extern "C" fn tts_set_rate(tts: *mut TTS, rate: c_float) -> bool {
 /// Returns the minimum pitch for this speech synthesizer.
 /// `tts` must be a valid pointer to a TTS object.
 #[no_mangle]
-pub unsafe extern "C" fn tts_min_pitch(tts: *mut TTS) -> c_float {
+pub unsafe extern "C" fn tts_min_pitch(tts: *const TTS) -> c_float {
     tts.as_ref().unwrap().min_pitch()
 }
 
 /// Returns the maximum pitch for this speech synthesizer.
 /// `tts` must be a valid pointer to a TTS object.
 #[no_mangle]
-pub unsafe extern "C" fn tts_max_pitch(tts: *mut TTS) -> c_float {
+pub unsafe extern "C" fn tts_max_pitch(tts: *const TTS) -> c_float {
     tts.as_ref().unwrap().max_pitch()
 }
 
 /// Returns the normal pitch for this speech synthesizer.
 /// `tts` must be a valid pointer to a TTS object.
 #[no_mangle]
-pub unsafe extern "C" fn tts_normal_pitch(tts: *mut TTS) -> c_float {
+pub unsafe extern "C" fn tts_normal_pitch(tts: *const TTS) -> c_float {
     tts.as_ref().unwrap().normal_pitch()
 }
 
@@ -229,7 +230,7 @@ pub unsafe extern "C" fn tts_normal_pitch(tts: *mut TTS) -> c_float {
 /// or if `tts` is NULL.
 /// Does nothing if `pitch` is NULL.
 #[no_mangle]
-pub unsafe extern "C" fn tts_get_pitch(tts: *mut TTS, pitch: *mut c_float) -> bool {
+pub unsafe extern "C" fn tts_get_pitch(tts: *const TTS, pitch: *mut c_float) -> bool {
     if tts.is_null() {
         return false;
     }
@@ -267,21 +268,21 @@ pub unsafe extern "C" fn tts_set_pitch(tts: *mut TTS, pitch: c_float) -> bool {
 /// Returns the minimum volume for this speech synthesizer.
 /// `tts` must be a valid pointer to a TTS object.
 #[no_mangle]
-pub unsafe extern "C" fn tts_min_volume(tts: *mut TTS) -> c_float {
+pub unsafe extern "C" fn tts_min_volume(tts: *const TTS) -> c_float {
     tts.as_ref().unwrap().min_volume()
 }
 
 /// Returns the maximum volume for this speech synthesizer.
 /// `tts` must be a valid pointer to a TTS object.
 #[no_mangle]
-pub unsafe extern "C" fn tts_max_volume(tts: *mut TTS) -> c_float {
+pub unsafe extern "C" fn tts_max_volume(tts: *const TTS) -> c_float {
     tts.as_ref().unwrap().max_volume()
 }
 
 /// Returns the normal volume for this speech synthesizer.
 /// `tts` must be a valid pointer to a TTS object.
 #[no_mangle]
-pub unsafe extern "C" fn tts_normal_volume(tts: *mut TTS) -> c_float {
+pub unsafe extern "C" fn tts_normal_volume(tts: *const TTS) -> c_float {
     tts.as_ref().unwrap().normal_volume()
 }
 
@@ -290,7 +291,7 @@ pub unsafe extern "C" fn tts_normal_volume(tts: *mut TTS) -> c_float {
 /// or if `tts` is NULL.
 /// Does nothing if `volume` is NULL.
 #[no_mangle]
-pub unsafe extern "C" fn tts_get_volume(tts: *mut TTS, volume: *mut c_float) -> bool {
+pub unsafe extern "C" fn tts_get_volume(tts: *const TTS, volume: *mut c_float) -> bool {
     if tts.is_null() {
         return false;
     }
@@ -331,7 +332,7 @@ pub unsafe extern "C" fn tts_set_volume(tts: *mut TTS, volume: c_float) -> bool
 /// If `speaking` is NULL, returns this value instead, meaning you can't tell the difference
 /// between an error occuring and the synth not speaking.
 #[no_mangle]
-pub unsafe extern "C" fn tts_is_speaking(tts: *mut TTS, speaking: *mut bool) -> bool {
+pub unsafe extern "C" fn tts_is_speaking(tts: *const TTS, speaking: *mut bool) -> bool {
     if tts.is_null() {
         return false;
     }