diff --git a/configure.ac b/configure.ac
index d0d80243..feb6046f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -275,6 +275,7 @@ dnl
 dnl Reference: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
 dnl ================================================================
 
+AX_CHECK_COMPILE_FLAG([-Wmissing-prototypes],    [CFLAGS="-Wmissing-prototypes $CFLAGS"])
 AX_CHECK_COMPILE_FLAG([-Wreturn-type],           [CFLAGS="-Wreturn-type $CFLAGS"])
 AX_CHECK_COMPILE_FLAG([-Wunused],                [CFLAGS="-Wunused $CFLAGS"])
 AX_CHECK_COMPILE_FLAG([-Wunused-parameter],      [CFLAGS="-Wunused-parameter $CFLAGS"])
diff --git a/tests/api.c b/tests/api.c
index b017e6ec..985b2543 100644
--- a/tests/api.c
+++ b/tests/api.c
@@ -34,7 +34,7 @@
 
 // region espeak_Initialize
 
-void
+static void
 test_espeak_terminate_without_initialize()
 {
 	printf("testing espeak_Terminate without espeak_Initialize\n");
@@ -49,7 +49,7 @@ test_espeak_terminate_without_initialize()
 	assert(p_decoder == NULL);
 }
 
-void
+static void
 test_espeak_initialize()
 {
 	printf("testing espeak_Initialize\n");
@@ -72,7 +72,7 @@ test_espeak_initialize()
 // endregion
 // region espeak_Synth
 
-void
+static void
 test_espeak_synth()
 {
 	printf("testing espeak_Synth\n");
@@ -103,7 +103,7 @@ test_espeak_synth()
 	assert(p_decoder == NULL);
 }
 
-void
+static void
 test_espeak_synth_no_voices(const char *path)
 {
 	printf("testing espeak_Synth in path with no voices\n");
@@ -135,7 +135,7 @@ test_espeak_synth_no_voices(const char *path)
 // endregion
 // region espeak_ng_Synthesize
 
-void
+static void
 test_espeak_ng_synthesize()
 {
 	printf("testing espeak_ng_Synthesize\n");
@@ -166,7 +166,7 @@ test_espeak_ng_synthesize()
 	assert(p_decoder == NULL);
 }
 
-void
+static void
 test_espeak_ng_synthesize_no_voices(const char *path)
 {
 	printf("testing espeak_ng_Synthesize in path with no voices\n");
@@ -198,7 +198,7 @@ test_espeak_ng_synthesize_no_voices(const char *path)
 // endregion
 // region espeak_SetVoiceByName
 
-void
+static void
 test_espeak_set_voice_by_name_null_voice()
 {
 	printf("testing espeak_SetVoiceByName(NULL)\n");
@@ -233,7 +233,7 @@ test_espeak_set_voice_by_name_null_voice()
 	assert(p_decoder == NULL);
 }
 
-void
+static void
 test_espeak_set_voice_by_name_blank_voice()
 {
 	printf("testing espeak_SetVoiceByName(\"\")\n");
@@ -268,7 +268,7 @@ test_espeak_set_voice_by_name_blank_voice()
 	assert(p_decoder == NULL);
 }
 
-void
+static void
 test_espeak_set_voice_by_name_valid_voice()
 {
 	printf("testing espeak_SetVoiceByName(\"de\")\n");
@@ -304,7 +304,7 @@ test_espeak_set_voice_by_name_valid_voice()
 	assert(p_decoder == NULL);
 }
 
-void
+static void
 test_espeak_set_voice_by_name_invalid_voice()
 {
 	printf("testing espeak_SetVoiceByName(\"zzz\")\n");
@@ -339,7 +339,7 @@ test_espeak_set_voice_by_name_invalid_voice()
 	assert(p_decoder == NULL);
 }
 
-void
+static void
 test_espeak_set_voice_by_name_language_variant_intonation_parameter()
 {
 	printf("testing espeak_SetVoiceByName(\"!v/Annie\") (language variant; intonation)\n");
@@ -378,7 +378,7 @@ test_espeak_set_voice_by_name_language_variant_intonation_parameter()
 // endregion
 // region espeak_SetVoiceByProperties
 
-void
+static void
 test_espeak_set_voice_by_properties_empty()
 {
 	printf("testing espeak_SetVoiceByProperties: (none)\n");
@@ -417,7 +417,7 @@ test_espeak_set_voice_by_properties_empty()
 	assert(p_decoder == NULL);
 }
 
-void
+static void
 test_espeak_set_voice_by_properties_blank_language()
 {
 	printf("testing espeak_SetVoiceByProperties: languages=\"\"\n");
@@ -457,7 +457,7 @@ test_espeak_set_voice_by_properties_blank_language()
 	assert(p_decoder == NULL);
 }
 
-void
+static void
 test_espeak_set_voice_by_properties_with_valid_language()
 {
 	printf("testing espeak_SetVoiceByProperties: languages=\"mk\" (valid)\n");
@@ -497,7 +497,7 @@ test_espeak_set_voice_by_properties_with_valid_language()
 	assert(p_decoder == NULL);
 }
 
-void
+static void
 test_espeak_set_voice_by_properties_with_invalid_language()
 {
 	printf("testing espeak_SetVoiceByProperties: languages=\"zzz\" (invalid)\n");
diff --git a/tests/encoding.c b/tests/encoding.c
index 52dffe2e..6ec97a84 100644
--- a/tests/encoding.c
+++ b/tests/encoding.c
@@ -26,7 +26,7 @@
 #include <espeak-ng/espeak_ng.h>
 #include <espeak-ng/encoding.h>
 
-void
+static void
 test_unbound_text_decoder()
 {
 	printf("testing unbound text decoder\n");
@@ -39,7 +39,7 @@ test_unbound_text_decoder()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_unknown_encoding()
 {
 	printf("testing unknown encodings\n");
@@ -57,7 +57,7 @@ test_unknown_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_us_ascii_encoding()
 {
 	printf("testing US-ASCII encoding\n");
@@ -91,7 +91,7 @@ test_us_ascii_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_koi8_r_encoding()
 {
 	printf("testing KOI8-R encoding\n");
@@ -117,7 +117,7 @@ test_koi8_r_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iscii_encoding()
 {
 	printf("testing ISCII encoding\n");
@@ -142,7 +142,7 @@ test_iscii_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_1_encoding()
 {
 	printf("testing ISO-8859-1 encoding\n");
@@ -175,7 +175,7 @@ test_iso_8859_1_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_2_encoding()
 {
 	printf("testing ISO-8859-2 encoding\n");
@@ -206,7 +206,7 @@ test_iso_8859_2_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_3_encoding()
 {
 	printf("testing ISO-8859-3 encoding\n");
@@ -237,7 +237,7 @@ test_iso_8859_3_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_4_encoding()
 {
 	printf("testing ISO-8859-4 encoding\n");
@@ -268,7 +268,7 @@ test_iso_8859_4_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_5_encoding()
 {
 	printf("testing ISO-8859-5 encoding\n");
@@ -298,7 +298,7 @@ test_iso_8859_5_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_6_encoding()
 {
 	printf("testing ISO-8859-6 encoding\n");
@@ -330,7 +330,7 @@ test_iso_8859_6_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_7_encoding()
 {
 	printf("testing ISO-8859-7 encoding\n");
@@ -363,7 +363,7 @@ test_iso_8859_7_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_8_encoding()
 {
 	printf("testing ISO-8859-8 encoding\n");
@@ -393,7 +393,7 @@ test_iso_8859_8_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_9_encoding()
 {
 	printf("testing ISO-8859-9 encoding\n");
@@ -424,7 +424,7 @@ test_iso_8859_9_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_10_encoding()
 {
 	printf("testing ISO-8859-10 encoding\n");
@@ -455,7 +455,7 @@ test_iso_8859_10_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_11_encoding()
 {
 	printf("testing ISO-8859-11 encoding\n");
@@ -482,7 +482,7 @@ test_iso_8859_11_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_13_encoding()
 {
 	printf("testing ISO-8859-13 encoding\n");
@@ -508,7 +508,7 @@ test_iso_8859_13_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_14_encoding()
 {
 	printf("testing ISO-8859-14 encoding\n");
@@ -540,7 +540,7 @@ test_iso_8859_14_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_15_encoding()
 {
 	printf("testing ISO-8859-15 encoding\n");
@@ -568,7 +568,7 @@ test_iso_8859_15_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_8859_16_encoding()
 {
 	printf("testing ISO-8859-16 encoding\n");
@@ -599,7 +599,7 @@ test_iso_8859_16_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_utf_8_encoding()
 {
 	printf("testing UTF-8 encoding\n");
@@ -702,7 +702,7 @@ test_utf_8_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_iso_10646_ucs_2_encoding()
 {
 	printf("testing ISO-10646-UCS-2 encoding\n");
@@ -728,7 +728,7 @@ test_iso_10646_ucs_2_encoding()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_char_decoder()
 {
 	printf("testing char decoder\n");
@@ -754,7 +754,7 @@ test_char_decoder()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_wchar_decoder()
 {
 	printf("testing wchar_t decoder\n");
@@ -796,7 +796,7 @@ test_wchar_decoder()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_auto_decoder()
 {
 	printf("testing auto decoder (UTF-8 + codepage-based fallback)\n");
@@ -846,7 +846,7 @@ test_auto_decoder()
 	destroy_text_decoder(decoder);
 }
 
-void
+static void
 test_peekc()
 {
 	printf("testing peekc\n");
diff --git a/tests/readclause.c b/tests/readclause.c
index 8b465118..673f28d1 100644
--- a/tests/readclause.c
+++ b/tests/readclause.c
@@ -57,7 +57,7 @@ set_text(const char *text, const char *voicename)
 	return text_decoder_decode_string(p_decoder, text, -1, ESPEAKNG_ENCODING_UTF_8);
 }
 
-void
+static void
 test_latin()
 {
 	printf("testing Latin (Latn)\n");
@@ -76,7 +76,7 @@ test_latin()
 	assert(clause_type_from_codepoint(0x2026) == (CLAUSE_SEMICOLON | CLAUSE_SPEAK_PUNCTUATION_NAME | CLAUSE_OPTIONAL_SPACE_AFTER));
 }
 
-void
+static void
 test_latin_sentence()
 {
 	printf("testing Latin (Latn) ... sentence\n");
@@ -108,7 +108,7 @@ test_latin_sentence()
 	assert(charix_top == 0);
 }
 
-void
+static void
 test_greek()
 {
 	printf("testing Greek (Grek)\n");
@@ -117,7 +117,7 @@ test_greek()
 	assert(clause_type_from_codepoint(0x0387) == CLAUSE_SEMICOLON);
 }
 
-void
+static void
 test_armenian()
 {
 	printf("testing Armenian (Armn)\n");
@@ -129,7 +129,7 @@ test_armenian()
 	assert(clause_type_from_codepoint(0x0589) == (CLAUSE_PERIOD | CLAUSE_OPTIONAL_SPACE_AFTER));
 }
 
-void
+static void
 test_arabic()
 {
 	printf("testing Arabic (Arab)\n");
@@ -140,7 +140,7 @@ test_arabic()
 	assert(clause_type_from_codepoint(0x06D4) == CLAUSE_PERIOD);
 }
 
-void
+static void
 test_devanagari()
 {
 	printf("testing Devanagari (Deva)\n");
@@ -148,7 +148,7 @@ test_devanagari()
 	assert(clause_type_from_codepoint(0x0964) == (CLAUSE_PERIOD | CLAUSE_OPTIONAL_SPACE_AFTER));
 }
 
-void
+static void
 test_tibetan()
 {
 	printf("testing Tibetan (Tibt)\n");
@@ -157,7 +157,7 @@ test_tibetan()
 	assert(clause_type_from_codepoint(0x0F0E) == CLAUSE_PARAGRAPH);
 }
 
-void
+static void
 test_sinhala()
 {
 	printf("testing Sinhala (Sinh)\n");
@@ -165,7 +165,7 @@ test_sinhala()
 	assert(clause_type_from_codepoint(0x0DF4) == (CLAUSE_PERIOD | CLAUSE_OPTIONAL_SPACE_AFTER));
 }
 
-void
+static void
 test_georgian()
 {
 	printf("testing Georgian (Geor)\n");
@@ -173,7 +173,7 @@ test_georgian()
 	assert(clause_type_from_codepoint(0x10FB) == CLAUSE_PARAGRAPH);
 }
 
-void
+static void
 test_ethiopic()
 {
 	printf("testing Ethiopic (Ethi)\n");
@@ -187,7 +187,7 @@ test_ethiopic()
 	assert(clause_type_from_codepoint(0x1368) == CLAUSE_PARAGRAPH);
 }
 
-void
+static void
 test_ideographic()
 {
 	printf("testing Ideographic (Hani)\n");
@@ -196,7 +196,7 @@ test_ideographic()
 	assert(clause_type_from_codepoint(0x3002) == (CLAUSE_PERIOD | CLAUSE_OPTIONAL_SPACE_AFTER));
 }
 
-void
+static void
 test_fullwidth()
 {
 	printf("testing Full Width\n");
@@ -209,7 +209,7 @@ test_fullwidth()
 	assert(clause_type_from_codepoint(0xFF1F) == (CLAUSE_QUESTION | CLAUSE_OPTIONAL_SPACE_AFTER));
 }
 
-void
+static void
 test_uts51_emoji_character()
 {
 	printf("testing Emoji ... UTS-51 ED-3. emoji character\n");
@@ -245,7 +245,7 @@ test_uts51_emoji_character()
 	assert(voice_change_name[0] == 0);
 }
 
-void
+static void
 test_uts51_text_presentation_sequence()
 {
 	printf("testing Emoji ... UTS-51 ED-8a. text presentation sequence\n");
@@ -278,7 +278,7 @@ test_uts51_text_presentation_sequence()
 	assert(voice_change_name[0] == 0);
 }
 
-void
+static void
 test_uts51_emoji_presentation_sequence()
 {
 	printf("testing Emoji ... UTS-51 ED-9a. emoji presentation sequence\n");
@@ -311,7 +311,7 @@ test_uts51_emoji_presentation_sequence()
 	assert(voice_change_name[0] == 0);
 }
 
-void
+static void
 test_uts51_emoji_modifier_sequence()
 {
 	printf("testing Emoji ... UTS-51 ED-13. emoji modifier sequence\n");
@@ -341,7 +341,7 @@ test_uts51_emoji_modifier_sequence()
 	assert(voice_change_name[0] == 0);
 }
 
-void
+static void
 test_uts51_emoji_flag_sequence()
 {
 	printf("testing Emoji ... UTS-51 ED-14. emoji flag sequence\n");
@@ -374,7 +374,7 @@ test_uts51_emoji_flag_sequence()
 	assert(voice_change_name[0] == 0);
 }
 
-void
+static void
 test_uts51_emoji_tag_sequence_emoji_character()
 {
 	printf("testing Emoji ... UTS-51 ED-14a. emoji tag sequence (emoji character)\n");
@@ -450,7 +450,7 @@ test_uts51_emoji_tag_sequence_emoji_character()
 	assert(voice_change_name[0] == 0);
 }
 
-void
+static void
 test_uts51_emoji_combining_sequence()
 {
 	printf("testing Emoji ... UTS-51 ED-14b. emoji combining sequence\n");
@@ -480,7 +480,7 @@ test_uts51_emoji_combining_sequence()
 	assert(voice_change_name[0] == 0);
 }
 
-void
+static void
 test_uts51_emoji_keycap_sequence()
 {
 	printf("testing Emoji ... UTS-51 ED-14c. emoji keycap sequence\n");