diff --git a/CMakeLists.txt b/CMakeLists.txt
index e3e7501..0ec1c1a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -44,7 +44,9 @@ function(cmark_add_compile_options target)
     endif()
     target_compile_definitions(${target} PRIVATE _CRT_SECURE_NO_WARNINGS)
   else()
-    target_compile_options(${target} PRIVATE -Wall -Wextra -pedantic)
+    target_compile_options(${target} PRIVATE
+            -Wall -Wextra -pedantic
+            $<$<COMPILE_LANGUAGE:C>:-Wstrict-prototypes>)
   endif()
   if(CMAKE_BUILD_TYPE MATCHES profile)
     target_compile_options(${target} PRIVATE -pg)
diff --git a/api_test/harness.c b/api_test/harness.c
index f6fd0bc..d27e7ca 100644
--- a/api_test/harness.c
+++ b/api_test/harness.c
@@ -5,7 +5,7 @@
 
 #include "harness.h"
 
-test_batch_runner *test_batch_runner_new() {
+test_batch_runner *test_batch_runner_new(void) {
   return (test_batch_runner *)calloc(1, sizeof(test_batch_runner));
 }
 
diff --git a/api_test/harness.h b/api_test/harness.h
index 53b9889..f352f07 100644
--- a/api_test/harness.h
+++ b/api_test/harness.h
@@ -12,7 +12,7 @@ typedef struct {
   int num_skipped;
 } test_batch_runner;
 
-test_batch_runner *test_batch_runner_new();
+test_batch_runner *test_batch_runner_new(void);
 
 void SKIP(test_batch_runner *runner, int num_tests);
 
diff --git a/api_test/main.c b/api_test/main.c
index 6d11e3d..c2db737 100644
--- a/api_test/main.c
+++ b/api_test/main.c
@@ -1067,7 +1067,7 @@ static void ref_source_pos(test_batch_runner *runner) {
   cmark_node_free(doc);
 }
 
-int main() {
+int main(void) {
   int retval;
   test_batch_runner *runner = test_batch_runner_new();
 
diff --git a/src/main.c b/src/main.c
index 0cc2b3d..e02a3c4 100644
--- a/src/main.c
+++ b/src/main.c
@@ -28,7 +28,7 @@ typedef enum {
   FORMAT_LATEX
 } writer_format;
 
-void print_usage() {
+void print_usage(void) {
   printf("Usage:   cmark [FILE*]\n");
   printf("Options:\n");
   printf("  --to, -t FORMAT  Specify output format (html, xml, man, "