diff --git a/src/arena.c b/src/arena.c
index 6373c0d01..da1a70e92 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -99,6 +99,6 @@ static void arena_free(void *ptr) {
 
 cmark_mem CMARK_ARENA_MEM_ALLOCATOR = {arena_calloc, arena_realloc, arena_free};
 
-cmark_mem *cmark_get_arena_mem_allocator() {
+cmark_mem *cmark_get_arena_mem_allocator(void) {
   return &CMARK_ARENA_MEM_ALLOCATOR;
 }
diff --git a/src/cmark-gfm.h b/src/cmark-gfm.h
index 6fb28693c..f7f7b8038 100644
--- a/src/cmark-gfm.h
+++ b/src/cmark-gfm.h
@@ -111,13 +111,13 @@ typedef struct cmark_mem {
  * realloc and free.
  */
 CMARK_GFM_EXPORT
-cmark_mem *cmark_get_default_mem_allocator();
+cmark_mem *cmark_get_default_mem_allocator(void);
 
 /** An arena allocator; uses system calloc to allocate large
  * slabs of memory.  Memory in these slabs is not reused at all.
  */
 CMARK_GFM_EXPORT
-cmark_mem *cmark_get_arena_mem_allocator();
+cmark_mem *cmark_get_arena_mem_allocator(void);
 
 /** Resets the arena allocator, quickly returning all used memory
  * to the operating system.
diff --git a/src/cmark.c b/src/cmark.c
index b3fad4b08..68c40c477 100644
--- a/src/cmark.c
+++ b/src/cmark.c
@@ -10,9 +10,9 @@
 cmark_node_type CMARK_NODE_LAST_BLOCK = CMARK_NODE_FOOTNOTE_DEFINITION;
 cmark_node_type CMARK_NODE_LAST_INLINE = CMARK_NODE_FOOTNOTE_REFERENCE;
 
-int cmark_version() { return CMARK_GFM_VERSION; }
+int cmark_version(void) { return CMARK_GFM_VERSION; }
 
-const char *cmark_version_string() { return CMARK_GFM_VERSION_STRING; }
+const char *cmark_version_string(void) { return CMARK_GFM_VERSION_STRING; }
 
 static void *xcalloc(size_t nmem, size_t size) {
   void *ptr = calloc(nmem, size);
@@ -38,7 +38,7 @@ static void xfree(void *ptr) {
 
 cmark_mem CMARK_DEFAULT_MEM_ALLOCATOR = {xcalloc, xrealloc, xfree};
 
-cmark_mem *cmark_get_default_mem_allocator() {
+cmark_mem *cmark_get_default_mem_allocator(void) {
   return &CMARK_DEFAULT_MEM_ALLOCATOR;
 }