diff --git a/api/boinc_api.h b/api/boinc_api.h
index e1665f5459..d1b49bc475 100644
--- a/api/boinc_api.h
+++ b/api/boinc_api.h
@@ -111,7 +111,7 @@ extern int boinc_get_status(BOINC_STATUS*);
 extern double boinc_get_fraction_done(void);
 extern void boinc_register_timer_callback(FUNC_PTR);
 extern double boinc_worker_thread_cpu_time(void);
-extern int boinc_init_parallel();
+extern int boinc_init_parallel(void);
 extern void boinc_web_graphics_url(char*);
 extern void boinc_remote_desktop_addr(char*);
 
@@ -130,7 +130,7 @@ extern int setMacIcon(char *filename, char *iconData, long iconSize);
 
 extern int boinc_get_init_data(APP_INIT_DATA&);
 extern int boinc_wu_cpu_time(double&);
-extern double boinc_elapsed_time();
+extern double boinc_elapsed_time(void);
 extern int boinc_upload_file(std::string& name);
 extern int boinc_upload_status(std::string& name);
 extern char* boinc_msg_prefix(char*, int);
@@ -155,7 +155,7 @@ extern APP_CLIENT_SHM *app_client_shm;
 extern HANDLE worker_thread_handle;
 #endif
 extern int boinc_init_options_general(BOINC_OPTIONS& opt);
-extern int start_timer_thread();
+extern int start_timer_thread(void);
 extern bool boinc_disable_timer_thread;
 
 inline void boinc_options_defaults(BOINC_OPTIONS& b) {
diff --git a/lib/diagnostics.h b/lib/diagnostics.h
index 7424bfa3ff..efdf5215a0 100644
--- a/lib/diagnostics.h
+++ b/lib/diagnostics.h
@@ -86,7 +86,7 @@ extern int boinc_finish_diag(void);
 extern int diagnostics_init(
     int flags, const char* stdout_prefix, const char* stderr_prefix
 );
-extern int diagnostics_thread_init();
+extern int diagnostics_thread_init(void);
 extern int diagnostics_finish(void);
 extern int diagnostics_is_initialized(void);
 extern int diagnostics_is_flag_set(int flags);