diff --git a/api/boinc_api.h b/api/boinc_api.h
index 9926b14ed8..e1665f5459 100644
--- a/api/boinc_api.h
+++ b/api/boinc_api.h
@@ -76,7 +76,7 @@ typedef struct BOINC_STATUS {
 
 extern volatile BOINC_STATUS boinc_status;
 
-typedef void (*FUNC_PTR)();
+typedef void (*FUNC_PTR)(void);
 
 struct APP_INIT_DATA;
 
@@ -93,13 +93,13 @@ extern int boinc_resume_other_activities(void);
 extern int boinc_report_app_status(
     double cpu_time, double checkpoint_cpu_time, double _fraction_done
 );
-extern int boinc_time_to_checkpoint();
-extern void boinc_begin_critical_section();
-extern int boinc_try_critical_section();
-extern void boinc_end_critical_section();
-extern void boinc_need_network();
-extern int boinc_network_poll();
-extern void boinc_network_done();
+extern int boinc_time_to_checkpoint(void);
+extern void boinc_begin_critical_section(void);
+extern int boinc_try_critical_section(void);
+extern void boinc_end_critical_section(void);
+extern void boinc_need_network(void);
+extern int boinc_network_poll(void);
+extern void boinc_network_done(void);
 extern void boinc_network_usage(double sent, double received);
 extern int boinc_is_standalone(void);
 extern void boinc_ops_per_cpu_sec(double fp, double integer);
@@ -108,9 +108,9 @@ extern void boinc_set_credit_claim(double credit);
 extern int boinc_receive_trickle_down(char* buf, int len);
 extern int boinc_init_options(BOINC_OPTIONS*);
 extern int boinc_get_status(BOINC_STATUS*);
-extern double boinc_get_fraction_done();
+extern double boinc_get_fraction_done(void);
 extern void boinc_register_timer_callback(FUNC_PTR);
-extern double boinc_worker_thread_cpu_time();
+extern double boinc_worker_thread_cpu_time(void);
 extern int boinc_init_parallel();
 extern void boinc_web_graphics_url(char*);
 extern void boinc_remote_desktop_addr(char*);
diff --git a/api/graphics_api.h b/api/graphics_api.h
index 6ef267b21a..314bbea57f 100644
--- a/api/graphics_api.h
+++ b/api/graphics_api.h
@@ -22,7 +22,7 @@
 extern "C" {
 #endif
 
-typedef void (*WORKER_FUNC_PTR)();
+typedef void (*WORKER_FUNC_PTR)(void);
 
 extern int boinc_init_graphics(WORKER_FUNC_PTR);
 
@@ -40,8 +40,8 @@ extern void boinc_app_mouse_button(int x, int y, int which, int is_down);
 extern void boinc_app_mouse_move(int x, int y, int left, int middle, int right);
 extern void boinc_app_key_press(int, int);
 extern void boinc_app_key_release(int, int);
-extern void boinc_suspend_graphics_thread();
-extern void boinc_resume_graphics_thread();
+extern void boinc_suspend_graphics_thread(void);
+extern void boinc_resume_graphics_thread(void);
 
 // C++ API follows here 
 #ifdef __cplusplus
@@ -50,7 +50,7 @@ extern void boinc_resume_graphics_thread();
 #include "boinc_api.h"
 
 extern int boinc_init_options_graphics(BOINC_OPTIONS&, WORKER_FUNC_PTR);
-extern bool boinc_graphics_possible();
+extern bool boinc_graphics_possible(void);
 
 // Implementation stuff
 //
diff --git a/lib/diagnostics.h b/lib/diagnostics.h
index e4468e15a2..7424bfa3ff 100644
--- a/lib/diagnostics.h
+++ b/lib/diagnostics.h
@@ -80,50 +80,50 @@ extern "C" {
 // These are functions common to all platforms
 extern int boinc_init_diagnostics( int flags );
 extern int boinc_init_graphics_diagnostics( int flags );
-extern int boinc_install_signal_handlers();
-extern int boinc_finish_diag();
+extern int boinc_install_signal_handlers(void);
+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_finish();
-extern int diagnostics_is_initialized();
+extern int diagnostics_finish(void);
+extern int diagnostics_is_initialized(void);
 extern int diagnostics_is_flag_set(int flags);
 
 // Properties
-extern char* diagnostics_get_boinc_dir();
-extern char* diagnostics_get_boinc_install_dir();
-extern char* diagnostics_get_symstore();
+extern char* diagnostics_get_boinc_dir(void);
+extern char* diagnostics_get_boinc_install_dir(void);
+extern char* diagnostics_get_symstore(void);
 extern int diagnostics_set_symstore(char* symstore);
-extern int diagnostics_is_proxy_enabled();
-extern char* diagnostics_get_proxy();
+extern int diagnostics_is_proxy_enabled(void);
+extern char* diagnostics_get_proxy(void);
 
-extern int diagnostics_is_aborted_via_gui();
-extern int diagnostics_set_aborted_via_gui();
+extern int diagnostics_is_aborted_via_gui(void);
+extern int diagnostics_set_aborted_via_gui(void);
 
 // Log rotation
-extern int diagnostics_cycle_logs();
+extern int diagnostics_cycle_logs(void);
 extern void diagnostics_set_max_file_sizes(int stdout_size, int stderr_size);
 
 // Thread Tracking
-extern int diagnostics_init_thread_list();
-extern int diagnostics_finish_thread_list();
-extern int diagnostics_update_thread_list();
-extern int diagnostics_set_thread_exempt_suspend();
+extern int diagnostics_init_thread_list(void);
+extern int diagnostics_finish_thread_list(void);
+extern int diagnostics_update_thread_list(void);
+extern int diagnostics_set_thread_exempt_suspend(void);
 extern int diagnostics_is_thread_exempt_suspend(long thread_id);
 
 // Message Monitoring (debugger viewport)
-extern int diagnostics_init_message_monitor();
-extern int diagnostics_finish_message_monitor();
+extern int diagnostics_init_message_monitor(void);
+extern int diagnostics_finish_message_monitor(void);
 #ifdef _WIN32
 extern UINT WINAPI diagnostics_message_monitor(LPVOID lpParameter);
 #endif
 extern int diagnostics_trace_to_debugger(const char* msg);
 
 // Unhandled exception monitor
-extern int diagnostics_init_unhandled_exception_monitor();
-extern int diagnostics_finish_unhandled_exception_monitor();
+extern int diagnostics_init_unhandled_exception_monitor(void);
+extern int diagnostics_finish_unhandled_exception_monitor(void);
 #ifdef _WIN32
 extern UINT WINAPI diagnostics_unhandled_exception_monitor(LPVOID lpParameter);
 extern LONG CALLBACK boinc_catch_signal(EXCEPTION_POINTERS *ExceptionInfo);