diff --git a/src/frontend/get_avail_mem_size.c b/src/frontend/get_avail_mem_size.c
index b5ac54e31..c40702baa 100644
--- a/src/frontend/get_avail_mem_size.c
+++ b/src/frontend/get_avail_mem_size.c
@@ -5,6 +5,7 @@
  */
 
 #include "ngspice/ngspice.h"
+#include "resource.h"
 
 #if defined(_WIN32)
 #undef BOOLEAN
@@ -29,7 +30,7 @@
 /**
  * Returns the size of available memory (RAM) in bytes.
  */
-unsigned long long getAvailableMemorySize( )
+unsigned long long getAvailableMemorySize(void)
 {
 #if defined(HAVE__PROC_MEMINFO)
     /* Cygwin , Linux--------------------------------- */
diff --git a/src/frontend/get_phys_mem_size.c b/src/frontend/get_phys_mem_size.c
index ef6c47b91..378f7115d 100644
--- a/src/frontend/get_phys_mem_size.c
+++ b/src/frontend/get_phys_mem_size.c
@@ -7,6 +7,7 @@
  */
 
 #include "ngspice/ngspice.h"
+#include "resource.h"
 
 #if defined(_WIN32)
 #undef BOOLEAN
@@ -29,7 +30,7 @@
 /**
  * Returns the size of physical memory (RAM) in bytes.
  */
-unsigned long long getMemorySize()
+unsigned long long getMemorySize(void)
 {
 #if defined(HAVE__PROC_MEMINFO)
     /* Cygwin , Linux--------------------------------- */
diff --git a/src/frontend/get_resident_set_size.c b/src/frontend/get_resident_set_size.c
index 888b022e3..aeff60558 100644
--- a/src/frontend/get_resident_set_size.c
+++ b/src/frontend/get_resident_set_size.c
@@ -7,6 +7,7 @@
  */
 
 #include "ngspice/ngspice.h"
+#include "resource.h"
 
 #if defined(_WIN32)
 #undef BOOLEAN
@@ -39,7 +40,7 @@
  * memory use) measured in bytes, or zero if the value cannot be
  * determined on this OS.
  */
-unsigned long long getPeakRSS()
+unsigned long long getPeakRSS(void)
 {
 #if defined(HAVE__PROC_MEMINFO)
     /* Linux ---------------------------------------------------- */
@@ -100,7 +101,7 @@ unsigned long long getPeakRSS()
  * Returns the current resident set size (physical memory use) measured
  * in bytes, or zero if the value cannot be determined on this OS.
  */
-unsigned long long getCurrentRSS( )
+unsigned long long getCurrentRSS(void)
 {
 #if defined(_WIN32)
     /* Windows -------------------------------------------------- */