diff --git a/doomgeneric/Makefile b/doomgeneric/Makefile
index f1d8df5..a35d5fb 100644
--- a/doomgeneric/Makefile
+++ b/doomgeneric/Makefile
@@ -15,7 +15,7 @@ endif
 CC=gcc  # gcc or g++
 CFLAGS+=-ggdb3 -Os
 LDFLAGS+=-Wl,--gc-sections
-CFLAGS+=-ggdb3 -Wall -DNORMALUNIX -DLINUX -DSNDSERV # -DUSEASM
+CFLAGS+=-ggdb3 -Wall -DNORMALUNIX -DLINUX -DSNDSERV -D_DEFAULT_SOURCE # -DUSEASM
 LIBS+=-lm -lc -lX11
 
 # subdirectory for objects
diff --git a/doomgeneric/doomgeneric.h b/doomgeneric/doomgeneric.h
index 2588710..c2dc4b5 100644
--- a/doomgeneric/doomgeneric.h
+++ b/doomgeneric/doomgeneric.h
@@ -1,6 +1,7 @@
 #ifndef DOOM_GENERIC
 #define DOOM_GENERIC
 
+#include <stdlib.h>
 #include <stdint.h>
 
 #define DOOMGENERIC_RESX 640
diff --git a/doomgeneric/doomgeneric_xlib.c b/doomgeneric/doomgeneric_xlib.c
index cf92fc7..f6d6a31 100644
--- a/doomgeneric/doomgeneric_xlib.c
+++ b/doomgeneric/doomgeneric_xlib.c
@@ -2,6 +2,7 @@
 
 #include "doomgeneric.h"
 
+#include <ctype.h>
 #include <stdio.h>
 #include <string.h>
 #include <unistd.h>