diff --git a/libio/Makefile b/libio/Makefile
new file mode 100644
index 0000000..710caaa
--- /dev/null
+++ b/libio/Makefile
@@ -0,0 +1,2 @@
+libio.o: libio.c libio.h
+	$(CC) -c -o libio.o libio.c
diff --git a/libio/libio.c b/libio/libio.c
index 35e988b..325270d 100644
--- a/libio/libio.c
+++ b/libio/libio.c
@@ -3,19 +3,11 @@
 
 /* Backups of standard library systems for the odd scenario where there is no
  * standard library but there is the write(2) system call. */
-static int _isdigit(int c);
-static int _strlen(char *s);
-
-/* Wrappers around write(2) */
-static int _write(int fd, char *s, int n);
-static int _write1(int fd, int c);
-
-/* Normally in <ctype.h> */
-static int
-_isdigit(int c){
-	return c >= '0' && c <= '9';
-}
 
+#if defined USE_SYSTEM_STRING_H
+#	include <string.h>
+#	define _strlen strlen
+#else
 /* Normally in <string.h> */
 static int
 _strlen(char *s){
@@ -25,6 +17,22 @@ _strlen(char *s){
 
 	return i;
 }
+#endif
+
+#if defined USE_SYSTEM_CTYPE_H
+#	include <ctype.h>
+#	define _isdigit isdigit
+#else
+/* Normally in <ctype.h> */
+static int
+_isdigit(int c){
+	return c >= '0' && c <= '9';
+}
+#endif
+
+/* Wrappers around write(2) */
+static int _write(int fd, char *s, int n);
+static int _write1(int fd, int c);
 
 /* Wrapper around write(2) - keeps re-running write(2) until the operation
  * succeeds in full. Usually unnecessary but still good. */
@@ -52,7 +60,7 @@ _write1(int fd, int c){
 
 int
 fdprint(int fd, char *s){
-	return _write(fd, s, strlen(s));
+	return _write(fd, s, _strlen(s));
 }
 
 int
diff --git a/retval/Makefile b/retval/Makefile
new file mode 100644
index 0000000..718a23c
--- /dev/null
+++ b/retval/Makefile
@@ -0,0 +1,8 @@
+retval: retval.o ../libio/libio.o
+	$(CC) -o retval ../libio/libio.o retval.o
+
+retval.o: retval.c ../libio/libio.h
+	$(CC) -c -I../libio -o retval.o retval.c
+
+../libio/libio.o: ../libio/libio.c ../libio/libio.h
+	$(MAKE) -C ../libio
diff --git a/retval/retval.c b/retval/retval.c
index ca3d68d..130bb56 100644
--- a/retval/retval.c
+++ b/retval/retval.c
@@ -1,4 +1,5 @@
 #include <ctype.h> /* isdigit(3) */
+#include <stdio.h> /* fprintf(3), stderr */
 #include <sysexits.h> /* EX_USAGE */
 #include <unistd.h> /* write(2) */
 #include "libio.h" /* fdprint(3), parse_uint(3) */
@@ -9,9 +10,7 @@ int main(int argc, char *argv[]){
 	unsigned int s;
 
 	if(argc < 2){
-usage:		write(2, "Usage: ", 7);
-		fdprint(2, argv[0] == NULL ? program_name : argv[0]);
-		write(2, " [status]\n", 10);
+usage:		fprintf(stderr, "Usage: %s [status]\n", argv[0] == NULL ? program_name : argv[0]);
 		return EX_USAGE;
 	}