diff --git a/src/villas-convert.cpp b/src/villas-convert.cpp
index e2c09322c..bd3e1ae22 100644
--- a/src/villas-convert.cpp
+++ b/src/villas-convert.cpp
@@ -50,8 +50,9 @@ int main(int argc, char *argv[])
 	const char *input_format = "villas.human";
 	const char *output_format = "villas.human";
 
-	char c, *endptr;
 	/* Parse optional command line arguments */
+	int c;
+	char *endptr;
 	while ((c = getopt(argc, argv, "Vhd:i:o:")) != -1) {
 		switch (c) {
 			case 'V':
diff --git a/src/villas-hook.cpp b/src/villas-hook.cpp
index 5866fa71d..9c6fbc342 100644
--- a/src/villas-hook.cpp
+++ b/src/villas-hook.cpp
@@ -121,8 +121,9 @@ int main(int argc, char *argv[])
 
 	json_t *cfg_cli = json_object();
 
-	char c, *endptr;
 	/* Parse optional command line arguments */
+	int c;
+	char *endptr;
 	while ((c = getopt(argc, argv, "Vhv:d:f:o:")) != -1) {
 		switch (c) {
 			case 'V':
diff --git a/src/villas-signal.cpp b/src/villas-signal.cpp
index 92752e7ef..553a8f42c 100644
--- a/src/villas-signal.cpp
+++ b/src/villas-signal.cpp
@@ -59,7 +59,8 @@ json_t * parse_cli(int argc, char *argv[])
 	int limit = -1;
 
 	/* Parse optional command line arguments */
-	char c, *endptr;
+	int c;
+	char *endptr;
 	while ((c = getopt(argc, argv, "v:r:f:l:a:D:no:")) != -1) {
 		switch (c) {
 			case 'n':