diff --git a/src/villas-convert.cpp b/src/villas-convert.cpp
index 455f72fab..7b8bccbcf 100644
--- a/src/villas-convert.cpp
+++ b/src/villas-convert.cpp
@@ -50,7 +50,8 @@ int main(int argc, char *argv[])
 	const char *input_format = "villas.human";
 	const char *output_format = "villas.human";
 
-	char c, *endptr;
+	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 8cca55003..3041339db 100644
--- a/src/villas-hook.cpp
+++ b/src/villas-hook.cpp
@@ -121,7 +121,8 @@ int main(int argc, char *argv[])
 
 	json_t *cfg_cli = json_object();
 
-	char c, *endptr;
+	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':
diff --git a/src/villas-test-cmp.cpp b/src/villas-test-cmp.cpp
index 1e27cd6d5..fcf623618 100644
--- a/src/villas-test-cmp.cpp
+++ b/src/villas-test-cmp.cpp
@@ -79,7 +79,8 @@ int main(int argc, char *argv[])
 	struct pool pool = { .state = STATE_DESTROYED };
 
 	/* Parse Arguments */
-	char c, *endptr;
+	int c;
+	char *endptr;
 	while ((c = getopt (argc, argv, "he:vtsf:V")) != -1) {
 		switch (c) {
 			case 'e':
diff --git a/src/villas-test-rtt.cpp b/src/villas-test-rtt.cpp
index e7292d409..c57c270ed 100644
--- a/src/villas-test-rtt.cpp
+++ b/src/villas-test-rtt.cpp
@@ -87,7 +87,8 @@ int main(int argc, char *argv[])
 	int ret;
 
 	/* Parse Arguments */
-	char c, *endptr;
+	int c;
+	char *endptr;
 	while ((c = getopt (argc, argv, "w:h:r:f:c:b:V")) != -1) {
 		switch (c) {
 			case 'c':