diff --git a/CMakeLists.txt b/CMakeLists.txt
index d40c6ae..6c619df 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2,13 +2,15 @@ cmake_minimum_required(VERSION 3.3)
 
 project(can-utils LANGUAGES C)
 
+include (CheckSymbolExists)
 include (GNUInstallDirs)
 
 if(NOT CMAKE_BUILD_TYPE)
   set(CMAKE_BUILD_TYPE Release)
 endif()
 
-if ("${CMAKE_C_COMPILER_ID}" STREQUAL "GNU")
+check_symbol_exists(__GNU_LIBRARY__ "features.h" _GNU_SOURCE)
+if(_GNU_SOURCE)
   add_definitions(-D_GNU_SOURCE)
 endif()
 
diff --git a/can-calc-bit-timing.c b/can-calc-bit-timing.c
index e3bee9f..9129844 100644
--- a/can-calc-bit-timing.c
+++ b/can-calc-bit-timing.c
@@ -444,7 +444,7 @@ static int can_calc_bittiming(struct net_device *dev, struct can_bittiming *bt,
 	unsigned int spt_nominal;	/* nominal sample point */
 	unsigned int best_tseg = 0;	/* current best value for tseg */
 	unsigned int best_brp = 0;	/* current best value for brp */
-	unsigned int brp, tsegall, tseg, tseg1, tseg2;
+	unsigned int brp, tsegall, tseg, tseg1 = 0, tseg2 = 0;
 	u64 v64;
 
 	/* Use CiA recommended sample points */
diff --git a/j1939cat.c b/j1939cat.c
index a37c5ba..5ae0c0b 100644
--- a/j1939cat.c
+++ b/j1939cat.c
@@ -645,7 +645,7 @@ static int j1939cat_parse_args(struct j1939cat_priv *priv, int argc, char *argv[
 int main(int argc, char *argv[])
 {
 	struct j1939cat_priv *priv;
-	int ret;
+	int ret = 0;
 
 	priv = malloc(sizeof(*priv));
 	if (!priv)
@@ -679,6 +679,6 @@ int main(int argc, char *argv[])
 	close(priv->infile);
 	close(priv->outfile);
 	close(priv->sock);
+
 	return ret;
 }
-
diff --git a/slcanpty.c b/slcanpty.c
index fc72dfe..d233c0d 100644
--- a/slcanpty.c
+++ b/slcanpty.c
@@ -24,17 +24,18 @@
  *
  */
 
+#include <fcntl.h>
+#include <libgen.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <libgen.h>
 #include <string.h>
-#include <unistd.h>
-#include <fcntl.h>
 #include <termios.h>
+#include <unistd.h>
 
 #include <net/if.h>
-#include <sys/socket.h>
 #include <sys/ioctl.h>
+#include <sys/select.h>
+#include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/types.h>