diff --git a/arch/platform/cooja/platform.c b/arch/platform/cooja/platform.c
index f9beff84c8b8f07312f09c388cd632f6fb566a10..3dea7d653869d16ca328dbd9c57e43a47f88275c 100644
--- a/arch/platform/cooja/platform.c
+++ b/arch/platform/cooja/platform.c
@@ -154,12 +154,9 @@ set_lladdr(void)
 
   memset(&addr, 0, sizeof(linkaddr_t));
 #if NETSTACK_CONF_WITH_IPV6
-  {
-    int i;
-    for(i = 0; i < sizeof(uip_lladdr.addr); i += 2) {
-      addr.u8[i + 1] = simMoteID & 0xff;
-      addr.u8[i + 0] = simMoteID >> 8;
-    }
+  for(size_t i = 0; i < sizeof(uip_lladdr.addr); i += 2) {
+    addr.u8[i + 1] = simMoteID & 0xff;
+    addr.u8[i + 0] = simMoteID >> 8;
   }
 #else /* NETSTACK_CONF_WITH_IPV6 */
   addr.u8[0] = simMoteID & 0xff;
diff --git a/os/lib/sensors.c b/os/lib/sensors.c
index ccfbdad5eff23e13fac97b38bfe0cc116bc9582b..91472ba2e9e150241c7e92cf3af78b849da4e9fc 100644
--- a/os/lib/sensors.c
+++ b/os/lib/sensors.c
@@ -29,8 +29,6 @@
  * This file is part of the Contiki operating system.
  *
  */
-/* exeperimental code, will be renamed to sensors.c when done */
-
 
 #include <string.h>
 
@@ -38,7 +36,7 @@
 
 #include "lib/sensors.h"
 
-const extern struct sensors_sensor *sensors[];
+extern const struct sensors_sensor *sensors[];
 extern unsigned char sensors_flags[];
 
 #define FLAG_CHANGED    0x80
diff --git a/os/net/ipv6/sicslowpan.c b/os/net/ipv6/sicslowpan.c
index 0b570e45e2c8ae5a533aa2f4e73590b2fe3ddf24..5ccacfd13375e67aa1426e82f17d2e845410efe3 100644
--- a/os/net/ipv6/sicslowpan.c
+++ b/os/net/ipv6/sicslowpan.c
@@ -434,7 +434,7 @@ copy_frags2uip(int context)
   for(i = 0; i < SICSLOWPAN_FRAGMENT_BUFFERS; i++) {
     /* And also copy all matching fragments */
     if(frag_buf[i].len > 0 && frag_buf[i].index == context) {
-      if((frag_buf[i].offset << 3) + frag_buf[i].len > sizeof(uip_buf)) {
+      if(((size_t)frag_buf[i].offset << 3) + frag_buf[i].len > sizeof(uip_buf)) {
         LOG_WARN("input: invalid fragment offset\n");
         clear_fragments(context);
         return false;
@@ -2026,12 +2026,12 @@ input(void)
 
   /* Sanity-check size of incoming packet to avoid buffer overflow */
   {
-    int req_size = uncomp_hdr_len + (uint16_t)(frag_offset << 3)
+    unsigned int req_size = uncomp_hdr_len + (uint16_t)(frag_offset << 3)
         + packetbuf_payload_len;
     if(req_size > sizeof(uip_buf)) {
 #if SICSLOWPAN_CONF_FRAG
       LOG_ERR(
-          "input: packet and fragment context %u dropped, minimum required IP_BUF size: %d+%d+%d=%d (current size: %u)\n",
+          "input: packet and fragment context %u dropped, minimum required IP_BUF size: %d+%d+%d=%u (current size: %u)\n",
           frag_context,
           uncomp_hdr_len, (uint16_t)(frag_offset << 3),
           packetbuf_payload_len, req_size, (unsigned)sizeof(uip_buf));
diff --git a/os/net/routing/rpl-lite/rpl-ext-header.c b/os/net/routing/rpl-lite/rpl-ext-header.c
index 8c7d936a4fef585b6973d9eff30bdfb746a3b8e6..6657795419bac41ce504a35562d1d983cad6ccf7 100644
--- a/os/net/routing/rpl-lite/rpl-ext-header.c
+++ b/os/net/routing/rpl-lite/rpl-ext-header.c
@@ -166,8 +166,7 @@ rpl_ext_header_srh_update(void)
 static int
 count_matching_bytes(const void *p1, const void *p2, size_t n)
 {
-  int i = 0;
-  for(i = 0; i < n; i++) {
+  for(size_t i = 0; i < n; i++) {
     if(((uint8_t *)p1)[i] != ((uint8_t *)p2)[i]) {
       return i;
     }
diff --git a/os/sys/process.h b/os/sys/process.h
index 5cffdb526295b718ebe768c509d61012eb2ee5b0..73e9535b3bb94fc670e2ff532e9742744204f690 100644
--- a/os/sys/process.h
+++ b/os/sys/process.h
@@ -302,12 +302,12 @@ static PT_THREAD(process_thread_##name(struct pt *process_pt,	\
 #define PROCESS(name, strname)				\
   PROCESS_THREAD(name, ev, data);			\
   struct process name = { NULL,		        \
-                          process_thread_##name }
+                          process_thread_##name, {0}, 0, 0 }
 #else
 #define PROCESS(name, strname)				\
   PROCESS_THREAD(name, ev, data);			\
   struct process name = { NULL, strname,		\
-                          process_thread_##name }
+                          process_thread_##name, {0}, 0, 0 }
 #endif
 
 /** @} */