diff -r d55232576214 include/lwip/api.h
--- a/include/lwip/api.h
+++ b/include/lwip/api.h
@@ -36,6 +36,8 @@
 
 #if LWIP_NETCONN /* don't build if not configured for use in lwipopts.h */
 
+#include <stddef.h> /* for size_t */
+
 #include "lwip/netbuf.h"
 #include "lwip/sys.h"
 #include "lwip/ip_addr.h"
diff -r d55232576214 include/lwip/api_msg.h
--- a/include/lwip/api_msg.h
+++ b/include/lwip/api_msg.h
@@ -36,6 +36,8 @@
 
 #if LWIP_NETCONN /* don't build if not configured for use in lwipopts.h */
 
+#include <stddef.h> /* for size_t */
+
 #include "lwip/ip_addr.h"
 #include "lwip/err.h"
 #include "lwip/sys.h"
diff -r d55232576214 include/lwip/netdb.h
--- a/include/lwip/netdb.h
+++ b/include/lwip/netdb.h
@@ -31,6 +31,8 @@
 
 #if LWIP_DNS && LWIP_SOCKET
 
+#include <stddef.h> /* for size_t */
+
 #include "lwip/sockets.h"
 
 /* some rarely used options */
diff -r d55232576214 include/lwip/sockets.h
--- a/include/lwip/sockets.h
+++ b/include/lwip/sockets.h
@@ -38,6 +38,8 @@
 
 #if LWIP_SOCKET /* don't build if not configured for use in lwipopts.h */
 
+#include <stddef.h> /* for size_t */
+
 #include "lwip/ip_addr.h"
 #include "lwip/inet.h"