diff --git a/src/include/lwip/mem.h b/src/include/lwip/mem.h
index e4f6a64d..67aa4c6a 100644
--- a/src/include/lwip/mem.h
+++ b/src/include/lwip/mem.h
@@ -45,6 +45,7 @@ extern "C" {
 
 #if MEM_LIBC_MALLOC
 
+#include <stdlib.h>
 #include <stddef.h> /* for size_t */
 typedef size_t mem_size_t;
 #define MEM_SIZE_F SZT_F