diff --git a/src/include/lwip/tcpip.h b/src/include/lwip/tcpip.h
index f2f6b469..fb55cebb 100644
--- a/src/include/lwip/tcpip.h
+++ b/src/include/lwip/tcpip.h
@@ -50,12 +50,14 @@ extern "C" {
 #endif
 
 #if LWIP_TCPIP_CORE_LOCKING
+#ifndef LOCK_TCPIP_CORE
 /** The global semaphore to lock the stack. */
 extern sys_mutex_t lock_tcpip_core;
 /** Lock lwIP core mutex (needs @ref LWIP_TCPIP_CORE_LOCKING 1) */
 #define LOCK_TCPIP_CORE()     sys_mutex_lock(&lock_tcpip_core)
 /** Unlock lwIP core mutex (needs @ref LWIP_TCPIP_CORE_LOCKING 1) */
 #define UNLOCK_TCPIP_CORE()   sys_mutex_unlock(&lock_tcpip_core)
+#endif
 #else /* LWIP_TCPIP_CORE_LOCKING */
 #define LOCK_TCPIP_CORE()
 #define UNLOCK_TCPIP_CORE()
diff --git a/test/unit/api/test_sockets.c b/test/unit/api/test_sockets.c
new file mode 100644
index 00000000..f0d889c3
--- /dev/null
+++ b/test/unit/api/test_sockets.c
@@ -0,0 +1,68 @@
+#include "test_sockets.h"
+
+#include "lwip/sockets.h"
+#include "lwip/stats.h"
+
+#include "lwip/tcpip.h"
+
+
+/* Setups/teardown functions */
+
+static void
+sockets_setup(void)
+{
+}
+
+static void
+sockets_teardown(void)
+{
+}
+
+#ifndef NUM_SOCKETS
+#define NUM_SOCKETS MEMP_NUM_NETCONN
+#endif
+
+/* Verify basic sockets functionality
+ */
+START_TEST(test_sockets_basics)
+{
+  int s, i, ret;
+  int s2[NUM_SOCKETS];
+  LWIP_UNUSED_ARG(_i);
+
+  s = lwip_socket(AF_INET, SOCK_STREAM, 0);
+  fail_unless(s >= 0);
+  lwip_close(s);
+
+  for (i = 0; i < NUM_SOCKETS; i++) {
+    s2[i] = lwip_socket(AF_INET, SOCK_STREAM, 0);
+    fail_unless(s2[i] >= 0);
+  }
+
+  /* all sockets used, now it should fail */
+  s = lwip_socket(AF_INET, SOCK_STREAM, 0);
+  fail_unless(s == -1);
+  /* close one socket */
+  ret = lwip_close(s2[0]);
+  fail_unless(ret == 0);
+  /* now it should succeed */
+  s2[0] = lwip_socket(AF_INET, SOCK_STREAM, 0);
+  fail_unless(s2[0] >= 0);
+
+  /* close all sockets */
+  for (i = 0; i < NUM_SOCKETS; i++) {
+    ret = lwip_close(s2[i]);
+    fail_unless(ret == 0);
+  }
+}
+END_TEST
+
+/** Create the suite including all tests for this module */
+Suite *
+sockets_suite(void)
+{
+  testfunc tests[] = {
+    TESTFUNC(test_sockets_basics)
+  };
+  return create_suite("SOCKETS", tests, sizeof(tests)/sizeof(testfunc), sockets_setup, sockets_teardown);
+}
diff --git a/test/unit/api/test_sockets.h b/test/unit/api/test_sockets.h
new file mode 100644
index 00000000..0abb7a30
--- /dev/null
+++ b/test/unit/api/test_sockets.h
@@ -0,0 +1,8 @@
+#ifndef LWIP_HDR_TEST_SOCKETS_H
+#define LWIP_HDR_TEST_SOCKETS_H
+
+#include "../lwip_check.h"
+
+Suite *sockets_suite(void);
+
+#endif
diff --git a/test/unit/arch/sys_arch.c b/test/unit/arch/sys_arch.c
new file mode 100644
index 00000000..85c62159
--- /dev/null
+++ b/test/unit/arch/sys_arch.c
@@ -0,0 +1,332 @@
+/*
+ * Copyright (c) 2017 Simon Goldschmidt
+ * All rights reserved. 
+ * 
+ * Redistribution and use in source and binary forms, with or without modification, 
+ * are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission. 
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED 
+ * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF 
+ * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT 
+ * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, 
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT 
+ * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING 
+ * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY 
+ * OF SUCH DAMAGE.
+ *
+ * This file is part of the lwIP TCP/IP stack.
+ * 
+ * Author: Simon Goldschmidt
+ *
+ */
+
+
+#include <lwip/opt.h>
+#include <lwip/arch.h>
+#include "sys_arch.h"
+#include <lwip/stats.h>
+#include <lwip/debug.h>
+#include <lwip/sys.h>
+
+#include <string.h>
+
+test_sys_arch_waiting_fn the_waiting_fn;
+
+void test_sys_arch_wait_callback(test_sys_arch_waiting_fn waiting_fn)
+{
+  the_waiting_fn = waiting_fn;
+}
+
+u32_t sys_jiffies(void)
+{
+  return (u32_t)0; /* todo */
+}
+
+u32_t sys_now(void)
+{
+  return (u32_t)0; /* todo */
+}
+
+void sys_init(void)
+{
+}
+
+#if !NO_SYS
+
+err_t sys_sem_new(sys_sem_t *sem, u8_t count)
+{
+  LWIP_ASSERT("sem != NULL", sem != NULL);
+  *sem = count + 1;
+  return ERR_OK;
+}
+
+void sys_sem_free(sys_sem_t *sem)
+{
+  LWIP_ASSERT("sem != NULL", sem != NULL);
+  *sem = 0;
+}
+
+void sys_sem_set_invalid(sys_sem_t *sem)
+{
+  LWIP_ASSERT("sem != NULL", sem != NULL);
+  *sem = 0;
+}
+
+/* semaphores are 1-based because RAM is initialized as 0, which would be valid */
+u32_t sys_arch_sem_wait(sys_sem_t *sem, u32_t timeout)
+{
+  u32_t ret = 0;
+  LWIP_ASSERT("sem != NULL", sem != NULL);
+  LWIP_ASSERT("*sem > 0", *sem > 0);
+  if (*sem == 1) {
+    /* need to wait */
+    if(!timeout)
+    {
+      /* wait infinite */
+      LWIP_ASSERT("cannot wait without waiting callback", the_waiting_fn != NULL);
+      do {
+        int expectSomething = the_waiting_fn(sem);
+        LWIP_ASSERT("*sem > 0", *sem > 0);
+        LWIP_ASSERT("expecting a semaphore count but it's 0", !expectSomething || (*sem > 1));
+        ret++;
+      } while(*sem == 1);
+    }
+    else
+    {
+      if (the_waiting_fn) {
+        int expectSomething = the_waiting_fn(sem);
+        LWIP_ASSERT("expecting a semaphore count but it's 0", !expectSomething || (*sem > 1));
+      }
+      LWIP_ASSERT("*sem > 0", *sem > 0);
+      if (*sem == 1) {
+        return SYS_ARCH_TIMEOUT;
+      }
+      ret = 1;
+    }
+  }
+  LWIP_ASSERT("*sem > 0", *sem > 0);
+  *sem--;
+  LWIP_ASSERT("*sem > 0", *sem > 0);
+  /* return the time we waited for the sem */
+  return ret;
+}
+
+void sys_sem_signal(sys_sem_t *sem)
+{
+  LWIP_ASSERT("sem != NULL", sem != NULL);
+  LWIP_ASSERT("*sem > 0", *sem > 0);
+  *sem++;
+  LWIP_ASSERT("*sem > 0", *sem > 0);
+}
+
+err_t sys_mutex_new(sys_mutex_t *mutex)
+{
+  LWIP_ASSERT("mutex != NULL", mutex != NULL);
+  *mutex = 1; /* 1 allocated */
+  return ERR_OK;
+}
+
+void sys_mutex_free(sys_mutex_t *mutex)
+{
+  /* parameter check */
+  LWIP_ASSERT("mutex != NULL", mutex != NULL);
+  LWIP_ASSERT("*mutex >= 1", *mutex >= 1);
+  *mutex = 0;
+}
+
+void sys_mutex_set_invalid(sys_mutex_t *mutex)
+{
+  LWIP_ASSERT("mutex != NULL", mutex != NULL);
+  *mutex = 0;
+}
+
+void sys_mutex_lock(sys_mutex_t *mutex)
+{
+  /* nothing to do, no multithreading supported */
+  LWIP_ASSERT("mutex != NULL", mutex != NULL);
+  LWIP_ASSERT("*mutex >= 1", *mutex >= 1);
+  /* we count up just to check the correct pairing of lock/unlock */
+  *mutex++;
+  LWIP_ASSERT("*mutex >= 1", *mutex >= 1);
+}
+
+void sys_mutex_unlock(sys_mutex_t *mutex)
+{
+  /* nothing to do, no multithreading supported */
+  LWIP_ASSERT("mutex != NULL", mutex != NULL);
+  LWIP_ASSERT("*mutex >= 1", *mutex >= 1);
+  /* we count down just to check the correct pairing of lock/unlock */
+  *mutex--;
+  LWIP_ASSERT("*mutex >= 1", *mutex >= 1);
+}
+
+
+sys_thread_t sys_thread_new(const char *name, lwip_thread_fn function, void *arg, int stacksize, int prio)
+{
+  LWIP_UNUSED_ARG(name);
+  LWIP_UNUSED_ARG(function);
+  LWIP_UNUSED_ARG(arg);
+  LWIP_UNUSED_ARG(stacksize);
+  LWIP_UNUSED_ARG(prio);
+  /* threads not supported */
+  return 0;
+}
+
+err_t sys_mbox_new(sys_mbox_t *mbox, int size)
+{
+  int mboxsize = size;
+  LWIP_ASSERT("mbox != NULL", mbox != NULL);
+  LWIP_ASSERT("size >= 0", size >= 0);
+  if (size == 0) {
+    mboxsize = 1024;
+  }
+  mbox->head = mbox->tail = 0;
+  mbox->sem = mbox; /* just point to something for sys_mbox_valid() */
+  mbox->q_mem = (void**)malloc(sizeof(void*)*mboxsize);
+  mbox->size = mboxsize;
+  mbox->used = 0;
+
+  memset(mbox->q_mem, 0, sizeof(void*)*mboxsize);
+  return ERR_OK;
+}
+
+void sys_mbox_free(sys_mbox_t *mbox)
+{
+  /* parameter check */
+  LWIP_ASSERT("mbox != NULL", mbox != NULL);
+  LWIP_ASSERT("mbox->sem != NULL", mbox->sem != NULL);
+  LWIP_ASSERT("mbox->sem == mbox", mbox->sem == mbox);
+  LWIP_ASSERT("mbox->q_mem != NULL", mbox->q_mem != NULL);
+  mbox->sem = NULL;
+  free(mbox->q_mem);
+  mbox->q_mem = NULL;
+}
+
+void sys_mbox_set_invalid(sys_mbox_t *mbox)
+{
+  LWIP_ASSERT("mbox != NULL", mbox != NULL);
+  LWIP_ASSERT("mbox->q_mem == NULL", mbox->q_mem == NULL);
+  mbox->sem = NULL;
+  mbox->q_mem = NULL;
+}
+
+void sys_mbox_post(sys_mbox_t *q, void *msg)
+{
+  LWIP_ASSERT("q != SYS_MBOX_NULL", q != SYS_MBOX_NULL);
+  LWIP_ASSERT("q->sem != q", q->sem != q);
+  LWIP_ASSERT("q->q_mem != NULL", q->q_mem != NULL);
+  LWIP_ASSERT("q->used >= 0", q->used >= 0);
+  LWIP_ASSERT("q->size > 0", q->size > 0);
+
+  LWIP_ASSERT("mbox already full", q->used < q->size);
+
+  q->q_mem[q->head] = msg;
+  q->head++;
+  if (q->head >= (unsigned int)q->size) {
+    q->head = 0;
+  }
+  LWIP_ASSERT("mbox is full!", q->head != q->tail);
+  q->used++;
+}
+
+err_t sys_mbox_trypost(sys_mbox_t *q, void *msg)
+{
+  LWIP_ASSERT("q != SYS_MBOX_NULL", q != SYS_MBOX_NULL);
+  LWIP_ASSERT("q->sem != q", q->sem != q);
+  LWIP_ASSERT("q->q_mem != NULL", q->q_mem != NULL);
+  LWIP_ASSERT("q->used >= 0", q->used >= 0);
+  LWIP_ASSERT("q->size > 0", q->size > 0);
+  LWIP_ASSERT("q->used <= q->size", q->used <= q->size);
+
+  if (q->used == q->size) {
+    return ERR_MEM;
+  }
+  sys_mbox_post(q, msg);
+  return ERR_OK;
+}
+
+u32_t sys_arch_mbox_fetch(sys_mbox_t *q, void **msg, u32_t timeout)
+{
+  u32_t ret = 0;
+  u32_t ret2;
+  LWIP_ASSERT("q != SYS_MBOX_NULL", q != SYS_MBOX_NULL);
+  LWIP_ASSERT("q->sem != q", q->sem != q);
+  LWIP_ASSERT("q->q_mem != NULL", q->q_mem != NULL);
+  LWIP_ASSERT("q->used >= 0", q->used >= 0);
+  LWIP_ASSERT("q->size > 0", q->size > 0);
+
+  if (q->used == 0) {
+    /* need to wait */
+    /* need to wait */
+    if(!timeout)
+    {
+      /* wait infinite */
+      LWIP_ASSERT("cannot wait without waiting callback", the_waiting_fn != NULL);
+      do {
+        int expectSomething = the_waiting_fn(q);
+        LWIP_ASSERT("q->used >= 0", q->used >= 0);
+        LWIP_ASSERT("expecting item available but it's 0", !expectSomething || (q->used > 0));
+        ret++;
+      } while(q->used == 0);
+    }
+    else
+    {
+      if (the_waiting_fn) {
+        int expectSomething = the_waiting_fn(q);
+        LWIP_ASSERT("expecting item available count but it's 0", !expectSomething || (q->used > 0));
+      }
+      LWIP_ASSERT("q->used >= 0", q->used >= 0);
+      if (q->used == 0) {
+        if(msg) {
+          *msg = NULL;
+        }
+        return SYS_ARCH_TIMEOUT;
+      }
+      ret = 1;
+    }
+  }
+  LWIP_ASSERT("q->used > 0", q->used > 0);
+  ret2 = sys_arch_mbox_tryfetch(q, msg);
+  LWIP_ASSERT("got no message", ret2 == 0);
+  return ret;
+}
+
+u32_t sys_arch_mbox_tryfetch(sys_mbox_t *q, void **msg)
+{
+  LWIP_ASSERT("q != SYS_MBOX_NULL", q != SYS_MBOX_NULL);
+  LWIP_ASSERT("q->sem == q", q->sem == q);
+  LWIP_ASSERT("q->q_mem != NULL", q->q_mem != NULL);
+  LWIP_ASSERT("q->used >= 0", q->used >= 0);
+  LWIP_ASSERT("q->size > 0", q->size > 0);
+
+  if (!q->used) {
+    return SYS_ARCH_TIMEOUT;
+  }
+  if(msg) {
+    *msg = q->q_mem[q->tail];
+  }
+
+  q->tail++;
+  if (q->tail >= (unsigned int)q->size) {
+    q->tail = 0;
+  }
+  q->used--;
+  LWIP_ASSERT("q->used >= 0", q->used >= 0);
+  return 0;
+}
+
+#if LWIP_NETCONN_SEM_PER_THREAD
+#error LWIP_NETCONN_SEM_PER_THREAD==1 not supported
+#endif /* LWIP_NETCONN_SEM_PER_THREAD */
+
+#endif /* !NO_SYS */
diff --git a/test/unit/arch/sys_arch.h b/test/unit/arch/sys_arch.h
new file mode 100644
index 00000000..9373c4c4
--- /dev/null
+++ b/test/unit/arch/sys_arch.h
@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2017 Simon Goldschmidt
+ * All rights reserved. 
+ * 
+ * Redistribution and use in source and binary forms, with or without modification, 
+ * are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission. 
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED 
+ * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF 
+ * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT 
+ * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, 
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT 
+ * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING 
+ * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY 
+ * OF SUCH DAMAGE.
+ *
+ * This file is part of the lwIP TCP/IP stack.
+ * 
+ * Author: Simon Goldschmidt
+ *
+ */
+#ifndef LWIP_HDR_TEST_SYS_ARCH_H
+#define LWIP_HDR_TEST_SYS_ARCH_H
+
+typedef int sys_sem_t;
+#define sys_sem_valid(sema) ((sema) != NULL)
+
+typedef int sys_mutex_t;
+#define sys_mutex_valid(mutex) (((mutex) != NULL)
+
+struct lwip_mbox {
+  void* sem;
+  void** q_mem;
+  unsigned int head, tail;
+  int size;
+  int used;
+};
+typedef struct lwip_mbox sys_mbox_t;
+#define SYS_MBOX_NULL NULL
+#define sys_mbox_valid(mbox) ((mbox != NULL) && ((mbox)->sem != NULL)  && ((mbox)->sem != (void*)-1))
+
+/* DWORD (thread id) is used for sys_thread_t but we won't include windows.h */
+typedef u32_t sys_thread_t;
+
+#define SYS_ARCH_DECL_PROTECT(lev)
+#define SYS_ARCH_PROTECT(lev)
+#define SYS_ARCH_UNPROTECT(lev)
+
+/* to implement doing something while waiting:
+ * pass a function to test_sys_arch_wait_callback() that returns
+ * '0' if waiting again and
+ * '1' if now there should be something to do (used for asserting)
+ */
+typedef int (*test_sys_arch_waiting_fn)(void* wait_element);
+void test_sys_arch_wait_callback(test_sys_arch_waiting_fn waiting_fn);
+
+/* no multithreading supported: define to nothing */
+#define LOCK_TCPIP_CORE()
+#define UNLOCK_TCPIP_CORE()
+
+#endif /* LWIP_HDR_TEST_SYS_ARCH_H */
+
diff --git a/test/unit/lwip_unittests.c b/test/unit/lwip_unittests.c
index 46fd4308..4f3dd1eb 100644
--- a/test/unit/lwip_unittests.c
+++ b/test/unit/lwip_unittests.c
@@ -8,6 +8,7 @@
 #include "etharp/test_etharp.h"
 #include "dhcp/test_dhcp.h"
 #include "mdns/test_mdns.h"
+#include "api/test_sockets.h"
 
 #include "lwip/init.h"
 
@@ -44,7 +45,8 @@ int main(void)
     pbuf_suite,
     etharp_suite,
     dhcp_suite,
-    mdns_suite
+    mdns_suite,
+    sockets_suite
   };
   size_t num = sizeof(suites)/sizeof(void*);
   LWIP_ASSERT("No suites defined", num > 0);
diff --git a/test/unit/lwipopts.h b/test/unit/lwipopts.h
index 25252b42..9c30f1d4 100644
--- a/test/unit/lwipopts.h
+++ b/test/unit/lwipopts.h
@@ -33,10 +33,10 @@
 #define LWIP_HDR_LWIPOPTS_H
 
 /* Prevent having to link sys_arch.c (we don't test the API layers in unit tests) */
-#define NO_SYS                          1
+#define NO_SYS                          0
 #define SYS_LIGHTWEIGHT_PROT            0
-#define LWIP_NETCONN                    0
-#define LWIP_SOCKET                     0
+#define LWIP_NETCONN                    1
+#define LWIP_SOCKET                     1
 
 /* Enable DHCP to test it, disable UDP checksum to easier inject packets */
 #define LWIP_DHCP                       1