<blockquote>
<p>By the way these are the warnings when i compile the program</p>
<pre><code>/data/riotbuild/RIOT-master/examples/tcp-simple-server/bin/esp32-wroom-32/lwip_api.a(tcpip.o): In function `tcpip_try_callback':
tcpip.c:(.text+0x32c): warning: undefined reference to `sys_mbox_trypost_fromisr'
/data/riotbuild/RIOT-master/examples/tcp-simple-server/bin/esp32-wroom-32/lwip_api.a(tcpip.o): In function `tcpip_send_msg_wait_sem':
tcpip.c:(.text+0x35e): warning: undefined reference to `sys_mbox_trypost_fromisr
</code></pre>
</blockquote>
<p>BTW, I was trying investige these undefined symbols a year ago, but I don't remeber what the problem was.</p>

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