> there doesn't seem to be any allowance for the SNTP timer.  SNTP support is conditionally included based on LWIP_UDP so I'd expect the above declaration should be
>
>  #define LWIP_NUM_SYS_TIMEOUT_INTERNAL   (LWIP_TCP + LWIP_UDP + IP_REASSEMBLY + LWIP_ARP + (2*LWIP_DHCP) + LWIP_AUTOIP + LWIP_IGMP + LWIP_DNS + PPP_NUM_TIMEOUTS + (LWIP_IPV6 * (1 + LWIP_IPV6_REASS + LWIP_IPV6_MLD)))

I agree with your assessment. I think your proposal is part of the solution.
Searching the code (at commit 86c9f799, which is a descendent of STABLE-2_2_0_RELEASE), I find:

$ git grep -w MEMP_NUM_SYS_TIMEOUT | grep -i increase
doc/mqtt_client.txt:You need to increase MEMP_NUM_SYS_TIMEOUT by one if you use MQTT!
src/apps/mdns/mdns.c: * You need to increase MEMP_NUM_SYS_TIMEOUT by one if you use MDNS!
src/apps/sntp/sntp.c: * You need to increase MEMP_NUM_SYS_TIMEOUT by one if you use SNTP!
src/apps/tftp/tftp.c: * You need to increase MEMP_NUM_SYS_TIMEOUT by one if you use TFTP!

So there are several lwIP "apps" which say that if you use them, you must "increase" MEMP_NUM_SYS_TIMEOUT, but lwIP provides no way to increase it relative to the default value (LWIP_NUM_SYS_TIMEOUT_INTERNAL). The only thing the user application can do is #define its own absolute value for MEMP_NUM_SYS_TIMEOUT (since LWIP_NUM_SYS_TIMEOUT_INTERNAL isn't exposed). But later if the user enables/uses more lwIP apps w/o remembering to also increase their MEMP_NUM_SYS_TIMEOUT, it will break.

IMO, a proper solution to this issue would include:

1. #defines to enable/disable the lwIP apps found by my grep above (mqtt, mdns, sntp, tftp)
2. Add those #defines as part of the calculation for the value of LWIP_NUM_SYS_TIMEOUT_INTERNAL (similar to as suggested in the original submission)
3. Remove those lines found by my grep above from the documentation
4. Probably also provide a new #define (e.g. MEMP_ADD_NUM_SYS_TIMEOUT), default value 0, which the user can optionally override. Then change MEMP_NUM_SYS_TIMEOUT definition to

  #define  MEMP_NUM_SYS_TIMEOUT  (LWIP_NUM_SYS_TIMEOUT_INTERNAL + MEMP_ADD_NUM_SYS_TIMEOUT)

Configured LWIP with pretty much default options apart from TCP/IP and sockets API disabled (UDP enabled, no DNS, DHCP etc).  Under these circumstances the LWIP_NUM_SYS_TIMEOUT_INTERNAL declaration in "./src/include/lwip/opt.h" evaluates to 2.

On trying to initialise the SNTP module with sntp_init() call, the sys_timeout() call within sntp_send_request() fails to find an available memory pool entry in MEMP_SYS_TIMEOUT pool and returns a NULL pointer, triggering the subsequent LWIP_ASSERT() statement.

looking at "opt.h"

  #define LWIP_NUM_SYS_TIMEOUT_INTERNAL   (LWIP_TCP + IP_REASSEMBLY + LWIP_ARP + (2*LWIP_DHCP) + LWIP_AUTOIP + LWIP_IGMP + LWIP_DNS + PPP_NUM_TIMEOUTS + (LWIP_IPV6 * (1 + LWIP_IPV6_REASS + LWIP_IPV6_MLD)))

there doesn't seem to be any allowance for the SNTP timer.  SNTP support is conditionally included based on LWIP_UDP so I'd expect the above declaration should be

  #define LWIP_NUM_SYS_TIMEOUT_INTERNAL   (LWIP_TCP + LWIP_UDP + IP_REASSEMBLY + LWIP_ARP + (2*LWIP_DHCP) + LWIP_AUTOIP + LWIP_IGMP + LWIP_DNS + PPP_NUM_TIMEOUTS + (LWIP_IPV6 * (1 + LWIP_IPV6_REASS + LWIP_IPV6_MLD)))




Encountered on 2.1.3 branch, but examination of declaration in master looks to me as though problem still exists.