Quality RTOS & Embedded Software

Delta Over-the-Air Updates
December 2021 Releases:
Extended Maintenance Plan (EMP):
FreeRTOS 202111.00 Release:
FreeRTOS LTS libraries:


[FreeRTOS+TCP API Reference]


Note: this hook needs to be defined only when ipconfigUSE_DHCP_HOOK is set to 1 in the FreeRTOSIPConfig.h file.

eDHCPCallbackAnswer_t xApplicationDHCPHook( eDHCPCallbackPhase_t eDHCPPhase,uint32_t ulIPAddress );

xApplicationDHCPHook is an application defined hook (or callback) function that is called by the FreeRTOS+TCP stack to check whether DHCP 'Discover' and 'Request' messages should be sent to complete the process of getting a dynamic IP-address or whether the device use the default static IP.

Callback functions are implemented by the application writer, but called by the TCP/IP stack. The prototype of the callback function must exactly match the prototype above (including the function name). The code in an application hook should not call FreeRTOS+TCP API's that are blocking. That could easily lead to a dead-lock.

When an application hook executes, it borrows the task priority and the stack of the IP-task. Therefore, we recommend that you keep application hooks short--it may want to wakeup some application task which will do further processing.



This parameter can be either eDHCPPhasePreRequest or eDHCPPhasePreDiscover when the TCP stack is about to send either a Request or a Discover message, respectively.


This parameter will contain the default IP address when the parameter eDHCPPhase is eDHCPPhasePreDiscover, otherwise (when eDHCPPhase is eDHCPPhasePreRequest) it will contain the IP address offered by the DHCP server.

Return value:

The return value can have these enum values:

  • eDHCPContinue if the application wants to continue the DHCP transaction.
  • eDHCPUseDefaults if the application wants to use the default network parameters.
  • eDHCPStopNoChanges in case the DHCP processing must be stopped, while keeping all network parameters as they were before.

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