Call back function is not calling


I am using DEV studio 2.2.1.
I was downloded the GPS simple Sample code from the samples given in the IDE,
in that code they are used one call back function in gps_simpleEventHandler(), but its not called. can you guide me what is the purpose of callback function, and when that functions should called?

the following code i am used
eRetStatus = gps_pvtSetOpts( GPS_OPT_PVT_HANDLER, gps_simplePvtHandler
GSM modue:SL6087
GPS module:XM0110
communication:I2c (Sl6087 communicate to xm0110 thriugh i2c communication)

not only in this sample code all the sample codes are not called these type of functions?
so can anyone explain me what is the exact definition of these functions, whether these functions are expect any unsolicited responses to executed it?



gps_simpleEventHandler is the the Library callback event handler that will manage all events from the Library
state machine.This call back is passed as parameter in gps_init function.
gps_init API will initialize the library interface and sets the Library in GPS_INITIALIZED state.

So, this handler will be called to manage all the GPS library plug in events on initialisation of library.


I get an idea from your reply, but if i am put some trace strings inside of these functions is not come in trace window.