User Defined Program for Homing and Auto tool changer

I don't know any details about your spindle and machine configuration. My experience is limited to newer HSK-63F router ATCs. All of the ones I have worked with have a series of sensors (usually two) that are used by the control to detect when the draw bar is open or closed, and if the tool holder is properly clamped in. With a bit of logic the control can decide if there is no tool there, a tool there, but not properly clamped, or the toolholder is in and properly clamped.

Fortunately the HSK setup doesn't have the problem ISO changers often have with a holder sticking in the taper. (You should never leave an the tool holders in an ISO taper spindle when you shut it down, they shrink as they cool and can get stuck.) Mostly because when the draw bar is opened to release the tool, it also presses the holder down and out of the taper. (There are some other reasons an HSK is less likely to stick to.) If the holder should happen to stick and the draw-bar can't eject the holder. The draw-bar open sensor will not be triggered and the tool change times out and sets an alarm.

Does your ISO spindle have comparable sensors that you should be using to monitor the steps of your tool changes?

As to writing conditional G-code, see the o-code section of the Linuxcnc manual.
linuxcnc.org/docs/html/gcode/o-code.html#ocode:conditional

Hi,
Newbie here, I have a custom program for homing and Tool Change in C/C++.
I want to replace the existing homing and Tool change to integrate my homing and tool change program with LinuxCNC .
I don't know how to approach this problem so need some help to figure it out.

Thanks and Regards
Abhishek