Getting the file location with extension

Hello All!
I suppose I have two applications such as app1.exe in C:\ and app2.exe in D:\ .
When I open appl.exe in C:\ ,   app1.exe call in app2.exe and then app2.exe will run
and app2.exe will give the file location of app1.exe with extension such as C:\app1.exe in run time.
Here I wanna know how to reveal the location of calling application (app1.exe) with extension from app2.exe.
I mean that app2.exe is called by app1.exe and when app2.exe is running, I wanna reveal the location of app1.exe
with extension from app2.exe.
Please give me any suggestions for that matter.
I'll thanks a million...