How to use win32 CreateProcess function to wait until the child done to write to file

Question

Hello im not win32 programmer and its all new to me. i like to open process from my parent win32 application ( ok this is i know how to do) the child process then write to text file and close it self . how can i detect in the parent application that the child application done writing to the text file . and then from the parent app to read the text file . this is all in win32 c++ Thanks