change Linux users run time, from a c/c++ code

I want to change Linux users run time, from a c/c++ code. How could I do the same?

Lets say, I am running a c/c++ binary from a Linux user "abc" which is a non root user. Inside the c/c++ code I want to switch over to user a "xyz" to perform a specific task, and then regain the privileges of "abc" back.