getlogin() c function returns NULL and error "No such file or directory"

Here is a good link I found explaining that it may not work: getlogin

Here is a quote from it:

Unfortunately, it is often rather easy to fool getlogin(). Sometimes it does not work at all, because some program messed up the utmp file

getlogin is an unsafe and deprecated way of determining the logged-in user. It's probably trying to open a record of logged-in users, perhaps utmp or something. The correct way to determine the user you're running as (which might not be the same as the logged-in user, but is almost always better to use anyway) is getpwuid(getuid()).