The user name will be in the environment and you can get it with getenv(). The environment variable will be called either USER or LOGNAME. (Posix requires LOGNAME) Whichever variable your system uses, it is set by the login program. And a decent cron will also set it for cron and "at" jobs. It won't get set if the user invokes your program via a remote service that did not properly log him in. But a clever user can set the env variable himself in that case. Which bring up a problem....a user could also clobber the variable, intentionally or otherwise.

Another approach is the inspect fd's 0, 1, and 2. They may be connected to the user's tty. isatty() can tell you if they are tty's or not. And ttyname() can provide the name of the tty. Once you have that, you can search the utmp file via getutmp() to get the user name as recorded by the login (or similiar) program. Again, not all remote services will set a utmp entry. And this will fail for cron or "at" jobs. This is how "who am i" works. And it is why
who am i < /dev/null 2 > /dev/null | cat
fails.

Another approach is to get the uid of the process with getuid() and then scan the passwd file via getpwnam(). This is how the program whoami works. But if user A signs on and su's to user B and then runs your program, you get user B.

As for the password, you can't get that. And I wonder why you want it.