int getlogin_r(char *buf, size_t bufsize);
char *cuserid(char *string);
getlogin_r() returns this same user name in the array buf of size bufsize.
cuserid() returns a pointer to a string containing a user name associated with the effective user ID of the process. If string is not a null pointer, it should be an array that can hold at least L_cuserid characters; the string is returned in this array. Otherwise, a pointer to a string in a static area is returned. This string is statically allocated and might be overwritten on subsequent calls to this function or to getlogin().
The macro L_cuserid is an integer constant that indicates how long an array you might need to store a user name. L_cuserid is declared in stdio.h.
These functions let your program identify positively the user who is running (cuserid()) or the user who logged in this session (getlogin()). (These can differ when set-user-ID programs are involved.)
For most purposes, it is more useful to use the environment variable LOGNAME to find out who the user is. This is more flexible precisely because the user can set LOGNAME arbitrarily.
Linux/glibc also has
/etc/passwd password database file
/var/run/utmp (traditionally /etc/utmp; some libc versions used /var/adm/utmp)
System V has a cuserid() function which uses the real user ID rather than the effective user ID. The cuserid() function was included in the 1988 version of POSIX, but removed from the 1990 version. It was present in SUSv2, but removed in POSIX.1-2001.
OpenBSD has getlogin() and setlogin(), and a username associated with a session, even if it has no controlling tty.
Note that glibc does not follow the POSIX spec and uses stdin instead of /dev/tty. A bug. (Other recent systems, like SunOS 5.8 and HP-UX 11.11 and FreeBSD 4.8 all return the login name also when stdin is redirected.)
Nobody knows precisely what cuserid() does; avoid it in portable programs. Or avoid it altogether: use getpwuid(geteuid()) instead, if that is what you meant. DO NOT USE cuserid().