int getlogin_r(char *buf, size_t bufsize);
char *cuserid(char *string);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
getlogin_r() returns this same username in the array buf of size bufsize.
cuserid() returns a pointer to a string containing a username 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 username. 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.)
Linux/glibc also has
MT-Unsafe race:getlogin race:utent
sig:ALRM timer locale
MT-Unsafe race:utent sig:ALRM timer
|cuserid()||Thread safety||MT-Unsafe race:cuserid/!string locale|
In the above table, utent in race:utent signifies that if any of the functions setutent(3), getutent(3), or endutent(3) are used in parallel in different threads of a program, then data races could occur. getlogin() and getlogin_r() call those functions, so we use race:utent to remind users.
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.
Note that glibc does not follow the POSIX specification 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.)