getlogin returns a pointer to a string containing the name of the user logged in on the controlling terminal of the process, or a null pointer if this information cannot be determined. The string is statically allocated and might be overwritten on subsequent calls to this function or to cuserid.
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 setuid 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.
getlogin returns a pointer to the user name when successful, and NULL on failure. getlogin_r returns 0 when successful, and nonzero on failure.