The cuserid() function generates a character-string representation of the login name under which the owner of the current process is logged in. If s is a
null pointer, this representation is generated in an internal static area whose address is returned. Otherwise, s is assumed to point to an array of at least L_cuserid
characters; the representation is left in this array. The constant L_cuserid is defined in the <stdio.h> header.
In multithreaded applications, the caller must always supply an array s for the return value.