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FreeBSD 11.0 - man page for cuserid (freebsd section 3)

CUSERID(3)						   BSD Library Functions Manual 						CUSERID(3)

cuserid -- get user name associated with effective UID
Compatibility Library (libcompat, -lcompat)
#include <stdio.h> char * cuserid(char *s);
The cuserid() function is made obsolete by getpwuid(3). The function cuserid() gets the user name associated with the effective UID of the current process. If the argument s is non-NULL, the name is copied to the buffer it points to, and that address is being returned. This buffer must provide space for at least L_cuserid characters. The L_cuserid constant is defined in <stdio.h>. If s is NULL, an internal array is used and its address will be returned.
The cuserid() function returns the address of an array in which the name has been stored. If the name associated with the effective UID of the current process could not be found, either a null pointer will be returned, or (if s is non-NULL) the buffer s will be filled with a null string.
geteuid(2), getpwuid(3)
April 10, 1995 BSD