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RedHat 9 (Linux i386) - man page for cuserid (redhat section 3)

GETLOGIN(3)				      Linux Programmer's Manual 				  GETLOGIN(3)

getlogin, cuserid - get user name
#include <unistd.h> char *getlogin(void); #include <stdio.h> char *cuserid(char *string);
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 allo- cated and might be overwritten on subsequent calls to this function or to cuserid. 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.
ENOMEM Insufficient memory to allocate passwd structure.
/etc/passwd password database file /var/run/utmp (traditionally /etc/utmp; some libc versions used /var/adm/utmp)
POSIX.1. 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.
Unfortunately, it is often rather easy to fool getlogin(). Sometimes it does not work at all, because some program messed up the utmp file. Often, it gives only the first 8 characters of the login name. The user cur- rently logged in on the controlling tty of our program need not be the user who started it. Avoid getlogin() for security-related purposes. Nobody knows precisely what cuserid() does - avoid it in portable programs - avoid it altogether - use getp- wuid(geteuid()) instead, if that is what you meant. DO NOT USE cuserid().
geteuid(2), getuid(2) Linux 1.2.13 1995-09-03 GETLOGIN(3)

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