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RedHat 9 (Linux i386) - man page for getlogin (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 allocated 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  arbi-

       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  char-
       acters  of the login name. The user currently logged in on the controlling tty of our pro-
       gram 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  getpwuid(geteuid())	instead,  if  that is what you meant.  DO NOT USE

       geteuid(2), getuid(2)

Linux 1.2.13				    1995-09-03				      GETLOGIN(3)

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