cuserid

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



NAME
       getlogin, getlogin_r, cuserid - get user name

SYNOPSIS
       #include <unistd.h>

       char *getlogin(void);
       int getlogin_r(char *buf, size_t bufsize);

       #include <stdio.h>

       char *cuserid(char *string);

DESCRIPTION
       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 stat-
       ically 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 buf-
       size.

       cuserid returns a pointer to a string containing a user	name  associ-
       ated  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 stat-
       ically  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.

RETURN VALUE
       getlogin returns a pointer to the user name when successful, and	 NULL
       on  failure.   getlogin_r  returns  0  when successful, and nonzero on
       failure.

ERRORS
       POSIX specifies

       ENXIO  The calling process has no controlling tty.

       EMFILE The calling process already has the maximum allowed  number  of
	      open files.

       ENFILE The  system  already  has	 the  maximum  allowed number of open
	      files.

       ERANGE (getlogin_r) The length of the user name, including final	 NUL,
	      is larger than bufsize.

       Linux/glibc also has

       ENOMEM Insufficient memory to allocate passwd structure.

       ENOENT There was no corresponding entry in the utmp-file.

FILES
       /etc/passwd    password database file
       /var/run/utmp  (traditionally /etc/utmp;
		      some libc versions used /var/adm/utmp)

CONFORMING TO
       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.

       OpenBSD	has  getlogin  and setlogin, and a username associated with a
       session, even if it has no controlling tty.

BUGS
       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  currently	logged	in on the controlling tty of our program need
       not be the user who started it.	Avoid getlogin() for security-related
       purposes.

       Note  that glibc does not follow the POSIX spec and uses stdin instead
       of /dev/tty.  A bug. (Other recent systems, like SunOS  5.8  and	 HPUX
       11.11  and  FreeBSD  4.8	 all return the login name also when stdin is
       redirected.)

       Nobody knows precisely what cuserid() does - avoid it in portable pro-
       grams - avoid it altogether - use getpwuid(geteuid()) instead, if that
       is what you meant.  DO NOT USE cuserid().

SEE ALSO
       geteuid(2), getuid(2)




Linux 2.4			  2003-08-24			  GETLOGIN(3)