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



NAME
       getlogin, getlogin_r, cuserid - get username

SYNOPSIS
       #include <unistd.h>

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

       #include <stdio.h>

       char *cuserid(char *string);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       getlogin_r(): _REENTRANT || _POSIX_C_SOURCE >= 199506L
       cuserid(): _XOPEN_SOURCE

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 statically allocated
       and might be overwritten on subsequent calls to this function or to cuserid().

       getlogin_r() returns this same username in the array buf of size bufsize.

       cuserid() returns a pointer to a string containing a username associated with the effective user ID of the pro-
       cess.  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 username.
       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 set-user-ID 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 username when successful, and NULL on failure.  getlogin_r() returns 0 when
       successful, and non-zero on failure.

ERRORS
       POSIX specifies

       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.

       ENXIO  The calling process has no controlling tty.

       ERANGE (getlogin_r) The length of the username, including the terminating null byte, is larger than bufsize.

       Linux/glibc also has

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

       ENOMEM Insufficient memory to allocate passwd structure.

       ENOTTY Standard input didn't refer to a terminal.  (See BUGS.)

FILES
       /etc/passwd
              password database file

       /var/run/utmp
              (traditionally /etc/utmp; some libc versions used /var/adm/utmp)

CONFORMING TO
       getlogin() and getlogin_r() specified in POSIX.1-2001.

       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.  It was present in
       SUSv2, but removed in POSIX.1-2001.

       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 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.

       Note that glibc does not follow the POSIX specification and uses stdin instead of  /dev/tty.   A  bug.   (Other
       recent  systems,  like  SunOS  5.8 and HP-UX 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 programs.  Or avoid it altogether:  use  getp-
       wuid(geteuid()) instead, if that is what you meant.  Do not use cuserid().

SEE ALSO
       geteuid(2), getuid(2), utmp(5)

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                               2008-06-29                       GETLOGIN(3)