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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);

   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  user  name  in the array buf of size
       bufsize.

       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  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 user name when successful, and NULL
       on failure.  getlogin_r() returns 0 when  successful,  and  nonzero  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  user  name,  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.

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 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 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 getpwuid(geteuid()) instead,  if
       that is what you meant.  Do not use cuserid().

SEE ALSO

       geteuid(2), getuid(2)

COLOPHON

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