getlogin, getlogin_r, cuserid – get user name

USAGE
       #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 stati-
       cally 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  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 (getlo-
       gin()).  (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 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 user name, including  the  termi-
              nating 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 pur-
       poses.

       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 pro-
       grams.  Or avoid it altogether:  use  getpwuid(geteuid())  instead,  if
       that is what you meant.  DO NOT USE cuserid().

SEE ALSO
       geteuid(2), getuid(2)