cuserid

       #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 stati-
       cally 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 buf-
       size.

       cuserid() returns a pointer to a string containing a  username  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 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 (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 username when successful, and  NULL
       on  failure, with errno set to indicate the cause of the error.  getlo-
       gin_r() returns 0 when successful, and nonzero on failure.

ERRORS
       POSIX specifies

       EMFILE The per-process limit on the number of open file descriptors has
              been reached.

       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)

ATTRIBUTES
       For   an   explanation   of   the  terms  used  in  this  section,  see
       attributes(7).

       +-------------+---------------+---------------------------------------+
       |Interface    | Attribute     | Value                                 |
       +-------------+---------------+---------------------------------------+
       |getlogin()   | Thread safety | MT-Unsafe race:getlogin race:utent    |
       |             |               | sig:ALRM timer locale                 |
       +-------------+---------------+---------------------------------------+
       |getlogin_r() | Thread safety | MT-Unsafe race:utent sig:ALRM timer   |
       |             |               | locale                                |
       +-------------+---------------+---------------------------------------+
       |cuserid()    | Thread safety | MT-Unsafe race:cuserid/!string locale |
       +-------------+---------------+---------------------------------------+
       In the above table, utent in race:utent signifies that if  any  of  the
       functions setutent(3), getutent(3), or endutent(3) are used in parallel
       in different threads of a program, then data races could occur.  getlo-
       gin()  and  getlogin_r()  call those functions, so we use race:utent to
       remind users.

CONFORMING TO
       getlogin() and getlogin_r(): POSIX.1-2001, POSIX.1-2008.

       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 terminal.

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 terminal 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
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       http://www.kernel.org/doc/man-pages/.

GNU                               2015-12-28                       GETLOGIN(3)
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