#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

       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-

       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

       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.

       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.

       POSIX specifies

       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.)

              password database file

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

   Multithreading (see pthreads(7))
       The getlogin() function is not thread-safe.

       The getlogin_r() function is thread-safe.

       The cuserid() function is  thread-safe  with  exceptions.   It  is  not
       thread-safe if called with a NULL parameter.

       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 terminal.

       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
       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().

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

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