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           #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
           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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