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CentOS 7.0 - man page for getlogin (centos section 3)

GETLOGIN(3)			    Linux Programmer's Manual			      GETLOGIN(3)

       getlogin, getlogin_r, cuserid - get username

       #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  statically 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 bufsize.

       cuserid() returns a pointer to a string containing a username associated with  the  effec-
       tive  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 (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  arbi-

       getlogin()  returns  a  pointer to the username when successful, and NULL on failure, with
       errno set to indicate the cause of the error.  getlogin_r() returns 0 when successful, and
       nonzero on failure.

       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 terminal.

       ERANGE (getlogin_r)  The  length  of  the  username,  including	the terminating null byte
	      ('\0'), 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.

       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 char-
       acters 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 pur-

       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 programs.   Or	avoid  it
       altogether:  use  getpwuid(geteuid())  instead,	if  that  is  what you meant.  Do not use

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

       This page is part of release 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at

GNU					    2013-04-19				      GETLOGIN(3)

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