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man:getlogin

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

NAME

     getlogin, getlogin_r, cuserid - get username

SYNOPSIS

     #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(): _POSIX_C_SOURCE >= 199506L
     cuserid():
         Since glibc 2.24:
             (_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
             || __GNU_SOURCE
         Up to and including glibc 2.23:
             _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.
     ENFILE The system-wide limit on the total number of open files has been
            reached.
     ENXIO  The calling process has no controlling terminal.
     ERANGE (getlogin_r) The length of the username, including the terminat-
            ing 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.)

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
     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

     logname(1), geteuid(2), getuid(2), utmp(5)

COLOPHON

     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at
     https://www.kernel.org/doc/man-pages/.

GNU 2017-09-15 GETLOGIN(3)

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