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
   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 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
   (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 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.
   getlogin_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
          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.)

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.
   getlogin() 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
   programs.  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 4.09 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/.

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