plucky (3) cuserid.3.gz

NAME

       getlogin, getlogin_r, cuserid - get username

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <unistd.h>

       char *getlogin(void);
       int getlogin_r(char buf[.bufsize], 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 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.

VERSIONS

       OpenBSD has getlogin() and setlogin(), and a username associated with  a  session,  even  if  it  has  no
       controlling terminal.

STANDARDS

       getlogin()
       getlogin_r()
              POSIX.1-2008.

       cuserid()
              None.

STANDARDS

       getlogin()
       getlogin_r():
              POSIX.1-2001.  OpenBSD.

       cuserid()
              System V, POSIX.1-1988.  Removed in POSIX.1-1990.  SUSv2.  Removed in POSIX.1-2001.

              System V has a cuserid() function which uses the real user ID rather than the effective user ID.

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)