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       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(): _POSIX_C_SOURCE >= 199506L

           Since glibc 2.24:
               (_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
               || _GNU_SOURCE
           Up to and including glibc 2.23:


       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


       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 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.)


              password database file

              (traditionally /etc/utmp; some libc versions used /var/adm/utmp)


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue                                 │
       │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.


       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.


       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

       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


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


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