getlogin(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | FILES | ATTRIBUTES | CONFORMING TO | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       getlogin, getlogin_r, cuserid - get username

SYNOPSIS         top

       #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         top

       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         top

       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         top

       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         top

       /etc/passwd
              password database file

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

ATTRIBUTES         top

       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         top

       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         top

       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         top

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

COLOPHON         top

       This page is part of release 5.13 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                            2021-03-22                    GETLOGIN(3)