GETLOGIN(2)               FreeBSD System Calls Manual              GETLOGIN(2)

NAME
     getlogin, getlogin_r, setlogin - get/set login name

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     char *
     getlogin(void);

     #include <sys/param.h>

     int
     getlogin_r(char *name, size_t len);

     int
     setlogin(const char *name);

DESCRIPTION
     The getlogin() routine returns the login name of the user associated with
     the current session, as previously set by setlogin().  The name is
     normally associated with a login shell at the time a session is created,
     and is inherited by all processes descended from the login shell.  (This
     is true even if some of those processes assume another user ID, for
     example when su(1) is used).

     The getlogin_r() function provides the same service as getlogin() except
     the caller must provide the buffer name with length len bytes to hold the
     result.  The buffer should be at least MAXLOGNAME bytes in length.

     The setlogin() system call sets the login name of the user associated
     with the current session to name.  This system call is restricted to the
     super-user, and is normally used only when a new session is being created
     on behalf of the named user (for example, at login time, or when a remote
     shell is invoked).

     NOTE: There is only one login name per session.

     It is CRITICALLY important to ensure that setlogin() is only ever called
     after the process has taken adequate steps to ensure that it is detached
     from its parent's session.  Making a setsid() system call is the ONLY way
     to do this.  The daemon(3) function calls setsid() which is an ideal way
     of detaching from a controlling terminal and forking into the background.

     In particular, doing a ioctl(ttyfd, TIOCNOTTY, ...) or setpgrp(...) is
     NOT sufficient.

     Once a parent process does a setsid() system call, it is acceptable for
     some child of that process to then do a setlogin() even though it is not
     the session leader, but beware that ALL processes in the session will
     change their login name at the same time, even the parent.

     This is not the same as the traditional UNIX behavior of inheriting
     privilege.

     Since the setlogin() system call is restricted to the super-user, it is
     assumed that (like all other privileged programs) the programmer has
     taken adequate precautions to prevent security violations.

RETURN VALUES
     If a call to getlogin() succeeds, it returns a pointer to a null-
     terminated string in a static buffer, or NULL if the name has not been
     set.  The getlogin_r() function returns zero if successful, or the error
     number upon failure.

     The setlogin() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS
     The following errors may be returned by these calls:

     [EFAULT]           The name argument gave an invalid address.

     [EINVAL]           The name argument pointed to a string that was too
                        long.  Login names are limited to MAXLOGNAME (from
                        <sys/param.h>) characters, currently 33 including
                        null.

     [EPERM]            The caller tried to set the login name and was not the
                        super-user.

     [ERANGE]           The size of the buffer is smaller than the result to
                        be returned.

SEE ALSO
     setsid(2), daemon(3)

STANDARDS
     The getlogin() system call and the getlogin_r() function conform to
     ISO/IEC 9945-1:1996 ("POSIX.1").

HISTORY
     The getlogin() system call first appeared in 4.4BSD.  The return value of
     getlogin_r() was changed from earlier versions of FreeBSD to be
     conformant with ISO/IEC 9945-1:1996 ("POSIX.1").

BUGS
     In earlier versions of the system, getlogin() failed unless the process
     was associated with a login terminal.  The current implementation (using
     setlogin()) allows getlogin to succeed even when the process has no
     controlling terminal.  In earlier versions of the system, the value
     returned by getlogin() could not be trusted without checking the user ID.
     Portable programs should probably still make this check.

FreeBSD 13.1-RELEASE-p6        September 9, 2020       FreeBSD 13.1-RELEASE-p6