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();
routine is a reentrant version of
getlogin();.
It is functionally identical to
getlogin();
except that the caller must provide a buffer,
name,
in which to store the user's login name and a corresponding
length parameter,
namelen,
that specifies the size of the buffer.
The buffer should be large enough to store the login name and a trailing NUL
(typically
LOGIN_NAME_MAX
bytes).
setlogin();
sets the login name of the user associated with the current session to
name.
This call is restricted to the superuser, 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.
The
ONLY
way to do this is via the
setsid();
function.
The
daemon();
function calls
setsid();
which is an ideal way of detaching from a controlling terminal and
forking into the background.
In particular, neither
ioctl(ttyfd TIOCNOTTY ...\&);
nor
setpgrp(...\&);
is sufficient to create a new session.
Once a parent process has called
setsid();,
it is acceptable for some child of that process to then call
setlogin();,
even though it is not the session leader.
Beware, however, that
ALL
processes in the session will change their login name at the same time,
even the parent.
This is different from traditional
.Ux
privilege inheritance and as such can be counter-intuitive.
Since the
setlogin();
routine 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 NUL-terminated string in a static buffer.
If the name has not been set, it returns
NULL.
If a call to
getlogin_r();
succeeds, a value of 0 is returned, else the error number is returned.
If a call to
setlogin();
succeeds, a value of 0 is returned.
If
setlogin();
fails, a value of \-1 is returned and an error code is
placed in the global location
errno.
ERRORS
The following errors may be returned by these calls:
[EFAULT]
The
name
parameter gave an
invalid address.
[EINVAL]
The
name
parameter
pointed to a string that was too long.
Login names are limited to
MAXLOGNAME-1
(from
‹sys/param.h ›)
characters, currently 31.
[EPERM]
The caller tried to set the login name and was not the superuser.
[ERANGE]
The buffer passed to
getlogin_r();
is not large enough to store the user's login name.
The
getlogin();
function first appeared in
4.2BSD.
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.