libc/sys/getlogin.2

.\"	$OpenBSD: getlogin.2,v 1.17 2005/02/25 03:12:44 cloder Exp $
.\"	$NetBSD: getlogin.2,v 1.4 1995/02/27 12:33:03 cgd Exp $
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.\"	@(#)getlogin.2	8.1 (Berkeley) 6/9/93
.\"
.Dd June 9, 1993
.Dt GETLOGIN 2
.Os
.Sh NAME
.Nm getlogin ,
.Nm setlogin
.Nd get/set login name
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft char *
.Fn getlogin void
.Ft int
.Fn getlogin_r "char *name" "size_t namelen"
.Ft int
.Fn setlogin "const char *name"
.Sh DESCRIPTION
The
.Fn getlogin
routine returns the login name of the user associated with the current
session, as previously set by
.Fn 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
.Xr su 1
is used.)
.Pp
The
.Fn getlogin_r
routine is a reentrant version of
.Fn getlogin .
It is functionally identical to
.Fn getlogin
except that the caller must provide a buffer,
.Fa name ,
in which to store the user's login name and a corresponding
length parameter,
.Fa namelen ,
that specifies the size of the buffer.
The buffer should be large enough to store the login name and a trailing NUL
(typically
.Ev LOGIN_NAME_MAX
bytes).
.Pp
.Fn setlogin
sets the login name of the user associated with the current session to
.Fa 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).
.Pp
.Em NOTE :
There is only one login name per session.
.Pp
It is
.Em CRITICALLY
important to ensure that
.Fn setlogin
is only ever called after the process has taken adequate steps to ensure
that it is detached from its parent's session.
The
.Em ONLY
way to do this is via the
.Fn setsid
function.
The
.Fn daemon
function calls
.Fn setsid
which is an ideal way of detaching from a controlling terminal and
forking into the background.
.Pp
In particular, neither
.Fn ioctl ttyfd TIOCNOTTY ...\&
nor
.Fn setpgrp ...\&
is sufficient to create a new session.
.Pp
Once a parent process has called
.Fn setsid ,
it is acceptable for some child of that process to then call
.Fn setlogin ,
even though it is not the session leader.
Beware, however, that
.Em ALL
processes in the session will change their login name at the same time,
even the parent.
.Pp
This is different from traditional
.Ux
privilege inheritance and as such can be counter-intuitive.
.Pp
Since the
.Fn 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.
.Sh RETURN VALUES
If a call to
.Fn getlogin
succeeds, it returns a pointer to a NUL-terminated string in a static buffer.
If the name has not been set, it returns
.Dv NULL .
If a call to
.Fn getlogin_r
succeeds, a value of 0 is returned, else the error number is returned.
If a call to
.Fn setlogin
succeeds, a value of 0 is returned.
If
.Fn setlogin
fails, a value of \-1 is returned and an error code is
placed in the global location
.Va errno .
.Sh ERRORS
The following errors may be returned by these calls:
.Bl -tag -width Er
.It Bq Er EFAULT
The
.Fa name
parameter gave an
invalid address.
.It Bq Er EINVAL
The
.Fa name
parameter
pointed to a string that was too long.
Login names are limited to
.Dv MAXLOGNAME-1
(from
.Ao Pa sys/param.h Ac )
characters, currently 31.
.It Bq Er EPERM
The caller tried to set the login name and was not the superuser.
.It Bq Er ERANGE
The buffer passed to
.Fn getlogin_r
is not large enough to store the user's login name.
.El
.Sh SEE ALSO
.Xr setsid 2
.Sh HISTORY
The
.Fn getlogin
function first appeared in
.Bx 4.2 .
.Sh BUGS
In earlier versions of the system,
.Fn getlogin
failed unless the process was associated with a login terminal.
The current implementation (using
.Fn setlogin )
allows getlogin to succeed even when the process has no controlling terminal.
In earlier versions of the system, the value returned by
.Fn getlogin
could not be trusted without checking the user ID.
Portable programs should probably still make this check.