NAME
getlogin,
getlogin_r, setlogin
— get or set login
name
SYNOPSIS
#include
<unistd.h>
char *
getlogin(void);
int
getlogin_r(char
*name, size_t
namelen);
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()
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 UNIX 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
getlogin_r() and
setlogin() will succeed unless:
- [
EFAULT] - The name argument points to an invalid address.
In addition, getlogin_r() may return the
following error:
- [
ERANGE] - The value of namelen is not large enough to store the user's login name and a trailing NUL.
setlogin() may return the following
errors:
- [
EINVAL] - The name argument pointed to a string that was too
long. Login names are limited to
LOGIN_NAME_MAX-1characters, currently 31. - [
EPERM] - The caller tried to set the login name and was not the superuser.
SEE ALSO
STANDARDS
The getlogin() and
getlogin_r() functions conform to
IEEE Std 1003.1-2008 (“POSIX.1”).
HISTORY
A getlogin() function which used
utmp(5) first
appeared in Version 7 AT&T UNIX. The
getlogin() and setlogin()
system calls first appeared in 4.3BSD-Reno.
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.