getlogin, setlogin - get/set login name
#include <unistd.h>
char *
getlogin(void);
int
getlogin_r(char *name, size_t namelen);
int
setlogin(const char *name);
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.
If a call to getlogin() succeeds, it returns a pointer to a
null-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.
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.
setsid(2)
The getlogin() function first appeared in 4.2BSD.
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.
OpenBSD 3.6 June 9, 1993
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