NAME
getlogin,
getlogin_r,
setlogin —
get/set login name
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
char *
getlogin(
void);
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(), however 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.
setlogin() sets the login name of the user associated with the
current session to
name. This 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. 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
setpgid(
...) 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
null-terminated string in a static buffer. If the name has not been set, it
returns
NULL
.
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
getlogin_r() function returns zero if successful, or the
error number upon failure.
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
(from
⟨sys/param.h⟩) characters, currently
16.
-
-
- [
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)
STANDARDS
The
getlogin() and
getlogin_r() functions
conform to
ISO/IEC 9945-1:1996
(“POSIX.1”).
HISTORY
The
getlogin() function first appeared in
4.4BSD.
BUGS
Login names are limited in length by
setlogin(). However,
lower limits are placed on login names elsewhere in the system
(
UT_NAMESIZE
in
⟨
utmp.h⟩).
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.