GETLOGIN(2) BSD System Calls Manual GETLOGIN(2)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 nor‐
mally 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(), how‐
ever 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 nor‐
mally 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-termi‐
nated 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 ALSOsetsid(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 con‐
trolling terminal. In earlier versions of the system, the value returned
by getlogin() could not be trusted without checking the user ID. Porta‐
ble programs should probably still make this check.
BSD January 6, 2009 BSD
