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);
#include <sys/param.h>
int
getlogin_r(char *name, int 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).
getlogin_r() provides the same service as getlogin() except 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. Making a setsid() system call is the ONLY way
to do this. The daemon() library call calls setsid() which is an ideal
way of detaching from a controlling terminal and forking into the back‐
ground.
In particular, doing a ioctl(ttyfd, TIOCNOTTY, ...) or setpgrp(...) is
NOT sufficient.
Once a parent process does a setsid() call, it is acceptable for some
child of that process to then do a setlogin() even though it is not the
session leader, but beware that ALL processes in the session will change
their login name at the same time, even the parent.
This is not the same as the traditional UNIX behavior of inheriting priv‐
ilege.
Since the setlogin() system call 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, or NULL if the name has not been set.
getlogin_r() returns zero if successful, or the error number upon fail‐
ure.
The setlogin() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
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 17 including
null.
[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), daemon(3)STANDARDSgetlogin() and getlogin_r() conform to ISO/IEC 9945-1:1996 (“POSIX.1”).
HISTORY
The getlogin() function first appeared in 4.4BSD. The return value of
getlogin_r() was changed from earlier versions of FreeBSD to be confor‐
mant with ISO/IEC 9945-1:1996 (“POSIX.1”).
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 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 June 9, 1993 BSD
