GETLOGIN(2) System Calls Manual GETLOGIN(2)
getlogin, getlogin_r, setlogin -- get/set login name
Standard C Library (libc, -lc)
getlogin_r(char *name, size_t len);
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() 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
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
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.
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
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
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
[ERANGE] The size of the buffer is smaller than the result to
The getlogin() and getlogin_r() functions conform to ISO/IEC 9945-1:1996
The getlogin() function first appeared in 4.4BSD.
Login names are limited in length by setlogin(). However, lower limits
are placed on login names elsewhere in the system (UT_NAMESIZE in
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.
NetBSD 6.1.5 January 6, 2009 NetBSD 6.1.5