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.