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 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.
The setlogin system call sets the login name of the user associated with the current session to name. This system 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 parents session. Making a setsid system call is the ONLY way to do this. The daemon(3) function calls setsid which is an ideal way of detaching from a controlling terminal and forking into the background.
In particular, doing a ioctl ttyfd TIOCNOTTY ... or setpgrp ... is NOT sufficient.
Once a parent process does a setsid system 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 privilege.
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.
If a call to getlogin succeeds, it returns a pointer to a null-terminated string in a static buffer, or NULL if the name has not been set. The getlogin_r function returns zero if successful, or the error number upon failure.
.Rv -std setlogin
The following errors may be returned by these calls: