GETLOGIN(2)

NAME
getlogin, getlogin_r, setlogin - get or set login name

SYNOPSIS
#include <unistd.h>
char *
getlogin(void);
int
getlogin_r(char *name, size_t namelen);
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 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() routine is a reentrant version of getlogin(). It is functionally identical to getlogin() except that the caller must provide a buffer, name, in which to store the user's login name and a corresponding length parameter, namelen, that specifies the size of the buffer. The buffer should be large enough to store the login name and a trailing NUL (typically LOGIN_NAME_MAX bytes).

setlogin() sets the login name of the user associated with the current session to name. This call is restricted to the superuser, 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 setpgrp(...) 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 NUL-terminated string in a static buffer. If the name has not been set, it returns NULL. If a call to getlogin_r() succeeds, a value of 0 is returned, else the error number is returned. 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.

ERRORS
getlogin_r() and setlogin() will succeed unless:

[EFAULT]
    The name argument points to an invalid address.

In addition, getlogin_r() may return the following error:

[ERANGE]
    The value of namelen is not large enough to store the user's login name and a trailing NUL.

setlogin() may return the following errors:

[EINVAL]
    The name argument pointed to a string that was too long. Login names are limited to LOGIN_NAME_MAX-1 characters, currently 31.

[EPERM]
    The caller tried to set the login name and was not the superuser.

SEE ALSO
setsid(2)

STANDARDS
The getlogin() and getlogin_r() functions conform to p1003.1-2008.

HISTORY
A getlogin() function which used utmp(5) first appeared in AT&T UNIX. The getlogin() and setlogin() system calls first appeared in 4.3BSD Reno.

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 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.