GETLOGIN(2)

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

LIBRARY
libc

SYNOPSIS
#include <unistd.h>
char *
getlogin(void);
#include <sys/param.h>
char *
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 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).

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 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. 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 background.

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 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.

RETURN VALUES
If a call to getlogin() succeeds, it returns a pointer to a null-terminated string in a static buffer. getlogin_r() returns a pointer to the buffer passed in by the caller on success. Both return NULL if the name has not been set.

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
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.

SEE ALSO
setsid(2), daemon(3)

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.

HISTORY
The getlogin() function first appeared in 4.4BSD.