NAME

getlogin, getlogin_r, setlogin — get/set login name

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <unistd.h>

char *
getlogin(void);

int
getlogin_r(char *name, size_t 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.)

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.

RETURN VALUES

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 returns NULL.

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 upon failure.

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 16.

[EPERM]

The caller tried to set the login name and was not the super-user.

[ERANGE]

The size of the buffer is smaller than the result to be returned.

SEE ALSO

setsid(2)

STANDARDS

The getlogin() and getlogin_r() functions conform to ISO/IEC 9945-1:1996 (“POSIX.1”).

HISTORY

The getlogin() function first appeared in 4.4BSD.

BUGS

Login names are limited in length by setlogin(). However, lower limits are placed on login names elsewhere in the system (UT_NAMESIZE in <utmp.h>).

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.