GETLOGIN(2) BSD System Calls Manual GETLOGIN(2)
getlogin, getlogin_r, setlogin -- get/set login name
Standard C Library (libc, -lc)
getlogin_r(char *name, size_t len);
setlogin(const char *name);
The getlogin() routine returns the login name of the user associated with the current ses-
sion, 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 cre-
ate 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-
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
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.
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.
The getlogin() and getlogin_r() functions conform to ISO/IEC 9945-1:1996 (``POSIX.1'').
The getlogin() function first appeared in 4.4BSD.
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.
BSD January 6, 2009 BSD