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getlogin, getlogin_r, setlogin — get/set login name
Standard C Library (libc, -lc)
getlogin_r(char *name, int len);
setlogin(const char *name);
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 parent's 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
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
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.
The setlogin() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
The following errors may be returned by these calls:
[EFAULT] The name argument gave an invalid address.
[EINVAL] The name argument pointed to a string that was too
long. Login names are limited to MAXLOGNAME (from
<sys/param.h>) characters, currently 17 including
[EPERM] The caller tried to set the login name and was not the
[ERANGE] The size of the buffer is smaller than the result to
The getlogin() system call and the getlogin_r() function conform to
ISO/IEC 9945-1:1996 (“POSIX.1”).
The getlogin() system call first appeared in 4.4BSD. The return value of
getlogin_r() was changed from earlier versions of FreeBSD to be
conformant with ISO/IEC 9945-1:1996 (“POSIX.1”).
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.