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GETLOGIN(2)		  FreeBSD System Calls Manual		   GETLOGIN(2)

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

     Standard C	Library	(libc, -lc)

     #include <unistd.h>

     char *

     #include <sys/param.h>

     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 session, as previously	set by setlogin().  The	name is	nor-
     mally 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
     NOT sufficient.

     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 priv-

     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-termi-
     nated 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 33	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
			be returned.

     setsid(2),	daemon(3)

     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 confor-
     mant 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 con-
     trolling terminal.	 In earlier versions of	the system, the	value returned
     by	getlogin() could not be	trusted	without	checking the user ID.  Porta-
     ble programs should probably still	make this check.

FreeBSD	13.0		       September 9, 2020		  FreeBSD 13.0


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