Document the getlogin_r function.

lib/libc/sys/getlogin.2

@@ -37,12 +37,16 @@
 .Os BSD 4.4
 .Sh NAME
 .Nm getlogin ,
+.Nm getlogin_r ,
 .Nm setlogin
 .Nd get/set login name
 .Sh SYNOPSIS
 .Fd #include <unistd.h>
 .Ft char *
 .Fn getlogin void
+.Fd #include <sys/param.h>
+.Ft char *
+.Fn getlogin_r "char *name" "int len"
 .Ft int
 .Fn setlogin "const char *name"
 .Sh DESCRIPTION
@@ -60,6 +64,18 @@ for example when
 .Xr su 1
 is used).
 .Pp
+.Fn getlogin_r
+provides the same service as
+.Fn getlogin
+except the caller must provide the buffer
+.Fa name
+with length
+.Fa len
+bytes
+to hold the result.  The buffer should be at least
+.Dv MAXLOGNAME
+bytes in length.
+.Pp
 .Fn Setlogin
 sets the login name of the user associated with the current session to
 .Fa name .
@@ -115,8 +131,12 @@ precautions to prevent security violations.
 If a call to
 .Fn getlogin
 succeeds, it returns a pointer to a null-terminated string in a static buffer.
-If the name has not been set, it returns
-.Dv NULL .
+.Fn getlogin_r
+returns a pointer to the buffer passed in by the caller on success.
+Both return
+.Dv NULL
+if the name has not been set.
+.Pp
 If a call to
 .Fn setlogin
 succeeds, a value of 0 is returned.  If