Add notes on the different system call service routines that have

appeared over time.

man2/stat.2

@@ -416,6 +416,31 @@ directory,
 does not return the file size in the
 .I st_size
 field; instead the field is returned with the value 0.
+.SS Underlying kernel interface
+Over time, increases in the size of the
+.I stat
+structure have led to three successive implementations of
+.BR stat ():
+.IR sys_stat ()
+(slot
+.IR __NR_oldstat ),
+.IR sys_newstat ()
+(slot
+.IR __NR_stat ),
+and
+.IR sys_stat64()
+(new in kernel 2.4; slot
+.IR __NR_stat64 ),
+with the last being the most current.
+Glibc hides these changes from applications and ensures that
+binary compatability is not broken for old binaries that
+call the
+.BR stat ()
+library wrapper function.
+Similar remarks apply for
+.BR fstat (2)
+and
+.BR lstat (2).
 .SH EXAMPLE
 The following program calls
 .BR stat (2)