Add notes on the different system call interfaces that

have appeared over time.

man2/uname.2

@@ -20,7 +20,9 @@
 .\" Formatted or processed versions of this manual, if unaccompanied by
 .\" the source, must acknowledge the copyright and authors of this work.
 .\"
-.TH UNAME 2 2001-12-15 "Linux" "Linux Programmer's Manual"
+.\" 2007-07-05 mtk: Added details on underlying system call interfaces
+.\"
+.TH UNAME 2 2007-07-05 "Linux" "Linux Programmer's Manual"
 .SH NAME
 uname \- get name and information about current kernel
 .SH SYNOPSIS
@@ -109,12 +111,6 @@ Clearly, it is a bad
 idea to use any of these constants; just use sizeof(...).
 Often 257 is chosen in order to have room for an internet hostname.
 .LP
-There have been three Linux system calls
-.BR uname ().
-The first one
-used length 9, the second one used 65, the third one also uses 65 but
-adds the \fIdomainname\fP field.
-.LP
 Part of the utsname information is also accessible via
 .BR sysctl (2)
 and via
@@ -123,6 +119,31 @@ and via
 .IR osrelease ,
 .IR version ,
 .IR domainname }.
+.SS Underlying kernel interface
+.LP
+Over time, increases in the size of the
+.I utsuname
+structure have led to three successive implementations of
+.BR uname ():
+.IR sys_olduname ()
+(slot
+.IR __NR_oldolduname ),
+.IR sys_uname ()
+(slot
+.IR __NR_olduname ),
+and
+.IR sys_newuname ()
+(slot
+.IR __NR_uname).
+The first one used length 9 for all fields;
+the second used 65;
+the third also uses 65 but adds the \fIdomainname\fP field.
+The glibc
+.BR uname ()
+wrapper function hides these details from applications,
+ensuring that new binaries use the current implementation,
+and that binary compatibility is not broken
+for older binaries.
 .SH "SEE ALSO"
 .BR uname (1),
 .BR getdomainname (2),