svn commit: r305022 - head/lib/libc/sys

[Konstantin Belousov]

Author: kib
Date: Mon Aug 29 19:33:07 2016
New Revision: 305022
URL: https://svnweb.freebsd.org/changeset/base/305022

Log:
  Restore the requirement of setting errno to zero before calling
  ptrace(2).  Describe the behaviour of automatically zeroing errno as
  historical feature.
  
  Requested by:	ache, jhb
  Reviewed by:	ache, bjk
  Sponsored by:	The FreeBSD Foundation
  MFC after:	1 week

Modified:
  head/lib/libc/sys/ptrace.2

Modified: head/lib/libc/sys/ptrace.2
==============================================================================
--- head/lib/libc/sys/ptrace.2	Mon Aug 29 19:14:52 2016	(r305021)
+++ head/lib/libc/sys/ptrace.2	Mon Aug 29 19:33:07 2016	(r305022)
@@ -2,7 +2,7 @@
 .\"	$NetBSD: ptrace.2,v 1.2 1995/02/27 12:35:37 cgd Exp $
 .\"
 .\" This file is in the public domain.
-.Dd August 28, 2016
+.Dd August 29, 2016
 .Dt PTRACE 2
 .Os
 .Sh NAME
@@ -900,19 +900,29 @@ argument is ignored.
 .Pp
 Additionally, other machine-specific requests can exist.
 .Sh RETURN VALUES
+Most requests return 0 on success and \-1 on error.
 Some requests can cause
 .Fn ptrace
 to return
 \-1
-as a non-error value; to disambiguate,
+as a non-error value, among them are
+.Dv PT_READ_I
+and
+.Dv PT_READ_D ,
+which return the value read from the process memory on success.
+To disambiguate,
 .Va errno
-is set to 0 in the libc wrapper for the
-.Fn ptrace
-system call and
+can be set to 0 before the call and checked afterwards.
+.Pp
+The current
 .Fn ptrace
-callers can reliably check
+implementation always sets
+.Va errno
+to 0 before calling into the kernel, both for historic reasons and for
+consistency with other operating systems.
+It is recommended to assign zero to
 .Va errno
-for non-zero value afterwards.
+explicitly for forward compatibility.
 .Sh ERRORS
 The
 .Fn ptrace