svn commit: r304928 - in head/lib/libc: amd64/sys i386/sys sys

[Konstantin Belousov]

On Mon, Aug 29, 2016 at 09:16:29PM +0300, Andrey Chernov wrote:
> Previous text is documented everywhere and describing usual good
> practice, so it should remains in place, in that means r304928 should be
> reverted, because replace proper way of doing things with obsoleted
> feature description.
> 
> What I suggest is not _replace_ old text with new, but _add_ new text as
> describing current and not recommended way, with the direct mention that
> this feature is obsoleted and not recommended for relay on it.
> 

diff --git a/lib/libc/sys/ptrace.2 b/lib/libc/sys/ptrace.2
index 48802f4..d406efc 100644
--- a/lib/libc/sys/ptrace.2
+++ b/lib/libc/sys/ptrace.2
@@ -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
+Current
 .Fn ptrace
-callers can reliably check
+implementation always sets
+.Va errno
+to 0 before calling into 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.
+explicitely for future compatibility.
 .Sh ERRORS
 The
 .Fn ptrace