3 files changed, 68 insertions, 2 deletions
diff --git a/lib/libc/sys/Makefile.inc b/lib/libc/sys/Makefile.inc
index c358e67..2778cad 100644
--- a/lib/libc/sys/Makefile.inc
+++ b/lib/libc/sys/Makefile.inc
@@ -48,6 +48,7 @@ INTERPOSED = \
 	poll \
 	ppoll \
 	pselect \
+	ptrace \
 	read \
 	readv \
 	recvfrom \
diff --git a/lib/libc/sys/ptrace.2 b/lib/libc/sys/ptrace.2
index b15b012..2cd26ee 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 July 28, 2016
+.Dd August 29, 2016
 .Dt PTRACE 2
 .Os
 .Sh NAME
@@ -900,13 +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
 can be set to 0 before the call and checked afterwards.
+.Pp
+The current
+.Fn ptrace
+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
+explicitly for forward compatibility.
 .Sh ERRORS
 The
 .Fn ptrace
diff --git a/lib/libc/sys/ptrace.c b/lib/libc/sys/ptrace.c
new file mode 100644
index 0000000..2bb615f
--- /dev/null
+++ b/lib/libc/sys/ptrace.c
@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2016 The FreeBSD Foundation.
+ * All rights reserved.
+ *
+ * Portions of this software were developed by Konstantin Belousov
+ * under sponsorship from the FreeBSD Foundation.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice(s), this list of conditions and the following disclaimer as
+ *    the first lines of this file unmodified other than the possible
+ *    addition of one or more copyright notices.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice(s), this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
+ * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
+ * OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
+ * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/types.h>
+#include <sys/ptrace.h>
+#include <errno.h>
+#include "libc_private.h"
+
+__weak_reference(_ptrace, ptrace);
+
+int
+_ptrace(int request, pid_t pid, caddr_t addr, int data)
+{
+
+	errno = 0;
+	return (__sys_ptrace(request, pid, addr, data));
+}