diff --git a/src/pkg/os/file_unix.go b/src/pkg/os/file_unix.go
index f6d76f289df460..699e4409eafadc 100644
--- a/src/pkg/os/file_unix.go
+++ b/src/pkg/os/file_unix.go
@@ -81,12 +81,7 @@ func OpenFile(name string, flag int, perm FileMode) (file *File, err error) {
 
 	// There's a race here with fork/exec, which we are
 	// content to live with.  See ../syscall/exec_unix.go.
-	// On OS X 10.6, the O_CLOEXEC flag is not respected.
-	// On OS X 10.7, the O_CLOEXEC flag works.
-	// Without a cheap & reliable way to detect 10.6 vs 10.7 at
-	// runtime, we just always call syscall.CloseOnExec on Darwin.
-	// Once >=10.7 is prevalent, this extra call can removed.
-	if syscall.O_CLOEXEC == 0 || runtime.GOOS == "darwin" { // O_CLOEXEC not supported
+	if !supportsCloseOnExec {
 		syscall.CloseOnExec(r)
 	}
 
diff --git a/src/pkg/os/sys_darwin.go b/src/pkg/os/sys_darwin.go
new file mode 100644
index 00000000000000..7a8330abb55127
--- /dev/null
+++ b/src/pkg/os/sys_darwin.go
@@ -0,0 +1,31 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package os
+
+import "syscall"
+
+// supportsCloseOnExec reports whether the platform supports the
+// O_CLOEXEC flag.
+var supportsCloseOnExec bool
+
+func init() {
+	// Seems like kern.osreldate is veiled on latest OS X. We use
+	// kern.osrelease instead.
+	osver, err := syscall.Sysctl("kern.osrelease")
+	if err != nil {
+		return
+	}
+	var i int
+	for i = range osver {
+		if osver[i] != '.' {
+			continue
+		}
+	}
+	// The O_CLOEXEC flag was introduced in OS X 10.7 (Darwin
+	// 11.0.0). See http://support.apple.com/kb/HT1633.
+	if i > 2 || i == 2 && osver[0] >= '1' && osver[1] >= '1' {
+		supportsCloseOnExec = true
+	}
+}
diff --git a/src/pkg/os/sys_freebsd.go b/src/pkg/os/sys_freebsd.go
new file mode 100644
index 00000000000000..273c2df1c12400
--- /dev/null
+++ b/src/pkg/os/sys_freebsd.go
@@ -0,0 +1,23 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package os
+
+import "syscall"
+
+// supportsCloseOnExec reports whether the platform supports the
+// O_CLOEXEC flag.
+var supportsCloseOnExec bool
+
+func init() {
+	osrel, err := syscall.SysctlUint32("kern.osreldate")
+	if err != nil {
+		return
+	}
+	// The O_CLOEXEC flag was introduced in FreeBSD 8.3.
+	// See http://www.freebsd.org/doc/en/books/porters-handbook/freebsd-versions.html.
+	if osrel >= 803000 {
+		supportsCloseOnExec = true
+	}
+}
diff --git a/src/pkg/os/sys_nacl.go b/src/pkg/os/sys_nacl.go
new file mode 100644
index 00000000000000..07907c84771f3e
--- /dev/null
+++ b/src/pkg/os/sys_nacl.go
@@ -0,0 +1,9 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package os
+
+// supportsCloseOnExec reports whether the platform supports the
+// O_CLOEXEC flag.
+const supportsCloseOnExec = false
diff --git a/src/pkg/os/sys_unix.go b/src/pkg/os/sys_unix.go
new file mode 100644
index 00000000000000..39c20dc739523c
--- /dev/null
+++ b/src/pkg/os/sys_unix.go
@@ -0,0 +1,11 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build dragonfly linux netbsd openbsd solaris
+
+package os
+
+// supportsCloseOnExec reports whether the platform supports the
+// O_CLOEXEC flag.
+const supportsCloseOnExec = true