code review 4151043: syscall: do not use NULL for zero-length read, write

src/pkg/syscall/syscall.go

 // Copyright 2009 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.
 
 // This package contains an interface to the low-level operating system
 // primitives.  The details vary depending on the underlying system.
 // Its primary use is inside other packages that provide a more portable
 // interface to the system, such as "os", "time" and "net".  Use those
 // packages rather than this one if you can.
 // For details of the functions and data types in this package consult
 // the manuals for the appropriate operating system.
 // These calls return errno == 0 to indicate success; otherwise
 // errno is an operating system error number describing the failure.
 package syscall
 
 // StringByteSlice returns a NUL-terminated slice of bytes
 // containing the text of s.
 func StringByteSlice(s string) []byte {
         a := make([]byte, len(s)+1)
         copy(a, s)
         return a
 }
 
 // StringBytePtr returns a pointer to a NUL-terminated array of bytes
 // containing the text of s.
 func StringBytePtr(s string) *byte { return &StringByteSlice(s)[0] }
+
+// Single-word zero for use when we need a valid pointer to 0 bytes.
+// See mksyscall.sh.
+var _zero uintptr