os: delete os.EINVAL and so on

The set of errors forwarded by the os package varied with system and
was therefore non-portable.
Three helpers added for portable error checking: IsExist, IsNotExist, and IsPermission.
One or two more may need to come, but let's keep the set very small to discourage
thinking about errors that way.

R=mikioh.mikioh, gustavo, r, rsc
CC=golang-dev
https://golang.org/cl/5672047

42 files changed, 278 insertions, 218 deletions

diff --git a/doc/go1.html b/doc/go1.html
index b1f92338da..13d74012bc 100644
--- a/doc/go1.html
+++ b/doc/go1.html
@@ -1494,12 +1494,31 @@ the i-number expression could be contracted to
 The vast majority of uses of <code>FileInfo</code> need only the methods
 of the standard interface.
 </p>
-	
+
+<p>
+The <code>os</code> package no longer contains wrappers for the POSIX errors
+such as <code>ENOENT</code>.
+For the few programs that need to verify particular error conditions, there are
+now the boolean functions
+<a href="/pkg/os/#IsExist"><code>IsExist</code></a>,
+<a href="/pkg/os/#IsNotExist"><code>IsNotExist</code></a>
+and
+<a href="/pkg/os/#IsPermission"><code>IsPermission</code></a>.
+</p>
+
+<pre><!--{{code "progs/go1.go" `/os\.Open/` `/}/`}}
+-->    f, err := os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600)
+    if os.IsExist(err) {
+        log.Printf(&#34;%s already exists&#34;, name)
+    }</pre>
+
 <p>
 <em>Updating</em>:
 Running <code>go fix</code> will update code that uses the old equivalent of the current <code>os.FileInfo</code>
 and <code>os.FileMode</code> API.
 Code that needs system-specific file details will need to be updated by hand.
+Code that uses the old POSIX error values from the <code>os</code> package
+will fail to compile and will also need to be updated by hand.
 </p>
 
 <h3 id="path_filepath">The path/filepath package</h3>
diff --git a/doc/go1.tmpl b/doc/go1.tmpl
index 32b166a8b1..a963b14984 100644
--- a/doc/go1.tmpl
+++ b/doc/go1.tmpl
@@ -1397,12 +1397,27 @@ the i-number expression could be contracted to
 The vast majority of uses of <code>FileInfo</code> need only the methods
 of the standard interface.
 </p>
-	
+
+<p>
+The <code>os</code> package no longer contains wrappers for the POSIX errors
+such as <code>ENOENT</code>.
+For the few programs that need to verify particular error conditions, there are
+now the boolean functions
+<a href="/pkg/os/#IsExist"><code>IsExist</code></a>,
+<a href="/pkg/os/#IsNotExist"><code>IsNotExist</code></a>
+and
+<a href="/pkg/os/#IsPermission"><code>IsPermission</code></a>.
+</p>
+
+{{code "progs/go1.go" `/os\.Open/` `/}/`}}
+
 <p>
 <em>Updating</em>:
 Running <code>go fix</code> will update code that uses the old equivalent of the current <code>os.FileInfo</code>
 and <code>os.FileMode</code> API.
 Code that needs system-specific file details will need to be updated by hand.
+Code that uses the old POSIX error values from the <code>os</code> package
+will fail to compile and will also need to be updated by hand.
 </p>
 
 <h3 id="path_filepath">The path/filepath package</h3>
diff --git a/doc/go_tutorial.html b/doc/go_tutorial.html
index eaa989a200..589262363f 100644
--- a/doc/go_tutorial.html
+++ b/doc/go_tutorial.html
@@ -623,7 +623,7 @@ each of which declares a receiver variable <code>file</code>.
 <pre><!--{{code "progs/file.go" `/Close/` "$"}}
 -->func (file *File) Close() error {
     if file == nil {
-        return os.EINVAL
+        return os.ErrInvalid
     }
     err := syscall.Close(file.fd)
     file.fd = -1 // so it can&#39;t be closed again
@@ -632,7 +632,7 @@ each of which declares a receiver variable <code>file</code>.
 
 func (file *File) Read(b []byte) (ret int, err error) {
     if file == nil {
-        return -1, os.EINVAL
+        return -1, os.ErrInvalid
     }
     r, err := syscall.Read(file.fd, b)
     return int(r), err
@@ -640,7 +640,7 @@ func (file *File) Read(b []byte) (ret int, err error) {
 
 func (file *File) Write(b []byte) (ret int, err error) {
     if file == nil {
-        return -1, os.EINVAL
+        return -1, os.ErrInvalid
     }
     r, err := syscall.Write(file.fd, b)
     return int(r), err
@@ -659,7 +659,7 @@ array, not just for <code>structs</code>.   We'll see an example with arrays lat
 The <code>String</code> method is so called because of a printing convention we'll
 describe later.
 <p>
-The methods use the public variable <code>os.EINVAL</code> to return the (<code>error</code>
+The methods use the public variable <code>os.ErrInvalid</code> to return the (<code>error</code>
 version of the) Unix error code <code>EINVAL</code>.  The <code>os</code> library defines a standard
 set of such error values.
 <p>
diff --git a/doc/go_tutorial.tmpl b/doc/go_tutorial.tmpl
index bde724ce31..33189189e4 100644
--- a/doc/go_tutorial.tmpl
+++ b/doc/go_tutorial.tmpl
@@ -538,7 +538,7 @@ array, not just for <code>structs</code>.   We'll see an example with arrays lat
 The <code>String</code> method is so called because of a printing convention we'll
 describe later.
 <p>
-The methods use the public variable <code>os.EINVAL</code> to return the (<code>error</code>
+The methods use the public variable <code>os.ErrInvalid</code> to return the (<code>error</code>
 version of the) Unix error code <code>EINVAL</code>.  The <code>os</code> library defines a standard
 set of such error values.
 <p>
diff --git a/doc/progs/file.go b/doc/progs/file.go
index e1aadaa7da..75f0f207ac 100644
--- a/doc/progs/file.go
+++ b/doc/progs/file.go
@@ -49,7 +49,7 @@ func Create(name string) (file *File, err error) {
 
 func (file *File) Close() error {
 	if file == nil {
-		return os.EINVAL
+		return os.ErrInvalid
 	}
 	err := syscall.Close(file.fd)
 	file.fd = -1 // so it can't be closed again
@@ -58,7 +58,7 @@ func (file *File) Close() error {
 
 func (file *File) Read(b []byte) (ret int, err error) {
 	if file == nil {
-		return -1, os.EINVAL
+		return -1, os.ErrInvalid
 	}
 	r, err := syscall.Read(file.fd, b)
 	return int(r), err
@@ -66,7 +66,7 @@ func (file *File) Read(b []byte) (ret int, err error) {
 
 func (file *File) Write(b []byte) (ret int, err error) {
 	if file == nil {
-		return -1, os.EINVAL
+		return -1, os.ErrInvalid
 	}
 	r, err := syscall.Write(file.fd, b)
 	return int(r), err
diff --git a/doc/progs/file_windows.go b/doc/progs/file_windows.go
index e6a355049a..8b79ee97de 100644
--- a/doc/progs/file_windows.go
+++ b/doc/progs/file_windows.go
@@ -49,7 +49,7 @@ func Create(name string) (file *File, err error) {
 
 func (file *File) Close() error {
 	if file == nil {
-		return os.EINVAL
+		return os.ErrInvalid
 	}
 	err := syscall.Close(file.fd)
 	file.fd = syscall.InvalidHandle // so it can't be closed again
@@ -58,7 +58,7 @@ func (file *File) Close() error {
 
 func (file *File) Read(b []byte) (ret int, err error) {
 	if file == nil {
-		return -1, os.EINVAL
+		return -1, os.ErrInvalid
 	}
 	r, err := syscall.Read(file.fd, b)
 	return int(r), err
@@ -66,7 +66,7 @@ func (file *File) Read(b []byte) (ret int, err error) {
 
 func (file *File) Write(b []byte) (ret int, err error) {
 	if file == nil {
-		return -1, os.EINVAL
+		return -1, os.ErrInvalid
 	}
 	r, err := syscall.Write(file.fd, b)
 	return int(r), err
diff --git a/doc/progs/go1.go b/doc/progs/go1.go
index 0348aa315e..653c97fbf5 100644
--- a/doc/progs/go1.go
+++ b/doc/progs/go1.go
@@ -11,6 +11,7 @@ import (
 	"flag"
 	"fmt"
 	"log"
+	"os"
 	"testing"
 	"time"
 	"unicode"
@@ -27,6 +28,7 @@ func main() {
 	runeType()
 	errorExample()
 	timePackage()
+	osIsExist()
 }
 
 var timeout = flag.Duration("timeout", 30*time.Second, "how long to wait for completion")
@@ -206,3 +208,12 @@ func BenchmarkSprintf(b *testing.B) {
 		fmt.Sprintf("%x", 23)
 	}
 }
+
+func osIsExist() {
+	name := "go1.go"
+	f, err := os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600)
+	if os.IsExist(err) {
+		log.Printf("%s already exists", name)
+	}
+	_ = f
+}
diff --git a/src/pkg/io/ioutil/tempfile.go b/src/pkg/io/ioutil/tempfile.go
index 645eed6abb..42d2e67586 100644
--- a/src/pkg/io/ioutil/tempfile.go
+++ b/src/pkg/io/ioutil/tempfile.go
@@ -49,7 +49,7 @@ func TempFile(dir, prefix string) (f *os.File, err error) {
 	for i := 0; i < 10000; i++ {
 		name := filepath.Join(dir, prefix+nextSuffix())
 		f, err = os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600)
-		if pe, ok := err.(*os.PathError); ok && pe.Err == os.EEXIST {
+		if os.IsExist(err) {
 			if nconflict++; nconflict > 10 {
 				rand = reseed()
 			}
@@ -76,7 +76,7 @@ func TempDir(dir, prefix string) (name string, err error) {
 	for i := 0; i < 10000; i++ {
 		try := filepath.Join(dir, prefix+nextSuffix())
 		err = os.Mkdir(try, 0700)
-		if pe, ok := err.(*os.PathError); ok && pe.Err == os.EEXIST {
+		if os.IsExist(err) {
 			if nconflict++; nconflict > 10 {
 				rand = reseed()
 			}
diff --git a/src/pkg/net/fd.go b/src/pkg/net/fd.go
index bf0a387775..334da7f22f 100644
--- a/src/pkg/net/fd.go
+++ b/src/pkg/net/fd.go
@@ -506,7 +506,7 @@ func (fd *netFD) Write(p []byte) (int, error) {
 	}
 	defer fd.decref()
 	if fd.sysfile == nil {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 
 	var err error
diff --git a/src/pkg/net/fd_windows.go b/src/pkg/net/fd_windows.go
index efd846e5d8..45f5c2d882 100644
--- a/src/pkg/net/fd_windows.go
+++ b/src/pkg/net/fd_windows.go
@@ -335,7 +335,7 @@ func (fd *netFD) Close() error {
 
 func (fd *netFD) shutdown(how int) error {
 	if fd == nil || fd.sysfd == syscall.InvalidHandle {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	err := syscall.Shutdown(fd.sysfd, how)
 	if err != nil {
@@ -369,7 +369,7 @@ func (o *readOp) Name() string {
 
 func (fd *netFD) Read(buf []byte) (int, error) {
 	if fd == nil {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	fd.rio.Lock()
 	defer fd.rio.Unlock()
@@ -378,7 +378,7 @@ func (fd *netFD) Read(buf []byte) (int, error) {
 	}
 	defer fd.decref()
 	if fd.sysfd == syscall.InvalidHandle {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	var o readOp
 	o.Init(fd, buf, 'r')
@@ -408,7 +408,7 @@ func (o *readFromOp) Name() string {
 
 func (fd *netFD) ReadFrom(buf []byte) (n int, sa syscall.Sockaddr, err error) {
 	if fd == nil {
-		return 0, nil, os.EINVAL
+		return 0, nil, syscall.EINVAL
 	}
 	if len(buf) == 0 {
 		return 0, nil, nil
@@ -447,7 +447,7 @@ func (o *writeOp) Name() string {
 
 func (fd *netFD) Write(buf []byte) (int, error) {
 	if fd == nil {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	fd.wio.Lock()
 	defer fd.wio.Unlock()
@@ -478,7 +478,7 @@ func (o *writeToOp) Name() string {
 
 func (fd *netFD) WriteTo(buf []byte, sa syscall.Sockaddr) (int, error) {
 	if fd == nil {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	if len(buf) == 0 {
 		return 0, nil
@@ -490,7 +490,7 @@ func (fd *netFD) WriteTo(buf []byte, sa syscall.Sockaddr) (int, error) {
 	}
 	defer fd.decref()
 	if fd.sysfd == syscall.InvalidHandle {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	var o writeToOp
 	o.Init(fd, buf, 'w')
@@ -578,10 +578,12 @@ func (fd *netFD) dup() (*os.File, error) {
 	return nil, os.NewSyscallError("dup", syscall.EWINDOWS)
 }
 
+var errNoSupport = errors.New("address family not supported")
+
 func (fd *netFD) ReadMsg(p []byte, oob []byte) (n, oobn, flags int, sa syscall.Sockaddr, err error) {
-	return 0, 0, 0, nil, os.EAFNOSUPPORT
+	return 0, 0, 0, nil, errNoSupport
 }
 
 func (fd *netFD) WriteMsg(p []byte, oob []byte, sa syscall.Sockaddr) (n int, oobn int, err error) {
-	return 0, 0, os.EAFNOSUPPORT
+	return 0, 0, errNoSupport
 }
diff --git a/src/pkg/net/file.go b/src/pkg/net/file.go
index f9546dc930..c95d16d64e 100644
--- a/src/pkg/net/file.go
+++ b/src/pkg/net/file.go
@@ -28,7 +28,7 @@ func newFileFD(f *os.File) (*netFD, error) {
 	switch sa.(type) {
 	default:
 		closesocket(fd)
-		return nil, os.EINVAL
+		return nil, syscall.EINVAL
 	case *syscall.SockaddrInet4:
 		family = syscall.AF_INET
 		if proto == syscall.SOCK_DGRAM {
@@ -84,7 +84,7 @@ func FileConn(f *os.File) (c Conn, err error) {
 		return newIPConn(fd), nil
 	}
 	fd.Close()
-	return nil, os.EINVAL
+	return nil, syscall.EINVAL
 }
 
 // FileListener returns a copy of the network listener corresponding
@@ -103,7 +103,7 @@ func FileListener(f *os.File) (l Listener, err error) {
 		return &UnixListener{fd, laddr.Name}, nil
 	}
 	fd.Close()
-	return nil, os.EINVAL
+	return nil, syscall.EINVAL
 }
 
 // FilePacketConn returns a copy of the packet network connection
@@ -122,5 +122,5 @@ func FilePacketConn(f *os.File) (c PacketConn, err error) {
 		return newUnixConn(fd), nil
 	}
 	fd.Close()
-	return nil, os.EINVAL
+	return nil, syscall.EINVAL
 }
diff --git a/src/pkg/net/http/fs_test.go b/src/pkg/net/http/fs_test.go
index f3e4a053e3..0409008b67 100644
--- a/src/pkg/net/http/fs_test.go
+++ b/src/pkg/net/http/fs_test.go
@@ -6,6 +6,7 @@ package http_test
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -131,7 +132,7 @@ func TestFileServerCleans(t *testing.T) {
 	ch := make(chan string, 1)
 	fs := FileServer(&testFileSystem{func(name string) (File, error) {
 		ch <- name
-		return nil, os.ENOENT
+		return nil, errors.New("file does not exist")
 	}})
 	tests := []struct {
 		reqPath, openArg string
diff --git a/src/pkg/net/http/httputil/persist.go b/src/pkg/net/http/httputil/persist.go
index c065ccfb49..32f4662cc0 100644
--- a/src/pkg/net/http/httputil/persist.go
+++ b/src/pkg/net/http/httputil/persist.go
@@ -13,12 +13,12 @@ import (
 	"net"
 	"net/http"
 	"net/textproto"
-	"os"
 	"sync"
 )
 
 var (
 	ErrPersistEOF = &http.ProtocolError{ErrorString: "persistent connection closed"}
+	ErrClosed     = &http.ProtocolError{ErrorString: "connection closed by user"}
 	ErrPipeline   = &http.ProtocolError{ErrorString: "pipeline error"}
 )
 
@@ -191,7 +191,7 @@ func (sc *ServerConn) Write(req *http.Request, resp *http.Response) error {
 	}
 	if sc.c == nil { // connection closed by user in the meantime
 		defer sc.lk.Unlock()
-		return os.EBADF
+		return ErrClosed
 	}
 	c := sc.c
 	if sc.nread <= sc.nwritten {
diff --git a/src/pkg/net/iprawsock_posix.go b/src/pkg/net/iprawsock_posix.go
index c34ffeb121..9caa86985a 100644
--- a/src/pkg/net/iprawsock_posix.go
+++ b/src/pkg/net/iprawsock_posix.go
@@ -66,7 +66,7 @@ func (c *IPConn) Read(b []byte) (int, error) {
 // Write implements the Conn Write method.
 func (c *IPConn) Write(b []byte) (int, error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	return c.fd.Write(b)
 }
@@ -74,7 +74,7 @@ func (c *IPConn) Write(b []byte) (int, error) {
 // Close closes the IP connection.
 func (c *IPConn) Close() error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	err := c.fd.Close()
 	c.fd = nil
@@ -100,7 +100,7 @@ func (c *IPConn) RemoteAddr() Addr {
 // SetDeadline implements the Conn SetDeadline method.
 func (c *IPConn) SetDeadline(t time.Time) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setDeadline(c.fd, t)
 }
@@ -108,7 +108,7 @@ func (c *IPConn) SetDeadline(t time.Time) error {
 // SetReadDeadline implements the Conn SetReadDeadline method.
 func (c *IPConn) SetReadDeadline(t time.Time) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setReadDeadline(c.fd, t)
 }
@@ -116,7 +116,7 @@ func (c *IPConn) SetReadDeadline(t time.Time) error {
 // SetWriteDeadline implements the Conn SetWriteDeadline method.
 func (c *IPConn) SetWriteDeadline(t time.Time) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setWriteDeadline(c.fd, t)
 }
@@ -125,7 +125,7 @@ func (c *IPConn) SetWriteDeadline(t time.Time) error {
 // receive buffer associated with the connection.
 func (c *IPConn) SetReadBuffer(bytes int) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setReadBuffer(c.fd, bytes)
 }
@@ -134,7 +134,7 @@ func (c *IPConn) SetReadBuffer(bytes int) error {
 // transmit buffer associated with the connection.
 func (c *IPConn) SetWriteBuffer(bytes int) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setWriteBuffer(c.fd, bytes)
 }
@@ -150,7 +150,7 @@ func (c *IPConn) SetWriteBuffer(bytes int) error {
 // SetReadDeadline.
 func (c *IPConn) ReadFromIP(b []byte) (int, *IPAddr, error) {
 	if !c.ok() {
-		return 0, nil, os.EINVAL
+		return 0, nil, syscall.EINVAL
 	}
 	// TODO(cw,rsc): consider using readv if we know the family
 	// type to avoid the header trim/copy
@@ -173,7 +173,7 @@ func (c *IPConn) ReadFromIP(b []byte) (int, *IPAddr, error) {
 // ReadFrom implements the PacketConn ReadFrom method.
 func (c *IPConn) ReadFrom(b []byte) (int, Addr, error) {
 	if !c.ok() {
-		return 0, nil, os.EINVAL
+		return 0, nil, syscall.EINVAL
 	}
 	n, uaddr, err := c.ReadFromIP(b)
 	return n, uaddr.toAddr(), err
@@ -187,7 +187,7 @@ func (c *IPConn) ReadFrom(b []byte) (int, Addr, error) {
 // On packet-oriented connections, write timeouts are rare.
 func (c *IPConn) WriteToIP(b []byte, addr *IPAddr) (int, error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	sa, err := addr.sockaddr(c.fd.family)
 	if err != nil {
@@ -199,11 +199,11 @@ func (c *IPConn) WriteToIP(b []byte, addr *IPAddr) (int, error) {
 // WriteTo implements the PacketConn WriteTo method.
 func (c *IPConn) WriteTo(b []byte, addr Addr) (int, error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	a, ok := addr.(*IPAddr)
 	if !ok {
-		return 0, &OpError{"write", c.fd.net, addr, os.EINVAL}
+		return 0, &OpError{"write", c.fd.net, addr, syscall.EINVAL}
 	}
 	return c.WriteToIP(b, a)
 }
diff --git a/src/pkg/net/ipsock_plan9.go b/src/pkg/net/ipsock_plan9.go
index 597b127754..385da036d4 100644
--- a/src/pkg/net/ipsock_plan9.go
+++ b/src/pkg/net/ipsock_plan9.go
@@ -83,7 +83,7 @@ func (c *plan9Conn) ok() bool { return c != nil && c.ctl != nil }
 // Read implements the Conn Read method.
 func (c *plan9Conn) Read(b []byte) (n int, err error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	if c.data == nil {
 		c.data, err = os.OpenFile(c.dir+"/data", os.O_RDWR, 0)
@@ -102,7 +102,7 @@ func (c *plan9Conn) Read(b []byte) (n int, err error) {
 // Write implements the Conn Write method.
 func (c *plan9Conn) Write(b []byte) (n int, err error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	if c.data == nil {
 		c.data, err = os.OpenFile(c.dir+"/data", os.O_RDWR, 0)
@@ -116,7 +116,7 @@ func (c *plan9Conn) Write(b []byte) (n int, err error) {
 // Close closes the connection.
 func (c *plan9Conn) Close() error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	err := c.ctl.Close()
 	if err != nil {
@@ -280,7 +280,7 @@ func (l *plan9Listener) Accept() (c Conn, err error) {
 
 func (l *plan9Listener) Close() error {
 	if l == nil || l.ctl == nil {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return l.ctl.Close()
 }
diff --git a/src/pkg/net/tcpsock_plan9.go b/src/pkg/net/tcpsock_plan9.go
index 128766144d..be89e537f3 100644
--- a/src/pkg/net/tcpsock_plan9.go
+++ b/src/pkg/net/tcpsock_plan9.go
@@ -36,7 +36,7 @@ func (c *TCPConn) SetWriteDeadline(t time.Time) error {
 // Most callers should just use Close.
 func (c *TCPConn) CloseRead() error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return os.EPLAN9
 }
@@ -45,7 +45,7 @@ func (c *TCPConn) CloseRead() error {
 // Most callers should just use Close.
 func (c *TCPConn) CloseWrite() error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return os.EPLAN9
 }
diff --git a/src/pkg/net/tcpsock_posix.go b/src/pkg/net/tcpsock_posix.go
index 200ce91566..f972043eeb 100644
--- a/src/pkg/net/tcpsock_posix.go
+++ b/src/pkg/net/tcpsock_posix.go
@@ -70,7 +70,7 @@ func (c *TCPConn) ok() bool { return c != nil && c.fd != nil }
 // Read implements the Conn Read method.
 func (c *TCPConn) Read(b []byte) (n int, err error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	return c.fd.Read(b)
 }
@@ -86,7 +86,7 @@ func (c *TCPConn) ReadFrom(r io.Reader) (int64, error) {
 // Write implements the Conn Write method.
 func (c *TCPConn) Write(b []byte) (n int, err error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	return c.fd.Write(b)
 }
@@ -94,7 +94,7 @@ func (c *TCPConn) Write(b []byte) (n int, err error) {
 // Close closes the TCP connection.
 func (c *TCPConn) Close() error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	err := c.fd.Close()
 	c.fd = nil
@@ -105,7 +105,7 @@ func (c *TCPConn) Close() error {
 // Most callers should just use Close.
 func (c *TCPConn) CloseRead() error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return c.fd.CloseRead()
 }
@@ -114,7 +114,7 @@ func (c *TCPConn) CloseRead() error {
 // Most callers should just use Close.
 func (c *TCPConn) CloseWrite() error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return c.fd.CloseWrite()
 }
@@ -138,7 +138,7 @@ func (c *TCPConn) RemoteAddr() Addr {
 // SetDeadline implements the Conn SetDeadline method.
 func (c *TCPConn) SetDeadline(t time.Time) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setDeadline(c.fd, t)
 }
@@ -146,7 +146,7 @@ func (c *TCPConn) SetDeadline(t time.Time) error {
 // SetReadDeadline implements the Conn SetReadDeadline method.
 func (c *TCPConn) SetReadDeadline(t time.Time) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setReadDeadline(c.fd, t)
 }
@@ -154,7 +154,7 @@ func (c *TCPConn) SetReadDeadline(t time.Time) error {
 // SetWriteDeadline implements the Conn SetWriteDeadline method.
 func (c *TCPConn) SetWriteDeadline(t time.Time) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setWriteDeadline(c.fd, t)
 }
@@ -163,7 +163,7 @@ func (c *TCPConn) SetWriteDeadline(t time.Time) error {
 // receive buffer associated with the connection.
 func (c *TCPConn) SetReadBuffer(bytes int) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setReadBuffer(c.fd, bytes)
 }
@@ -172,7 +172,7 @@ func (c *TCPConn) SetReadBuffer(bytes int) error {
 // transmit buffer associated with the connection.
 func (c *TCPConn) SetWriteBuffer(bytes int) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setWriteBuffer(c.fd, bytes)
 }
@@ -190,7 +190,7 @@ func (c *TCPConn) SetWriteBuffer(bytes int) error {
 // data to be sent and acknowledged.
 func (c *TCPConn) SetLinger(sec int) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setLinger(c.fd, sec)
 }
@@ -199,7 +199,7 @@ func (c *TCPConn) SetLinger(sec int) error {
 // keepalive messages on the connection.
 func (c *TCPConn) SetKeepAlive(keepalive bool) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setKeepAlive(c.fd, keepalive)
 }
@@ -210,7 +210,7 @@ func (c *TCPConn) SetKeepAlive(keepalive bool) error {
 // that data is sent as soon as possible after a Write.
 func (c *TCPConn) SetNoDelay(noDelay bool) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setNoDelay(c.fd, noDelay)
 }
@@ -294,7 +294,7 @@ func ListenTCP(net string, laddr *TCPAddr) (*TCPListener, error) {
 // and the remote address.
 func (l *TCPListener) AcceptTCP() (c *TCPConn, err error) {
 	if l == nil || l.fd == nil || l.fd.sysfd < 0 {
-		return nil, os.EINVAL
+		return nil, syscall.EINVAL
 	}
 	fd, err := l.fd.accept(sockaddrToTCP)
 	if err != nil {
@@ -317,7 +317,7 @@ func (l *TCPListener) Accept() (c Conn, err error) {
 // Already Accepted connections are not closed.
 func (l *TCPListener) Close() error {
 	if l == nil || l.fd == nil {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return l.fd.Close()
 }
@@ -329,7 +329,7 @@ func (l *TCPListener) Addr() Addr { return l.fd.laddr }
 // A zero time value disables the deadline.
 func (l *TCPListener) SetDeadline(t time.Time) error {
 	if l == nil || l.fd == nil {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setDeadline(l.fd, t)
 }
diff --git a/src/pkg/net/udpsock_plan9.go b/src/pkg/net/udpsock_plan9.go
index f90a5fe9ab..d1f83d3848 100644
--- a/src/pkg/net/udpsock_plan9.go
+++ b/src/pkg/net/udpsock_plan9.go
@@ -43,7 +43,7 @@ func (c *UDPConn) SetWriteDeadline(t time.Time) error {
 // after a fixed time limit; see SetDeadline and SetReadDeadline.
 func (c *UDPConn) ReadFromUDP(b []byte) (n int, addr *UDPAddr, err error) {
 	if !c.ok() {
-		return 0, nil, os.EINVAL
+		return 0, nil, syscall.EINVAL
 	}
 	if c.data == nil {
 		c.data, err = os.OpenFile(c.dir+"/data", os.O_RDWR, 0)
@@ -69,7 +69,7 @@ func (c *UDPConn) ReadFromUDP(b []byte) (n int, addr *UDPAddr, err error) {
 // ReadFrom implements the PacketConn ReadFrom method.
 func (c *UDPConn) ReadFrom(b []byte) (n int, addr Addr, err error) {
 	if !c.ok() {
-		return 0, nil, os.EINVAL
+		return 0, nil, syscall.EINVAL
 	}
 	return c.ReadFromUDP(b)
 }
@@ -82,7 +82,7 @@ func (c *UDPConn) ReadFrom(b []byte) (n int, addr Addr, err error) {
 // On packet-oriented connections, write timeouts are rare.
 func (c *UDPConn) WriteToUDP(b []byte, addr *UDPAddr) (n int, err error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	if c.data == nil {
 		c.data, err = os.OpenFile(c.dir+"/data", os.O_RDWR, 0)
@@ -106,11 +106,11 @@ func (c *UDPConn) WriteToUDP(b []byte, addr *UDPAddr) (n int, err error) {
 // WriteTo implements the PacketConn WriteTo method.
 func (c *UDPConn) WriteTo(b []byte, addr Addr) (n int, err error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	a, ok := addr.(*UDPAddr)
 	if !ok {
-		return 0, &OpError{"write", c.dir, addr, os.EINVAL}
+		return 0, &OpError{"write", c.dir, addr, syscall.EINVAL}
 	}
 	return c.WriteToUDP(b, a)
 }
diff --git a/src/pkg/net/udpsock_posix.go b/src/pkg/net/udpsock_posix.go
index 6108373568..8ee58ed4c6 100644
--- a/src/pkg/net/udpsock_posix.go
+++ b/src/pkg/net/udpsock_posix.go
@@ -63,7 +63,7 @@ func (c *UDPConn) ok() bool { return c != nil && c.fd != nil }
 // Read implements the Conn Read method.
 func (c *UDPConn) Read(b []byte) (int, error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	return c.fd.Read(b)
 }
@@ -71,7 +71,7 @@ func (c *UDPConn) Read(b []byte) (int, error) {
 // Write implements the Conn Write method.
 func (c *UDPConn) Write(b []byte) (int, error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	return c.fd.Write(b)
 }
@@ -79,7 +79,7 @@ func (c *UDPConn) Write(b []byte) (int, error) {
 // Close closes the UDP connection.
 func (c *UDPConn) Close() error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	err := c.fd.Close()
 	c.fd = nil
@@ -105,7 +105,7 @@ func (c *UDPConn) RemoteAddr() Addr {
 // SetDeadline implements the Conn SetDeadline method.
 func (c *UDPConn) SetDeadline(t time.Time) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setDeadline(c.fd, t)
 }
@@ -113,7 +113,7 @@ func (c *UDPConn) SetDeadline(t time.Time) error {
 // SetReadDeadline implements the Conn SetReadDeadline method.
 func (c *UDPConn) SetReadDeadline(t time.Time) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setReadDeadline(c.fd, t)
 }
@@ -121,7 +121,7 @@ func (c *UDPConn) SetReadDeadline(t time.Time) error {
 // SetWriteDeadline implements the Conn SetWriteDeadline method.
 func (c *UDPConn) SetWriteDeadline(t time.Time) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setWriteDeadline(c.fd, t)
 }
@@ -130,7 +130,7 @@ func (c *UDPConn) SetWriteDeadline(t time.Time) error {
 // receive buffer associated with the connection.
 func (c *UDPConn) SetReadBuffer(bytes int) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setReadBuffer(c.fd, bytes)
 }
@@ -139,7 +139,7 @@ func (c *UDPConn) SetReadBuffer(bytes int) error {
 // transmit buffer associated with the connection.
 func (c *UDPConn) SetWriteBuffer(bytes int) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setWriteBuffer(c.fd, bytes)
 }
@@ -154,7 +154,7 @@ func (c *UDPConn) SetWriteBuffer(bytes int) error {
 // after a fixed time limit; see SetDeadline and SetReadDeadline.
 func (c *UDPConn) ReadFromUDP(b []byte) (n int, addr *UDPAddr, err error) {
 	if !c.ok() {
-		return 0, nil, os.EINVAL
+		return 0, nil, syscall.EINVAL
 	}
 	n, sa, err := c.fd.ReadFrom(b)
 	switch sa := sa.(type) {
@@ -169,7 +169,7 @@ func (c *UDPConn) ReadFromUDP(b []byte) (n int, addr *UDPAddr, err error) {
 // ReadFrom implements the PacketConn ReadFrom method.
 func (c *UDPConn) ReadFrom(b []byte) (int, Addr, error) {
 	if !c.ok() {
-		return 0, nil, os.EINVAL
+		return 0, nil, syscall.EINVAL
 	}
 	n, uaddr, err := c.ReadFromUDP(b)
 	return n, uaddr.toAddr(), err
@@ -183,7 +183,7 @@ func (c *UDPConn) ReadFrom(b []byte) (int, Addr, error) {
 // On packet-oriented connections, write timeouts are rare.
 func (c *UDPConn) WriteToUDP(b []byte, addr *UDPAddr) (int, error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	if c.fd.isConnected {
 		return 0, &OpError{"write", c.fd.net, addr, ErrWriteToConnected}
@@ -198,11 +198,11 @@ func (c *UDPConn) WriteToUDP(b []byte, addr *UDPAddr) (int, error) {
 // WriteTo implements the PacketConn WriteTo method.
 func (c *UDPConn) WriteTo(b []byte, addr Addr) (int, error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	a, ok := addr.(*UDPAddr)
 	if !ok {
-		return 0, &OpError{"write", c.fd.net, addr, os.EINVAL}
+		return 0, &OpError{"write", c.fd.net, addr, syscall.EINVAL}
 	}
 	return c.WriteToUDP(b, a)
 }
diff --git a/src/pkg/net/unixsock_posix.go b/src/pkg/net/unixsock_posix.go
index 10b7966851..238cefe718 100644
--- a/src/pkg/net/unixsock_posix.go
+++ b/src/pkg/net/unixsock_posix.go
@@ -123,7 +123,7 @@ func (c *UnixConn) ok() bool { return c != nil && c.fd != nil }
 // Read implements the Conn Read method.
 func (c *UnixConn) Read(b []byte) (n int, err error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	return c.fd.Read(b)
 }
@@ -131,7 +131,7 @@ func (c *UnixConn) Read(b []byte) (n int, err error) {
 // Write implements the Conn Write method.
 func (c *UnixConn) Write(b []byte) (n int, err error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	return c.fd.Write(b)
 }
@@ -139,7 +139,7 @@ func (c *UnixConn) Write(b []byte) (n int, err error) {
 // Close closes the Unix domain connection.
 func (c *UnixConn) Close() error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	err := c.fd.Close()
 	c.fd = nil
@@ -168,7 +168,7 @@ func (c *UnixConn) RemoteAddr() Addr {
 // SetDeadline implements the Conn SetDeadline method.
 func (c *UnixConn) SetDeadline(t time.Time) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setDeadline(c.fd, t)
 }
@@ -176,7 +176,7 @@ func (c *UnixConn) SetDeadline(t time.Time) error {
 // SetReadDeadline implements the Conn SetReadDeadline method.
 func (c *UnixConn) SetReadDeadline(t time.Time) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setReadDeadline(c.fd, t)
 }
@@ -184,7 +184,7 @@ func (c *UnixConn) SetReadDeadline(t time.Time) error {
 // SetWriteDeadline implements the Conn SetWriteDeadline method.
 func (c *UnixConn) SetWriteDeadline(t time.Time) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setWriteDeadline(c.fd, t)
 }
@@ -193,7 +193,7 @@ func (c *UnixConn) SetWriteDeadline(t time.Time) error {
 // receive buffer associated with the connection.
 func (c *UnixConn) SetReadBuffer(bytes int) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setReadBuffer(c.fd, bytes)
 }
@@ -202,7 +202,7 @@ func (c *UnixConn) SetReadBuffer(bytes int) error {
 // transmit buffer associated with the connection.
 func (c *UnixConn) SetWriteBuffer(bytes int) error {
 	if !c.ok() {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setWriteBuffer(c.fd, bytes)
 }
@@ -216,7 +216,7 @@ func (c *UnixConn) SetWriteBuffer(bytes int) error {
 // see SetDeadline and SetReadDeadline.
 func (c *UnixConn) ReadFromUnix(b []byte) (n int, addr *UnixAddr, err error) {
 	if !c.ok() {
-		return 0, nil, os.EINVAL
+		return 0, nil, syscall.EINVAL
 	}
 	n, sa, err := c.fd.ReadFrom(b)
 	switch sa := sa.(type) {
@@ -229,7 +229,7 @@ func (c *UnixConn) ReadFromUnix(b []byte) (n int, addr *UnixAddr, err error) {
 // ReadFrom implements the PacketConn ReadFrom method.
 func (c *UnixConn) ReadFrom(b []byte) (n int, addr Addr, err error) {
 	if !c.ok() {
-		return 0, nil, os.EINVAL
+		return 0, nil, syscall.EINVAL
 	}
 	n, uaddr, err := c.ReadFromUnix(b)
 	return n, uaddr.toAddr(), err
@@ -243,10 +243,10 @@ func (c *UnixConn) ReadFrom(b []byte) (n int, addr Addr, err error) {
 // On packet-oriented connections, write timeouts are rare.
 func (c *UnixConn) WriteToUnix(b []byte, addr *UnixAddr) (n int, err error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	if addr.Net != sotypeToNet(c.fd.sotype) {
-		return 0, os.EAFNOSUPPORT
+		return 0, syscall.EAFNOSUPPORT
 	}
 	sa := &syscall.SockaddrUnix{Name: addr.Name}
 	return c.fd.WriteTo(b, sa)
@@ -255,18 +255,18 @@ func (c *UnixConn) WriteToUnix(b []byte, addr *UnixAddr) (n int, err error) {
 // WriteTo implements the PacketConn WriteTo method.
 func (c *UnixConn) WriteTo(b []byte, addr Addr) (n int, err error) {
 	if !c.ok() {
-		return 0, os.EINVAL
+		return 0, syscall.EINVAL
 	}
 	a, ok := addr.(*UnixAddr)
 	if !ok {
-		return 0, &OpError{"write", c.fd.net, addr, os.EINVAL}
+		return 0, &OpError{"write", c.fd.net, addr, syscall.EINVAL}
 	}
 	return c.WriteToUnix(b, a)
 }
 
 func (c *UnixConn) ReadMsgUnix(b, oob []byte) (n, oobn, flags int, addr *UnixAddr, err error) {
 	if !c.ok() {
-		return 0, 0, 0, nil, os.EINVAL
+		return 0, 0, 0, nil, syscall.EINVAL
 	}
 	n, oobn, flags, sa, err := c.fd.ReadMsg(b, oob)
 	switch sa := sa.(type) {
@@ -278,11 +278,11 @@ func (c *UnixConn) ReadMsgUnix(b, oob []byte) (n, oobn, flags int, addr *UnixAdd
 
 func (c *UnixConn) WriteMsgUnix(b, oob []byte, addr *UnixAddr) (n, oobn int, err error) {
 	if !c.ok() {
-		return 0, 0, os.EINVAL
+		return 0, 0, syscall.EINVAL
 	}
 	if addr != nil {
 		if addr.Net != sotypeToNet(c.fd.sotype) {
-			return 0, 0, os.EAFNOSUPPORT
+			return 0, 0, syscall.EAFNOSUPPORT
 		}
 		sa := &syscall.SockaddrUnix{Name: addr.Name}
 		return c.fd.WriteMsg(b, oob, sa)
@@ -339,7 +339,7 @@ func ListenUnix(net string, laddr *UnixAddr) (*UnixListener, error) {
 // and the remote address.
 func (l *UnixListener) AcceptUnix() (*UnixConn, error) {
 	if l == nil || l.fd == nil {
-		return nil, os.EINVAL
+		return nil, syscall.EINVAL
 	}
 	fd, err := l.fd.accept(sockaddrToUnix)
 	if err != nil {
@@ -363,7 +363,7 @@ func (l *UnixListener) Accept() (c Conn, err error) {
 // Already accepted connections are not closed.
 func (l *UnixListener) Close() error {
 	if l == nil || l.fd == nil {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 
 	// The operating system doesn't clean up
@@ -391,7 +391,7 @@ func (l *UnixListener) Addr() Addr { return l.fd.laddr }
 // A zero time value disables the deadline.
 func (l *UnixListener) SetDeadline(t time.Time) (err error) {
 	if l == nil || l.fd == nil {
-		return os.EINVAL
+		return syscall.EINVAL
 	}
 	return setDeadline(l.fd, t)
 }
diff --git a/src/pkg/os/dir_plan9.go b/src/pkg/os/dir_plan9.go
index f2dc15409d..7fa4c7f444 100644
--- a/src/pkg/os/dir_plan9.go
+++ b/src/pkg/os/dir_plan9.go
@@ -10,6 +10,9 @@ import (
 	"syscall"
 )
 
+var errShortStat = errors.New("short stat message")
+var errBadStat = errors.New("bad stat message format")
+
 func (file *File) readdir(n int) (fi []FileInfo, err error) {
 	// If this file has no dirinfo, create one.
 	if file.dirinfo == nil {
@@ -35,7 +38,7 @@ func (file *File) readdir(n int) (fi []FileInfo, err error) {
 				break
 			}
 			if d.nbuf < syscall.STATFIXLEN {
-				return result, &PathError{"readdir", file.name, Eshortstat}
+				return result, &PathError{"readdir", file.name, errShortStat}
 			}
 		}
 
@@ -43,7 +46,7 @@ func (file *File) readdir(n int) (fi []FileInfo, err error) {
 		m, _ := gbit16(d.buf[d.bufp:])
 		m += 2
 		if m < syscall.STATFIXLEN {
-			return result, &PathError{"readdir", file.name, Eshortstat}
+			return result, &PathError{"readdir", file.name, errShortStat}
 		}
 		dir, e := UnmarshalDir(d.buf[d.bufp : d.bufp+int(m)])
 		if e != nil {
@@ -138,7 +141,7 @@ func UnmarshalDir(b []byte) (d *Dir, err error) {
 	n, b = gbit16(b)
 
 	if int(n) != len(b) {
-		return nil, Ebadstat
+		return nil, errBadStat
 	}
 
 	d = new(Dir)
@@ -155,7 +158,7 @@ func UnmarshalDir(b []byte) (d *Dir, err error) {
 	d.Muid, b = gstring(b)
 
 	if len(b) != 0 {
-		return nil, Ebadstat
+		return nil, errBadStat
 	}
 
 	return d, nil
diff --git a/src/pkg/os/env.go b/src/pkg/os/env.go
index 59350510cc..207e0a0ec7 100644
--- a/src/pkg/os/env.go
+++ b/src/pkg/os/env.go
@@ -84,7 +84,7 @@ var ENOENV = errors.New("no such environment variable")
 // It returns the value and an error, if any.
 func Getenverror(key string) (value string, err error) {
 	if len(key) == 0 {
-		return "", EINVAL
+		return "", ErrInvalid
 	}
 	val, found := syscall.Getenv(key)
 	if !found {
diff --git a/src/pkg/os/error.go b/src/pkg/os/error.go
index 135cdae1f9..5baeba475b 100644
--- a/src/pkg/os/error.go
+++ b/src/pkg/os/error.go
@@ -4,6 +4,18 @@
 
 package os
 
+import (
+	"errors"
+)
+
+// Portable analogs of some common system call errors.
+var (
+	ErrInvalid    = errors.New("invalid argument")
+	ErrPermission = errors.New("permission denied")
+	ErrExist      = errors.New("file already exists")
+	ErrNotExist   = errors.New("file does not exit")
+)
+
 // PathError records an error and the operation and file path that caused it.
 type PathError struct {
 	Op   string
diff --git a/src/pkg/os/error_plan9.go b/src/pkg/os/error_plan9.go
index cc847e0774..159d685e7c 100644
--- a/src/pkg/os/error_plan9.go
+++ b/src/pkg/os/error_plan9.go
@@ -4,34 +4,38 @@
 
 package os
 
-import (
-	"errors"
-	"syscall"
-)
+// IsExist returns whether the error is known to report that a file already exists.
+func IsExist(err error) bool {
+	if pe, ok := err.(*PathError); ok {
+		err = pe.Err
+	}
+	return contains(err.Error(), " exists")
+}
 
-var (
-	Eshortstat = errors.New("stat buffer too small")
-	Ebadstat   = errors.New("malformed stat buffer")
-	Ebadfd     = errors.New("fd out of range or not open")
-	Ebadarg    = errors.New("bad arg in system call")
-	Enotdir    = errors.New("not a directory")
-	Enonexist  = errors.New("file does not exist")
-	Eexist     = errors.New("file already exists")
-	Eio        = errors.New("i/o error")
-	Eperm      = errors.New("permission denied")
+// IsNotExist returns whether the error is known to report that a file does not exist.
+func IsNotExist(err error) bool {
+	if pe, ok := err.(*PathError); ok {
+		err = pe.Err
+	}
+	return contains(err.Error(), "does not exist")
+}
 
-	EINVAL  = Ebadarg
-	ENOTDIR = Enotdir
-	ENOENT  = Enonexist
-	EEXIST  = Eexist
-	EIO     = Eio
-	EACCES  = Eperm
-	EPERM   = Eperm
-	EISDIR  = syscall.EISDIR
+// IsPermission returns whether the error is known to report that permission is denied.
+func IsPermission(err error) bool {
+	if pe, ok := err.(*PathError); ok {
+		err = pe.Err
+	}
+	return contains(err.Error(), "permission denied")
+}
 
-	EBADF        = errors.New("bad file descriptor")
-	ENAMETOOLONG = errors.New("file name too long")
-	ERANGE       = errors.New("math result not representable")
-	EPIPE        = errors.New("Broken Pipe")
-	EPLAN9       = errors.New("not supported by plan 9")
-)
+// contains is a local version of strings.Contains. It knows len(sep) > 1.
+func contains(s, sep string) bool {
+	n := len(sep)
+	c := sep[0]
+	for i := 0; i+n <= len(s); i++ {
+		if s[i] == c && s[i:i+n] == sep {
+			return true
+		}
+	}
+	return false
+}
diff --git a/src/pkg/os/error_posix.go b/src/pkg/os/error_posix.go
index 57c9b6f278..74b75d1121 100644
--- a/src/pkg/os/error_posix.go
+++ b/src/pkg/os/error_posix.go
@@ -8,44 +8,29 @@ package os
 
 import "syscall"
 
-// Commonly known Unix errors.
-var (
-	EPERM        error = syscall.EPERM
-	ENOENT       error = syscall.ENOENT
-	ESRCH        error = syscall.ESRCH
-	EINTR        error = syscall.EINTR
-	EIO          error = syscall.EIO
-	E2BIG        error = syscall.E2BIG
-	ENOEXEC      error = syscall.ENOEXEC
-	EBADF        error = syscall.EBADF
-	ECHILD       error = syscall.ECHILD
-	EDEADLK      error = syscall.EDEADLK
-	ENOMEM       error = syscall.ENOMEM
-	EACCES       error = syscall.EACCES
-	EFAULT       error = syscall.EFAULT
-	EBUSY        error = syscall.EBUSY
-	EEXIST       error = syscall.EEXIST
-	EXDEV        error = syscall.EXDEV
-	ENODEV       error = syscall.ENODEV
-	ENOTDIR      error = syscall.ENOTDIR
-	EISDIR       error = syscall.EISDIR
-	EINVAL       error = syscall.EINVAL
-	ENFILE       error = syscall.ENFILE
-	EMFILE       error = syscall.EMFILE
-	ENOTTY       error = syscall.ENOTTY
-	EFBIG        error = syscall.EFBIG
-	ENOSPC       error = syscall.ENOSPC
-	ESPIPE       error = syscall.ESPIPE
-	EROFS        error = syscall.EROFS
-	EMLINK       error = syscall.EMLINK
-	EPIPE        error = syscall.EPIPE
-	EAGAIN       error = syscall.EAGAIN
-	EDOM         error = syscall.EDOM
-	ERANGE       error = syscall.ERANGE
-	EADDRINUSE   error = syscall.EADDRINUSE
-	ECONNREFUSED error = syscall.ECONNREFUSED
-	ENAMETOOLONG error = syscall.ENAMETOOLONG
-	EAFNOSUPPORT error = syscall.EAFNOSUPPORT
-	ETIMEDOUT    error = syscall.ETIMEDOUT
-	ENOTCONN     error = syscall.ENOTCONN
-)
+// IsExist returns whether the error is known to report that a file already exists.
+// It is satisfied by ErrExist as well as some syscall errors.
+func IsExist(err error) bool {
+	if pe, ok := err.(*PathError); ok {
+		err = pe.Err
+	}
+	return err == syscall.EEXIST || err == ErrExist
+}
+
+// IsNotExist returns whether the error is known to report that a file does not exist.
+// It is satisfied by ErrNotExist as well as some syscall errors.
+func IsNotExist(err error) bool {
+	if pe, ok := err.(*PathError); ok {
+		err = pe.Err
+	}
+	return err == syscall.ENOENT || err == ErrNotExist
+}
+
+// IsPermission returns whether the error is known to report that permission is denied.
+// It is satisfied by ErrPermission as well as some syscall errors.
+func IsPermission(err error) bool {
+	if pe, ok := err.(*PathError); ok {
+		err = pe.Err
+	}
+	return err == syscall.EACCES || err == syscall.EPERM || err == ErrPermission
+}
diff --git a/src/pkg/os/exec/lp_unix.go b/src/pkg/os/exec/lp_unix.go
index 2d3a919dc6..2163221997 100644
--- a/src/pkg/os/exec/lp_unix.go
+++ b/src/pkg/os/exec/lp_unix.go
@@ -23,7 +23,7 @@ func findExecutable(file string) error {
 	if m := d.Mode(); !m.IsDir() && m&0111 != 0 {
 		return nil
 	}
-	return os.EPERM
+	return os.ErrPermission
 }
 
 // LookPath searches for an executable binary named file
diff --git a/src/pkg/os/exec/lp_windows.go b/src/pkg/os/exec/lp_windows.go
index b7efcd68b8..d8351d7e6d 100644
--- a/src/pkg/os/exec/lp_windows.go
+++ b/src/pkg/os/exec/lp_windows.go
@@ -19,7 +19,7 @@ func chkStat(file string) error {
 		return err
 	}
 	if d.IsDir() {
-		return os.EPERM
+		return os.ErrPermission
 	}
 	return nil
 }
@@ -39,7 +39,7 @@ func findExecutable(file string, exts []string) (string, error) {
 			return f, nil
 		}
 	}
-	return ``, os.ENOENT
+	return ``, os.ErrNotExist
 }
 
 // LookPath searches for an executable binary named file
diff --git a/src/pkg/os/exec_plan9.go b/src/pkg/os/exec_plan9.go
index b725aeb8d6..c57c4dc6d6 100644
--- a/src/pkg/os/exec_plan9.go
+++ b/src/pkg/os/exec_plan9.go
@@ -76,7 +76,7 @@ func (p *Process) Wait(options int) (w *Waitmsg, err error) {
 	var waitmsg syscall.Waitmsg
 
 	if p.Pid == -1 {
-		return nil, EINVAL
+		return nil, ErrInvalid
 	}
 
 	for true {
diff --git a/src/pkg/os/exec_unix.go b/src/pkg/os/exec_unix.go
index 7fe7c2fe8c..a5c22812a2 100644
--- a/src/pkg/os/exec_unix.go
+++ b/src/pkg/os/exec_unix.go
@@ -29,7 +29,7 @@ const (
 // (WNOHANG etc.) affect the behavior of the Wait call.
 func (p *Process) Wait(options int) (w *Waitmsg, err error) {
 	if p.Pid == -1 {
-		return nil, EINVAL
+		return nil, syscall.EINVAL
 	}
 	var status syscall.WaitStatus
 	var rusage *syscall.Rusage
diff --git a/src/pkg/os/exec_windows.go b/src/pkg/os/exec_windows.go
index f357a3034b..2a7affa284 100644
--- a/src/pkg/os/exec_windows.go
+++ b/src/pkg/os/exec_windows.go
@@ -48,7 +48,7 @@ func (p *Process) Signal(sig Signal) error {
 // Release releases any resources associated with the Process.
 func (p *Process) Release() error {
 	if p.handle == uintptr(syscall.InvalidHandle) {
-		return EINVAL
+		return syscall.EINVAL
 	}
 	e := syscall.CloseHandle(syscall.Handle(p.handle))
 	if e != nil {
diff --git a/src/pkg/os/file.go b/src/pkg/os/file.go
index 85f151e284..439164241a 100644
--- a/src/pkg/os/file.go
+++ b/src/pkg/os/file.go
@@ -55,7 +55,7 @@ const (
 // EOF is signaled by a zero count with err set to io.EOF.
 func (f *File) Read(b []byte) (n int, err error) {
 	if f == nil {
-		return 0, EINVAL
+		return 0, ErrInvalid
 	}
 	n, e := f.read(b)
 	if n < 0 {
@@ -76,7 +76,7 @@ func (f *File) Read(b []byte) (n int, err error) {
 // At end of file, that error is io.EOF.
 func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
 	if f == nil {
-		return 0, EINVAL
+		return 0, ErrInvalid
 	}
 	for len(b) > 0 {
 		m, e := f.pread(b, off)
@@ -99,7 +99,7 @@ func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
 // Write returns a non-nil error when n != len(b).
 func (f *File) Write(b []byte) (n int, err error) {
 	if f == nil {
-		return 0, EINVAL
+		return 0, ErrInvalid
 	}
 	n, e := f.write(b)
 	if n < 0 {
@@ -119,7 +119,7 @@ func (f *File) Write(b []byte) (n int, err error) {
 // WriteAt returns a non-nil error when n != len(b).
 func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
 	if f == nil {
-		return 0, EINVAL
+		return 0, ErrInvalid
 	}
 	for len(b) > 0 {
 		m, e := f.pwrite(b, off)
@@ -153,7 +153,7 @@ func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
 // an array of bytes.
 func (f *File) WriteString(s string) (ret int, err error) {
 	if f == nil {
-		return 0, EINVAL
+		return 0, ErrInvalid
 	}
 	return f.Write([]byte(s))
 }
diff --git a/src/pkg/os/file_plan9.go b/src/pkg/os/file_plan9.go
index c28ea3471c..70041f22aa 100644
--- a/src/pkg/os/file_plan9.go
+++ b/src/pkg/os/file_plan9.go
@@ -5,11 +5,14 @@
 package os
 
 import (
+	"errors"
 	"runtime"
 	"syscall"
 	"time"
 )
 
+var ErrPlan9 = errors.New("unimplemented on Plan 9")
+
 // File represents an open file descriptor.
 type File struct {
 	*file
@@ -140,7 +143,7 @@ func (file *File) Close() error {
 
 func (file *file) close() error {
 	if file == nil || file.fd < 0 {
-		return Ebadfd
+		return ErrInvalid
 	}
 	var err error
 	syscall.ForkLock.RLock()
@@ -203,7 +206,7 @@ func (f *File) Chmod(mode FileMode) error {
 // of recently written data to disk.
 func (f *File) Sync() (err error) {
 	if f == nil {
-		return EINVAL
+		return ErrInvalid
 	}
 
 	var d Dir
@@ -338,27 +341,27 @@ func Pipe() (r *File, w *File, err error) {
 
 // Link creates a hard link.
 func Link(oldname, newname string) error {
-	return EPLAN9
+	return ErrPlan9
 }
 
 func Symlink(oldname, newname string) error {
-	return EPLAN9
+	return ErrPlan9
 }
 
 func Readlink(name string) (string, error) {
-	return "", EPLAN9
+	return "", ErrPlan9
 }
 
 func Chown(name string, uid, gid int) error {
-	return EPLAN9
+	return ErrPlan9
 }
 
 func Lchown(name string, uid, gid int) error {
-	return EPLAN9
+	return ErrPlan9
 }
 
 func (f *File) Chown(uid, gid int) error {
-	return EPLAN9
+	return ErrPlan9
 }
 
 // TempDir returns the default directory to use for temporary files.
diff --git a/src/pkg/os/file_posix.go b/src/pkg/os/file_posix.go
index 8d3a00b6c5..8861af1c7d 100644
--- a/src/pkg/os/file_posix.go
+++ b/src/pkg/os/file_posix.go
@@ -160,7 +160,7 @@ func (f *File) Truncate(size int64) error {
 // of recently written data to disk.
 func (f *File) Sync() (err error) {
 	if f == nil {
-		return EINVAL
+		return syscall.EINVAL
 	}
 	if e := syscall.Fsync(f.fd); e != nil {
 		return NewSyscallError("fsync", e)
diff --git a/src/pkg/os/file_unix.go b/src/pkg/os/file_unix.go
index 0a422f4e88..6aa0280f4a 100644
--- a/src/pkg/os/file_unix.go
+++ b/src/pkg/os/file_unix.go
@@ -90,7 +90,7 @@ func (f *File) Close() error {
 
 func (file *file) close() error {
 	if file == nil || file.fd < 0 {
-		return EINVAL
+		return syscall.EINVAL
 	}
 	var err error
 	if e := syscall.Close(file.fd); e != nil {
diff --git a/src/pkg/os/file_windows.go b/src/pkg/os/file_windows.go
index 350d2a72cf..82c7429945 100644
--- a/src/pkg/os/file_windows.go
+++ b/src/pkg/os/file_windows.go
@@ -98,7 +98,7 @@ func OpenFile(name string, flag int, perm FileMode) (file *File, err error) {
 	if e == nil {
 		if flag&O_WRONLY != 0 || flag&O_RDWR != 0 {
 			r.Close()
-			return nil, &PathError{"open", name, EISDIR}
+			return nil, &PathError{"open", name, syscall.EISDIR}
 		}
 		return r, nil
 	}
@@ -117,7 +117,7 @@ func (file *File) Close() error {
 
 func (file *file) close() error {
 	if file == nil || file.fd == syscall.InvalidHandle {
-		return EINVAL
+		return syscall.EINVAL
 	}
 	var e error
 	if file.isdir() {
@@ -138,10 +138,10 @@ func (file *file) close() error {
 
 func (file *File) readdir(n int) (fi []FileInfo, err error) {
 	if file == nil || file.fd == syscall.InvalidHandle {
-		return nil, EINVAL
+		return nil, syscall.EINVAL
 	}
 	if !file.isdir() {
-		return nil, &PathError{"Readdir", file.name, ENOTDIR}
+		return nil, &PathError{"Readdir", file.name, syscall.ENOTDIR}
 	}
 	wantAll := n <= 0
 	size := n
diff --git a/src/pkg/os/getwd.go b/src/pkg/os/getwd.go
index 56836434db..81d8fed926 100644
--- a/src/pkg/os/getwd.go
+++ b/src/pkg/os/getwd.go
@@ -52,7 +52,7 @@ func Getwd() (pwd string, err error) {
 	pwd = ""
 	for parent := ".."; ; parent = "../" + parent {
 		if len(parent) >= 1024 { // Sanity check
-			return "", ENAMETOOLONG
+			return "", syscall.ENAMETOOLONG
 		}
 		fd, err := Open(parent)
 		if err != nil {
@@ -74,7 +74,7 @@ func Getwd() (pwd string, err error) {
 			}
 		}
 		fd.Close()
-		return "", ENOENT
+		return "", ErrNotExist
 
 	Found:
 		pd, err := fd.Stat()
diff --git a/src/pkg/os/os_test.go b/src/pkg/os/os_test.go
index 9a95407ad5..e02d7a43a3 100644
--- a/src/pkg/os/os_test.go
+++ b/src/pkg/os/os_test.go
@@ -13,6 +13,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+	"syscall"
 	"testing"
 	"time"
 )
@@ -769,7 +770,7 @@ func TestSeek(t *testing.T) {
 	for i, tt := range tests {
 		off, err := f.Seek(tt.in, tt.whence)
 		if off != tt.out || err != nil {
-			if e, ok := err.(*PathError); ok && e.Err == EINVAL && tt.out > 1<<32 {
+			if e, ok := err.(*PathError); ok && e.Err == syscall.EINVAL && tt.out > 1<<32 {
 				// Reiserfs rejects the big seeks.
 				// http://code.google.com/p/go/issues/detail?id=91
 				break
@@ -789,17 +790,17 @@ var openErrorTests = []openErrorTest{
 	{
 		sfdir + "/no-such-file",
 		O_RDONLY,
-		ENOENT,
+		syscall.ENOENT,
 	},
 	{
 		sfdir,
 		O_WRONLY,
-		EISDIR,
+		syscall.EISDIR,
 	},
 	{
 		sfdir + "/" + sfname + "/no-such-file",
 		O_WRONLY,
-		ENOTDIR,
+		syscall.ENOTDIR,
 	},
 }
 
diff --git a/src/pkg/os/path.go b/src/pkg/os/path.go
index e962f3e397..02a77ec805 100644
--- a/src/pkg/os/path.go
+++ b/src/pkg/os/path.go
@@ -4,7 +4,10 @@
 
 package os
 
-import "io"
+import (
+	"io"
+	"syscall"
+)
 
 // MkdirAll creates a directory named path,
 // along with any necessary parents, and returns nil,
@@ -20,7 +23,7 @@ func MkdirAll(path string, perm FileMode) error {
 		if dir.IsDir() {
 			return nil
 		}
-		return &PathError{"mkdir", path, ENOTDIR}
+		return &PathError{"mkdir", path, syscall.ENOTDIR}
 	}
 
 	// Doesn't already exist; make sure parent does.
@@ -70,7 +73,7 @@ func RemoveAll(path string) error {
 	// Otherwise, is this a directory we need to recurse into?
 	dir, serr := Lstat(path)
 	if serr != nil {
-		if serr, ok := serr.(*PathError); ok && (serr.Err == ENOENT || serr.Err == ENOTDIR) {
+		if serr, ok := serr.(*PathError); ok && (IsNotExist(serr.Err) || serr.Err == syscall.ENOTDIR) {
 			return nil
 		}
 		return serr
diff --git a/src/pkg/os/path_test.go b/src/pkg/os/path_test.go
index 8a786008c0..c1e3fb3543 100644
--- a/src/pkg/os/path_test.go
+++ b/src/pkg/os/path_test.go
@@ -8,6 +8,7 @@ import (
 	. "os"
 	"path/filepath"
 	"runtime"
+	"syscall"
 	"testing"
 )
 
@@ -201,7 +202,7 @@ func TestMkdirAllAtSlash(t *testing.T) {
 	if err != nil {
 		pathErr, ok := err.(*PathError)
 		// common for users not to be able to write to /
-		if ok && pathErr.Err == EACCES {
+		if ok && pathErr.Err == syscall.EACCES {
 			return
 		}
 		t.Fatalf(`MkdirAll "/_go_os_test/dir": %v`, err)
diff --git a/src/pkg/os/stat_plan9.go b/src/pkg/os/stat_plan9.go
index 00622581f4..a7990a359e 100644
--- a/src/pkg/os/stat_plan9.go
+++ b/src/pkg/os/stat_plan9.go
@@ -62,7 +62,7 @@ func dirstat(arg interface{}) (d *Dir, err error) {
 			return nil, &PathError{"stat", name, err}
 		}
 		if n < syscall.STATFIXLEN {
-			return nil, &PathError{"stat", name, Eshortstat}
+			return nil, &PathError{"stat", name, errShortStat}
 		}
 
 		// Pull the real size out of the stat message.
@@ -79,7 +79,7 @@ func dirstat(arg interface{}) (d *Dir, err error) {
 			return
 		}
 	}
-	return nil, &PathError{"stat", name, Ebadstat}
+	return nil, &PathError{"stat", name, errBadStat}
 }
 
 // Stat returns a FileInfo structure describing the named file.
diff --git a/src/pkg/os/stat_windows.go b/src/pkg/os/stat_windows.go
index c8bfc3f6d4..ffb679f417 100644
--- a/src/pkg/os/stat_windows.go
+++ b/src/pkg/os/stat_windows.go
@@ -14,7 +14,7 @@ import (
 // If there is an error, it will be of type *PathError.
 func (file *File) Stat() (fi FileInfo, err error) {
 	if file == nil || file.fd < 0 {
-		return nil, EINVAL
+		return nil, syscall.EINVAL
 	}
 	if file.isdir() {
 		// I don't know any better way to do that for directory
diff --git a/test/fixedbugs/bug262.go b/test/fixedbugs/bug262.go
index f5f2c35532..ebca7905f9 100644
--- a/test/fixedbugs/bug262.go
+++ b/test/fixedbugs/bug262.go
@@ -7,7 +7,7 @@
 package main
 
 import (
-	"os"
+	"errors"
 	"strconv"
 )
 
@@ -44,7 +44,7 @@ func main() {
 	}
 	mm := make(map[string]error)
 	trace = ""
-	mm["abc"] = os.EINVAL
+	mm["abc"] = errors.New("invalid")
 	*i(), mm[f()] = strconv.Atoi(h())
 	if mm["abc"] != nil || trace != "ifh" {
 		println("BUG1", mm["abc"], trace)