go/doc/go1.html

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<h2 id="introduction">Introduction to Go 1</h2>

<p>
For a full explanation of the motivation and design of Go 1, see XXX.
Here follows a summary.
</p>

<p>
Go 1 is intended to be a stable language and core library set that
will form a reliable foundation for people and organizations that
want to make a long-term commitment to developing in the Go programming
language. Go will continue to develop, but in a way that guarantees
code written to the Go 1 specification will continue to work. For
instance, Go 1 will be a supported platform on Google App Engine
for the next few years. Incompatible changes to the environment,
should they arise, will be done in a distinct version.
</p>

<p>
This document describes the changes in the language and libraries
in Go 1, relative to the previous release, r60 (at the time of
writing, tagged as r60.3). It also explains how to update code at
r60 to compile and run under Go 1. Finally, it outlines the new
<code>go</code> command for building Go programs and the new binary
release process being introduced. Most of these topics have more
thorough presentations elsewhere; such documents are linked below.
</p>

<h2 id="language">Changes to the language</h2>

<h3 id="append">Append</h3>

<p>
The <code>append</code> built-in function is variadic, so one can
append to a byte slice using the <code>...</code> syntax in the
call.
</p>

<pre><!--{{code "progs/go1.go" `/greeting := ..byte/` `/append.*hello/`}}
-->    greeting := []byte{}
    greeting = append(greeting, []byte(&#34;hello &#34;)...)</pre>

<p>
By analogy with the similar property of <code>copy</code>, Go 1
permits a string to be appended (byte-wise) directly to a byte
slice; the conversion is no longer necessary:
</p>

<pre><!--{{code "progs/go1.go" `/append.*world/`}}
-->    greeting = append(greeting, &#34;world&#34;...)</pre>

<p>
<em>Updating</em>:
This is a new feature, so existing code needs no changes.
</p>

<h3 id="close">Close</h3>

<p>
The <code>close</code> built-in function lets a sender tell a receiver
that no more data will be transmitted on the channel.  In Go 1 the
type system enforces the directionality when possible: it is illegal
to call <code>close</code> on a receive-only channel:
</p>

<pre>
    var c chan int
    var csend chan&lt;- int = c
    var crecv &lt;-chan int = c
    close(c)     // legal
    close(csend) // legal
    close(crecv) // illegal
</pre>

<p>
<em>Updating</em>:
Existing code that attempts to close a receive-only channel was
erroneous even before Go 1 and should be fixed.  The compiler will
now reject such code.
</p>

<h3 id="literals">Composite literals</h3>

<p>
In Go 1, a composite literal of array, slice, or map type can elide the
type specification for the elements' initializers if they are of pointer type.
All four of the initializations in this example are legal; the last one was illegal before Go 1.
</p>

<pre><!--{{code "progs/go1.go" `/type Date struct/` `/STOP/`}}
-->    type Date struct {
        month string
        day   int
    }
    // Struct values, fully qualified; always legal.
    holiday1 := []Date{
        Date{&#34;Feb&#34;, 14},
        Date{&#34;Nov&#34;, 11},
        Date{&#34;Dec&#34;, 25},
    }
    // Struct values, type name elided; always legal.
    holiday2 := []Date{
        {&#34;Feb&#34;, 14},
        {&#34;Nov&#34;, 11},
        {&#34;Dec&#34;, 25},
    }
    // Pointers, fully qualified, always legal.
    holiday3 := []*Date{
        &amp;Date{&#34;Feb&#34;, 14},
        &amp;Date{&#34;Nov&#34;, 11},
        &amp;Date{&#34;Dec&#34;, 25},
    }
    // Pointers, type name elided; legal in Go 1.
    holiday4 := []*Date{
        {&#34;Feb&#34;, 14},
        {&#34;Nov&#34;, 11},
        {&#34;Dec&#34;, 25},
    }</pre>

<p>
<em>Updating</em>:
This change has no effect on existing code, but the command
<code>gofmt</code> <code>-s</code> applied to existing source
will, among other things, elide explicit element types wherever permitted.
</p>


<h3 id="init">Goroutines during init</h3>

<p>
Go 1 allows goroutines to be created and run during initialization.
(They used to be created but were not run until after initialization
completed.) Code that uses goroutines can now be called from
<code>init</code> routines and global initialization expressions
without introducing a deadlock.
</p>

<pre><!--{{code "progs/go1.go" `/PackageGlobal/` `/^}/`}}
-->var PackageGlobal int

func init() {
    c := make(chan int)
    go initializationFunction(c)
    PackageGlobal = &lt;-c
}</pre>

<p>
<em>Updating</em>:
This is a new feature, so existing code needs no changes,
although it's possible that code that depends on goroutines not starting before <code>main</code> will break.
There was no such code in the standard repository.
</p>

<h3 id="rune">The rune type</h3>

<p>
Go 1 introduces a new basic type, <code>rune</code>, to be used to represent
individual Unicode code points.
It is an alias for <code>int32</code>, analogous to <code>byte</code>
as an alias for <code>uint8</code>.
</p>

<p>
Character literals such as <code>'a'</code>, <code>'語'</code>, and <code>'\u0345'</code>
now have default type <code>rune</code>,
analogous to <code>1.0</code> having default type <code>float64</code>.
A variable initialized to a character constant will therefore
have type <code>rune</code> unless otherwise specified.
</p>

<p>
Libraries have been updated to use <code>rune</code> rather than <code>int</code>
when appropriate. For instance, the functions <code>unicode.ToLower</code> and
relatives now take and return a <code>rune</code>.
</p>

<pre><!--{{code "progs/go1.go" `/STARTRUNE/` `/ENDRUNE/`}}
-->    delta := &#39;δ&#39; // delta has type rune.
    var DELTA rune
    DELTA = unicode.ToUpper(delta)
    epsilon := unicode.ToLower(DELTA + 1)
    if epsilon != &#39;δ&#39;+1 {
        log.Fatal(&#34;inconsistent casing for Greek&#34;)
    }</pre>

<p>
<em>Updating</em>:
Most source code will be unaffected by this because the type inference from
<code>:=</code> initializers introduces the new type silently, and it propagates
from there.
Some code may get type errors that a trivial conversion will resolve.
</p>

<h3 id="error">The error type</h3>

<p>
Go 1 introduces a new built-in type, <code>error</code>, which has the following definition:
</p>

<pre>
    type error interface {
        Error() string
    }
</pre>

<p>
Since the consequences of this type are all in the package library,
it is discussed <a href="#errors">below</a>.
</p>

<h3 id="delete">Deleting from maps</h3>

<p>
The original syntax for deleting an element in a map was:
</p>

<pre>
    m[k] = ignored, false
</pre>

<p>
In Go 1, that syntax has gone; instead there is a new built-in
function, <code>delete</code>.  The call
</p>

<pre><!--{{code "progs/go1.go" `/delete\(m, k\)/`}}
-->    delete(m, k)</pre>

<p>
will delete the map entry retrieved by the expression <code>m[k]</code>.
There is no return value. Deleting a non-existent entry is a no-op.
</p>

<p>
<em>Updating</em>:
Running <code>go fix</code> will convert expressions of the form <code>m[k] = ignored,
false</code> into <code>delete(m, k)</code> when it is clear that
the ignored value can be safely discarded from the program and
<code>false</code> refers to the predefined boolean constant.
The fix tool
will flag other uses of the syntax for inspection by the programmer.
</p>

<h3 id="iteration">Iterating in maps</h3>

<p>
In Go 1, the order in which elements are visited when iterating
over a map using a <code>for</code> <code>range</code> statement
is defined to be unpredictable, even if the same loop is run multiple
times with the same map.
Code should not assume that the elements are visited in any particular order.
</p>

<pre><!--{{code "progs/go1.go" `/Sunday/` `/^	}/`}}
-->    m := map[string]int{&#34;Sunday&#34;: 0, &#34;Monday&#34;: 1}
    for name, value := range m {
        // This loop should not assume Sunday will be visited first.
        f(name, value)
    }</pre>

<p>
<em>Updating</em>:
This is one change where tools cannot help.  Most existing code
will be unaffected, but some programs may break or misbehave; we
recommend manual checking of all range statements over maps to
verify they do not depend on iteration order. There were a few such
examples in the standard repository; they have been fixed.
Note that it was already incorrect to depend on the iteration order, which
was unspecified. This change codifies the unpredictability.
</p>

<h3 id="multiple_assignment">Multiple assignment</h3>

<p>
Go 1 fully specifies the evaluation order in multiple assignment
statements. In particular, if the left-hand side of the assignment
statement contains expressions that require evaluation, such as
function calls or array indexing operations, these will all be done
using the usual left-to-right rule before any variables are assigned
their value.  Once everything is evaluated, the actual assignments
proceed in left-to-right order.
</p>

<p>
These examples illustrate the behavior.
</p>

<pre><!--{{code "progs/go1.go" `/sa :=/` `/then sc.0. = 2/`}}
-->    sa := []int{1, 2, 3}
    i := 0
    i, sa[i] = 1, 2 // sets i = 1, sa[0] = 2

    sb := []int{1, 2, 3}
    j := 0
    sb[j], j = 2, 1 // sets sb[0] = 2, j = 1

    sc := []int{1, 2, 3}
    sc[0], sc[0] = 1, 2 // sets sc[0] = 1, then sc[0] = 2 (so sc[0] = 2 at end)</pre>

<p>
<em>Updating</em>:
This is one change where tools cannot help, but breakage is unlikely.
No code in the standard repository was broken by this change, and code
that depended on the previous unspecified behavior was already incorrect.
</p>

<h3 id="shadowing">Returns and shadowed variables</h3>

<p>
A shadowed variable is one that has the same name as another variable in an inner scope.
In functions with named return values,
the Go 1 compilers disallow return statements without arguments if any of the named return values is shadowed at the point of the return statement.
(It isn't part of the specification, because this is one area we are still exploring;
the situation is analogous to the compilers rejecting functions that do not end with an explicit return statement.)
</p>

<p>
This function implicitly returns a shadowed return value and will be rejected by the compiler:
</p>

<pre>
    func Bug() (i, j, k int) {
        for i = 0; i &lt; 5; i++ {
            for j := 0; j &lt; 5; j++ { // Redeclares j.
                k += i*j
                if k > 100 {
                    return // Rejected: j is shadowed here.
                }
            }
        }
        return // OK: j is not shadowed here.
    }
</pre>

<p>
<em>Updating</em>:
Code that shadows return values in this way will be rejected by the compiler and will need to be fixed by hand.
The few cases that arose in the standard repository were mostly bugs.
</p>

<h3 id="unexported">Copying structs with unexported fields</h3>

<p>
Go 1 relaxes the rules about accessing structs with unexported (lower-case) fields,
permitting a client package to assign (and therefore copy) such a struct.
Of course, the client package still cannot access such fields individually.
</p>

<p>
As an example, if package <code>p</code> includes the definitions,
</p>

<pre>
    type Struct struct {
        Public int
        secret int
    }
    func NewStruct(a int) Struct {  // Note: not a pointer.
        return Struct{a, f(a)}
    }
    func (s Struct) String() string {
        return fmt.Sprintf("{%d (secret %d)}", s.Public, s.secret)
    }
</pre>

<p>
a package that imports <code>p</code> can assign and copy values of type
<code>p.Struct</code> at will.
Behind the scenes the unexported fields will be assigned and copied just
as if they were exported,
but the client code will never be aware of them. The code
</p>

<pre>
    import "p"

    myStruct := p.NewStruct(23)
    copyOfMyStruct := myStruct
    fmt.Println(myStruct, copyOfMyStruct)
</pre>

<p>
will show that the secret field of the struct has been copied to the new value.
</p>

<p>
<em>Updating</em>:
This is a new feature, so existing code needs no changes.
</p>

<h3 id="equality">Equality of structs and arrays</h3>

<p>
Go 1 defines equality and inequality (<code>==</code> and
<code>!=</code>) for struct and array values, respectively, provided
the elements of the data structures can themselves be compared.
That is, if equality is defined for all the fields of a struct (or
an array element), then it is defined for the struct (or array).
</p>

<p>
As a result, structs and arrays can now be used as map keys:
</p>

<pre><!--{{code "progs/go1.go" `/type Day struct/` `/Printf/`}}
-->    type Day struct {
        long  string
        short string
    }
    Christmas := Day{&#34;Christmas&#34;, &#34;XMas&#34;}
    Thanksgiving := Day{&#34;Thanksgiving&#34;, &#34;Turkey&#34;}
    holiday := map[Day]bool{
        Christmas:    true,
        Thanksgiving: true,
    }
    fmt.Printf(&#34;Christmas is a holiday: %t\n&#34;, holiday[Christmas])</pre>

<p>
Note that equality is still undefined for slices, for which the
calculation is in general infeasible.  Also note that the ordered
comparison operators (<code>&lt;</code> <code>&lt;=</code>
<code>&gt;</code> <code>&gt;=</code>) are still undefined for
structs and arrays.

<p>
<em>Updating</em>:
This is a new feature, so existing code needs no changes.
</p>

<h3 id="funcs">Function and map equality</h3>

<p>
Go 1 disallows checking for equality of functions and maps,
respectively, except to compare them directly to <code>nil</code>.
</p>

<p>
<em>Updating</em>:
Existing code that depends on function or map equality will be
rejected by the compiler and will need to be fixed by hand.
Few programs will be affected, but the fix may require some
redesign.
</p>

<h2 id="packages">The package hierarchy</h2>

<p>
This section describes how the packages have been rearranged in Go 1.
Some have moved, some have been renamed, some have been deleted.
New packages are described in later sections.
</p>

<h3 id="hierarchy">The package hierarchy</h3>

<p>
Go 1 has a rearranged package hierarchy that groups related items
into subdirectories. For instance, <code>utf8</code> and
<code>utf16</code> now occupy subdirectories of <code>unicode</code>.
Also, <a href="#subrepo">some packages</a> have moved into
subrepositories of
<a href="http://code.google.com/p/go"><code>code.google.com/p/go</code></a>
while <a href="#deleted">others</a> have been deleted outright.
</p>

<table class="codetable" frame="border" summary="Moved packages">
<colgroup align="left" width="60%"></colgroup>
<colgroup align="left" width="40%"></colgroup>
<tr>
<th align="left">Old path</th>
<th align="left">New path</th>
</tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>asn1</td> <td>encoding/asn1</td></tr>
<tr><td>csv</td> <td>encoding/csv</td></tr>
<tr><td>gob</td> <td>encoding/gob</td></tr>
<tr><td>json</td> <td>encoding/json</td></tr>
<tr><td>xml</td> <td>encoding/xml</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>exp/template/html</td> <td>html/template</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>big</td> <td>math/big</td></tr>
<tr><td>cmath</td> <td>math/cmplx</td></tr>
<tr><td>rand</td> <td>math/rand</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>http</td> <td>net/http</td></tr>
<tr><td>http/cgi</td> <td>net/http/cgi</td></tr>
<tr><td>http/fcgi</td> <td>net/http/fcgi</td></tr>
<tr><td>http/httptest</td> <td>net/http/httptest</td></tr>
<tr><td>http/pprof</td> <td>net/http/pprof</td></tr>
<tr><td>mail</td> <td>net/mail</td></tr>
<tr><td>rpc</td> <td>net/rpc</td></tr>
<tr><td>rpc/jsonrpc</td> <td>net/rpc/jsonrpc</td></tr>
<tr><td>smtp</td> <td>net/smtp</td></tr>
<tr><td>url</td> <td>net/url</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>exec</td> <td>os/exec</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>scanner</td> <td>text/scanner</td></tr>
<tr><td>tabwriter</td> <td>text/tabwriter</td></tr>
<tr><td>template</td> <td>text/template</td></tr>
<tr><td>template/parse</td> <td>text/template/parse</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>utf8</td> <td>unicode/utf8</td></tr>
<tr><td>utf16</td> <td>unicode/utf16</td></tr>
</table>

<p>
Note that the package names for the old <code>cmath</code> and
<code>exp/template/html</code> packages have changed to <code>cmplx</code>
and <code>template</code>.
</p>

<p>
<em>Updating</em>:
Running <code>go fix</code> will update all imports and package renames for packages that
remain inside the standard repository.  Programs that import packages
that are no longer in the standard repository will need to be edited
by hand.
</p>

<h3 id="exp">The package tree exp</h3>

<p>
Because they are not standardized, the packages under the <code>exp</code> directory will not be available in the
standard Go 1 release distributions, although they will be available in source code form
in <a href="http://code.google.com/p/go/">the repository</a> for
developers who wish to use them.
</p>

<p>
Several packages have moved under <code>exp</code> at the time of Go 1's release:
</p>

<ul>
<li><code>ebnf</code></li>
<li><code>go/types</code></li>
<li><code>os/signal</code></li>
</ul>

<p>
All these packages are available under the same names, with the prefix <code>exp/</code>: <code>exp/ebnf</code> etc.
</p>

<p>
Also, the <code>utf8.String</code> type has been moved to its own package, <code>exp/utf8string</code>.
</p>

<p>
Finally, the <code>gotype</code> command now resides in <code>exp/gotype</code>, while
<code>ebnflint</code> is now in <code>exp/ebnflint</code>.
If they are installed, they now reside in <code>$GOROOT/bin/tool</code>.
</p>

<p>
<em>Updating</em>:
Code that uses packages in <code>exp</code> will need to be updated by hand,
or else compiled from an installation that has <code>exp</code> available.
The <code>go fix</code> tool or the compiler will complain about such uses.
</p>

<h3 id="old">The package tree old</h3>

<p>
Because they are deprecated, the packages under the <code>old</code> directory will not be available in the
standard Go 1 release distributions, although they will be available in source code form for
developers who wish to use them.
</p>

<p>
The packages in their new locations are:
</p>

<ul>
<li><code>old/netchan</code></li>
<li><code>old/regexp</code></li>
<li><code>old/template</code></li>
</ul>

<p>
<em>Updating</em>:
Code that uses packages now in <code>old</code> will need to be updated by hand,
or else compiled from an installation that has <code>old</code> available.
The <code>go fix</code> tool will warn about such uses.
</p>

<h3 id="deleted">Deleted packages</h3>

<p>
Go 1 deletes several packages outright:
</p>

<ul>
<li><code>container/vector</code></li>
<li><code>exp/datafmt</code></li>
<li><code>go/typechecker</code></li>
<li><code>try</code></li>
</ul>

<p>
and also the command <code>gotry</code>.
</p>

<p>
<em>Updating</em>:
Code that uses <code>container/vector</code> should be updated to use
slices directly.  See
<a href="http://code.google.com/p/go-wiki/wiki/SliceTricks">the Go
Language Community Wiki</a> for some suggestions.
Code that uses the other packages (there should be almost zero) will need to be rethought.
</p>

<h3 id="subrepo">Packages moving to subrepositories</h3>

<p>
Go 1 has moved a number of packages into sub-repositories of
<a href="http://code.google.com/p/go/">the main Go repository</a>.
This table lists the old and new import paths:

<table class="codetable" frame="border" summary="Sub-repositories">
<colgroup align="left" width="40%"></colgroup>
<colgroup align="left" width="60%"></colgroup>
<tr>
<th align="left">Old</th>
<th align="left">New</th>
</tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>crypto/bcrypt</td> <td>code.google.com/p/go.crypto/bcrypt</tr>
<tr><td>crypto/blowfish</td> <td>code.google.com/p/go.crypto/blowfish</tr>
<tr><td>crypto/cast5</td> <td>code.google.com/p/go.crypto/cast5</tr>
<tr><td>crypto/md4</td> <td>code.google.com/p/go.crypto/md4</tr>
<tr><td>crypto/ocsp</td> <td>code.google.com/p/go.crypto/ocsp</tr>
<tr><td>crypto/openpgp</td> <td>code.google.com/p/go.crypto/openpgp</tr>
<tr><td>crypto/openpgp/armor</td> <td>code.google.com/p/go.crypto/openpgp/armor</tr>
<tr><td>crypto/openpgp/elgamal</td> <td>code.google.com/p/go.crypto/openpgp/elgamal</tr>
<tr><td>crypto/openpgp/errors</td> <td>code.google.com/p/go.crypto/openpgp/errors</tr>
<tr><td>crypto/openpgp/packet</td> <td>code.google.com/p/go.crypto/openpgp/packet</tr>
<tr><td>crypto/openpgp/s2k</td> <td>code.google.com/p/go.crypto/openpgp/s2k</tr>
<tr><td>crypto/ripemd160</td> <td>code.google.com/p/go.crypto/ripemd160</tr>
<tr><td>crypto/twofish</td> <td>code.google.com/p/go.crypto/twofish</tr>
<tr><td>crypto/xtea</td> <td>code.google.com/p/go.crypto/xtea</tr>
<tr><td>exp/ssh</td> <td>code.google.com/p/go.crypto/ssh</tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>image/bmp</td> <td>code.google.com/p/go.image/bmp</tr>
<tr><td>image/tiff</td> <td>code.google.com/p/go.image/tiff</tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>net/dict</td> <td>code.google.com/p/go.net/dict</tr>
<tr><td>net/websocket</td> <td>code.google.com/p/go.net/websocket</tr>
<tr><td>exp/spdy</td> <td>code.google.com/p/go.net/spdy</tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>encoding/git85</td> <td>code.google.com/p/go.codereview/git85</tr>
<tr><td>patch</td> <td>code.google.com/p/go.codereview/patch</tr>
</table>

<p>
<em>Updating</em>:
Running <code>go fix</code> will update imports of these packages to use the new import paths.
Installations that depend on these packages will need to install them using
a <code>go install</code> command.
</p>

<h2 id="major">Major changes to the library</h2>

<p>
This section describes significant changes to the core libraries, the ones that
affect the most programs.
</p>

<h3 id="errors">The error type and errors package</h3>

<p>
As mentioned above, Go 1 introduces a new built-in interface type called <code>error</code>.
Its intent is to replace the old <code>os.Error</code> type with a more central concept.
So the widely-used <code>String</code> method does not cause accidental satisfaction
of the <code>error</code> interface, the <code>error</code> interface uses instead
the name <code>Error</code> for that method:
</p>

<pre>
    type error interface {
        Error() string
    }
</pre>

<p>
The <code>fmt</code> library automatically invokes <code>Error</code>, as it already
does for <code>String</code>, for easy printing of error values.
</p>

<pre><!--{{code "progs/go1.go" `/START ERROR EXAMPLE/` `/END ERROR EXAMPLE/`}}
-->type SyntaxError struct {
    File    string
    Line    int
    Message string
}

func (se *SyntaxError) Error() string {
    return fmt.Sprintf(&#34;%s:%d: %s&#34;, se.File, se.Line, se.Message)
}</pre>

<p>
All standard packages have been updated to use the new interface; the old <code>os.Error</code> is gone.
</p>

<p>
A new package, <a href="/pkg/errors/"><code>errors</code></a>, contains the function
</p>

<pre>
func New(text string) error
</pre>

<p>
to turn a string into an error. It replaces the old <code>os.NewError</code>.
</p>

<pre><!--{{code "progs/go1.go" `/ErrSyntax/`}}
-->    var ErrSyntax = errors.New(&#34;syntax error&#34;)</pre>
		
<p>
<em>Updating</em>:
Running <code>go fix</code> will update almost all code affected by the change.
Code that defines error types with a <code>String</code> method will need to be updated
by hand to rename the methods to <code>Error</code>.
</p>

<h3 id="errno">System call errors</h3>

<p>
In Go 1, the
<a href="/pkg/syscall/"><code>syscall</code></a>
package returns an <code>error</code> for system call errors,
rather than plain integer <code>errno</code> values.
On Unix, the implementation is done by a 
<a href="/pkg/syscall/#Errno"><code>syscall.Errno</code></a> type
that satisfies <code>error</code> and replaces the old <code>os.Errno</code>.
</p>

<p>
<em>Updating</em>:
Running <code>go fix</code> will update almost all code affected by the change.
Regardless, most code should use the <code>os</code> package
rather than <code>syscall</code> and so will be unaffected.
</p>

<h3 id="time">Time</h3>

<p>
One of the most sweeping changes in the Go 1 library is the
complete redesign of the 
<a href="/pkg/time/"><code>time</code></a> package.
Instead of an integer number of nanoseconds as an <code>int64</code>,
and a separate <code>*time.Time</code> type to deal with human
units such as hours and years,
there are now two fundamental types:
<a href="/pkg/time/#Time"><code>time.Time</code></a>
(a value, so the <code>*</code> is gone), which represents a moment in time;
and <a href="/pkg/time/#Duration"><code>time.Duration</code></a>,
which represents an interval.
Both have nanosecond resolution.
A <code>Time</code> can represent any time into the ancient
past and remote future, while a <code>Duration</code> can
span plus or minus only about 290 years.
There are methods on these types, plus a number of helpful
predefined constant durations such as <code>time.Second</code>.
</p>

<p>
Among the new methods are things like
<a href="/pkg/time/#Time.Add"><code>Time.Add</code></a>,
which adds a <code>Duration</code> to a <code>Time</code>, and
<a href="/pkg/time/#Time.Sub"><code>Time.Sub</code></a>,
which subtracts two <code>Times</code> to yield a <code>Duration</code>.
</p>

<p>
The most important semantic change is that the Unix epoch (Jan 1, 1970) is now
relevant only for those functions and methods that mention Unix:
<a href="/pkg/time/#Unix"><code>time.Unix</code></a>
and the <a href="/pkg/time/#Time.Unix"><code>Unix</code></a>
and <a href="/pkg/time/#Time.UnixNano"><code>UnixNano</code></a> methods
of the <code>Time</code> type.
In particular,
<a href="/pkg/time/#Now"><code>time.Now</code></a>
returns a <code>time.Time</code> value rather than, in the old
API, an integer nanosecond count since the Unix epoch.
</p>

<pre><!--{{code "progs/go1.go" `/sleepUntil/` `/^}/`}}
-->// sleepUntil sleeps until the specified time. It returns immediately if it&#39;s too late.
func sleepUntil(wakeup time.Time) {
    now := time.Now() // A Time.
    if !wakeup.After(now) {
        return
    }
    delta := wakeup.Sub(now) // A Duration.
    log.Printf(&#34;Sleeping for %.3fs&#34;, delta.Seconds())
    time.Sleep(delta)
}</pre>

<p>
The new types, methods, and constants have been propagated through
all the standard packages that use time, such as <code>os</code> and
its representation of file time stamps.
</p>

<p>
<em>Updating</em>:
The <code>go fix</code> tool will update many uses of the old <code>time</code> package to use the new
types and methods, although it does not replace values such as <code>1e9</code>
representing nanoseconds per second.
Also, because of type changes in some of the values that arise,
some of the expressions rewritten by the fix tool may require
further hand editing; in such cases the rewrite will include
the correct function or method for the old functionality, but
may have the wrong type or require further analysis.
</p>

<h2 id="minor">Minor changes to the library</h2>

<p>
This section describes smaller changes, such as those to less commonly
used packages or that affect
few programs beyond the need to run <code>go fix</code>.
This category includes packages that are new in Go 1.
</p>

<h3 id="archive_zip">The archive/zip package</h3>

<p>
In Go 1, <a href="/pkg/archive/zip/#Writer"><code>*zip.Writer</code></a> no
longer has a <code>Write</code> method. Its presence was a mistake.
</p>

<p>
<em>Updating</em>:
What little code is affected will be caught by the compiler and must be updated by hand.
</p>

<h3 id="bufio">The bufio package</h3>

<p>
In Go 1, <a href="/pkg/bufio/#NewReaderSize"><code>bufio.NewReaderSize</code></a>
and
<a href="/pkg/bufio/#NewWriterSize"><code>bufio.NewWriterSize</code></a>
functions no longer return an error for invalid sizes.
If the argument size is too small or invalid, it is adjusted.
</p>

<p>
<em>Updating</em>:
Running <code>go fix</code> will update calls that assign the error to _.
Calls that aren't fixed will be caught by the compiler and must be updated by hand.
</p>

<h3 id="compress">The compress/flate, compress/gzip and compress/zlib packages</h3>

<p>
In Go 1, the <code>NewWriterXxx</code> functions in
<a href="/pkg/compress/flate"><code>compress/flate</code></a>,
<a href="/pkg/compress/gzip"><code>compress/gzip</code></a> and
<a href="/pkg/compress/zlib"><code>compress/zlib</code></a>
all return <code>(*Writer, error)</code> if they take a compression level,
and <code>*Writer</code> otherwise. Package <code>gzip</code>'s
<code>Compressor</code> and <code>Decompressor</code> types have been renamed
to <code>Writer</code> and <code>Reader</code>. Package <code>flate</code>'s
<code>WrongValueError</code> type has been removed.
</p>

<p>
<em>Updating</em>
Running <code>go fix</code> will update old names and calls that assign the error to _.
Calls that aren't fixed will be caught by the compiler and must be updated by hand.
</p>

<h3 id="crypto_aes_des">The crypto/aes and crypto/des packages</h3>

<p>
In Go 1, the <code>Reset</code> method has been removed. Go does not guarantee
that memory is not copied and therefore this method was misleading.
</p>

<p>
The cipher-specific types <code>*aes.Cipher</code>, <code>*des.Cipher</code>,
and <code>*des.TripleDESCipher</code> have been removed in favor of
<code>cipher.Block</code>.
</p>

<p>
<em>Updating</em>:
Remove the calls to Reset. Replace uses of the specific cipher types with
cipher.Block.
</p>

<h3 id="crypto_elliptic">The crypto/elliptic package</h3>

<p>
In Go 1, <a href="/pkg/crypto/elliptic/#Curve"><code>elliptic.Curve</code></a>
has been made an interface to permit alternative implementations. The curve
parameters have been moved to the
<a href="/pkg/crypto/elliptic/#CurveParams"><code>elliptic.CurveParams</code></a>
structure.
</p>

<p>
<em>Updating</em>:
Existing users of <code>*elliptic.Curve</code> will need to change to
simply <code>elliptic.Curve</code>. Calls to <code>Marshal</code>,
<code>Unmarshal</code> and <code>GenerateKey</code> are now functions
in <code>crypto/elliptic</code> that take an <code>elliptic.Curve</code>
as their first argument.
</p>

<h3 id="crypto_hmac">The crypto/hmac package</h3>

<p>
In Go 1, the hash-specific functions, such as <code>hmac.NewMD5</code>, have
been removed from <code>crypto/hmac</code>. Instead, <code>hmac.New</code> takes
a function that returns a <code>hash.Hash</code>, such as <code>md5.New</code>.
</p>

<p>
<em>Updating</em>:
Running <code>go fix</code> will perform the needed changes.
</p>

<h3 id="crypto_x509">The crypto/x509 package</h3>

<p>
In Go 1, the
<a href="/pkg/crypto/x509/#CreateCertificate"><code>CreateCertificate</code></a>
and
<a href="/pkg/crypto/x509/#CreateCRL"><code>CreateCRL</code></a>
functions in <code>crypto/x509</code> have been altered to take an
<code>interface{}</code> where they previously took a <code>*rsa.PublicKey</code>
or <code>*rsa.PrivateKey</code>. This will allow other public key algorithms
to be implemented in the future.
</p>

<p>
<em>Updating</em>:
No changes will be needed.
</p>

<h3 id="encoding_binary">The encoding/binary package</h3>               

<p>
In Go 1, the <code>binary.TotalSize</code> function has been replaced by
<a href="/pkg/encoding/binary/#Size"><code>Size</code></a>,
which takes an <code>interface{}</code> argument rather than
a <code>reflect.Value</code>.
</p>

<p>
<em>Updating</em>:
What little code is affected will be caught by the compiler and must be updated by hand.
</p>

<h3 id="encoding_xml">The encoding/xml package</h3>

<p>
In Go 1, the <a href="/pkg/encoding/xml/"><code>xml</code></a> package
has been brought closer in design to the other marshaling packages such
as <a href="/pkg/encoding/gob/"><code>encoding/gob</code></a>.
</p>

<p>
The old <code>Parser</code> type is renamed
<a href="/pkg/encoding/xml/#Decoder"><code>Decoder</code></a> and has a new
<a href="/pkg/encoding/xml/#Decoder.Decode"><code>Decode</code></a> method. An
<a href="/pkg/encoding/xml/#Encoder"><code>Encoder</code></a> type was also     introducted.
</p>

<p>
The functions <a href="/pkg/encoding/xml/#Marshal"><code>Marshal</code></a>
and <a href="/pkg/encoding/xml/#Unmarshal"><code>Unmarshal</code></a>
work with <code>[]byte</code> values now. To work with streams,
use the new <a href="/pkg/encoding/xml/#Encoder"><code>Encoder</code></a>
and <a href="/pkg/encoding/xml/#Decoder"><code>Decoder</code></a> types.
</p>

<p>
When marshaling or unmarshaling values, the format of supported flags in
field tags has changed to be closer to the
<a href="/pkg/encoding/json"><code>json</code></a> package
(<code>`xml:"name,flag"`</code>). The matching done between field tags, field
names, and the XML attribute and element names is now case-sensitive.
The <code>XMLName</code> field tag, if present, must also match the name
of the XML element being marshaled.
</p>

<p>
<em>Updating</em>:
Running <code>go fix</code> will update most uses of the package except for some calls to
<code>Unmarshal</code>. Special care must be taken with field tags,
since the fix tool will not update them and if not fixed by hand they will
misbehave silently in some cases. For example, the old
<code>"attr"</code> is now written <code>",attr"</code> while plain
<code>"attr"</code> remains valid but with a different meaning.
</p>

<h3 id="expvar">The expvar package</h3>

<p>
In Go 1, the <code>RemoveAll</code> function has been removed.
The <code>Iter</code> function and Iter method on <code>*Map</code> have
been replaced by
<a href="/pkg/expvar/#Do"><code>Do</code></a>
and
<a href="/pkg/expvar/#Map.Do"><code>(*Map).Do</code></a>.
</p>

<p>
<em>Updating</em>:
Most code using <code>expvar</code> will not need changing. The rare code that used
<code>Iter</code> can be updated to pass a closure to <code>Do</code> to achieve the same effect.
</p>

<h3 id="flag">The flag package</h3>

<p>
In Go 1, the interface <a href="/pkg/flag/#Value"><code>flag.Value</code></a> has changed slightly.
The <code>Set</code> method now returns an <code>error</code> instead of
a <code>bool</code> to indicate success or failure.
</p>

<p>
There is also a new kind of flag, <code>Duration</code>, to support argument
values specifying time intervals.
Values for such flags must be given units, just as <code>time.Duration</code>
formats them: <code>10s</code>, <code>1h30m</code>, etc.
</p>

<pre><!--{{code "progs/go1.go" `/timeout/`}}
-->var timeout = flag.Duration(&#34;timeout&#34;, 30*time.Second, &#34;how long to wait for completion&#34;)</pre>

<p>
<em>Updating</em>:
Programs that implement their own flags will need minor manual fixes to update their
<code>Set</code> methods.
The <code>Duration</code> flag is new and affects no existing code.
</p>


<h3 id="go">The go/* packages</h3>

<p>
Several packages under <code>go</code> have slightly revised APIs.
</p>

<p>
A concrete <code>Mode</code> type was introduced for configuration mode flags
in the packages
<a href="/pkg/go/scanner/"><code>go/scanner</code></a>,
<a href="/pkg/go/parser/"><code>go/parser</code></a>,
<a href="/pkg/go/printer/"><code>go/printer</code></a>, and
<a href="/pkg/go/doc/"><code>go/doc</code></a>.
</p>

<p>
The modes <code>AllowIllegalChars</code> and <code>InsertSemis</code> have been removed
from the <a href="/pkg/go/scanner/"><code>go/scanner</code></a> package. They were mostly
useful for scanning text other then Go source files. Instead, the
<a href="/pkg/text/scanner/"><code>text/scanner</code></a> package should be used
for that purpose.
</p>

<p>
The <a href="/pkg/go/scanner/#ErrorHandler"><code>ErrorHandler</code></a> provided
to the scanner's <a href="/pkg/go/scanner/#Scanner.Init"><code>Init</code></a> method is
now simply a function rather than an interface. The <code>ErrorVector</code> type has
been removed in favor of the (existing) <a href="/pkg/go/scanner/#ErrorList"><code>ErrorList</code></a>
type, and the <code>ErrorVector</code> methods have been migrated. Instead of embedding
an <code>ErrorVector</code> in a client of the scanner, now a client should maintain
an <code>ErrorList</code>.
</p>

<p>
The set of parse functions provided by the <a href="/pkg/go/parser/"><code>go/parser</code></a>
package has been reduced to the primary parse function
<a href="/pkg/go/parser/#ParseFile"><code>ParseFile</code></a>, and a couple of
convenience functions <a href="/pkg/go/parser/#ParseDir"><code>ParseDir</code></a>
and <a href="/pkg/go/parser/#ParseExpr"><code>ParseExpr</code></a>.
</p>

<p>
The <a href="/pkg/go/printer/"><code>go/printer</code></a> package supports an additional
configuration mode <a href="/pkg/go/printer/#Mode"><code>SourcePos</code></a>;
if set, the printer will emit <code>//line</code> comments such that the generated
output contains the original source code position information. The new type
<a href="/pkg/go/printer/#CommentedNode"><code>CommentedNode</code></a> can be  
used to provide comments associated with an arbitrary
<a href="/pkg/go/ast/#Node"><code>ast.Node</code></a> (until now only
<a href="/pkg/go/ast/#File"><code>ast.File</code></a> carried comment information).
</p>

<p>
The type names of the <a href="/pkg/go/doc/"><code>go/doc</code></a> package have been
streamlined by removing the <code>Doc</code> suffix: <code>PackageDoc</code>
is now <code>Package</code>, <code>ValueDoc</code> is <code>Value</code>, etc.
Also, all types now consistently have a <code>Name</code> field (or <code>Names</code>,
in the case of type <code>Value</code>) and <code>Type.Factories</code> has become
<code>Type.Funcs</code>.
Instead of calling <code>doc.NewPackageDoc(pkg, importpath)</code>,
documentation for a package is created with:
</p>

<pre>
    doc.New(pkg, importpath, mode)
</pre>

<p>
where the new <code>mode</code> parameter specifies the operation mode:
if set to <a href="/pkg/go/doc/#AllDecls"><code>AllDecls</code></a>, all declarations
(not just exported ones) are considered.
The function <code>NewFileDoc</code> was removed, and the function
<code>CommentText</code> has become the method
<a href="/pkg/go/ast/#Text"><code>Text</code></a> of
<a href="/pkg/go/ast/#CommentGroup"><code>ast.CommentGroup</code></a>.
</p>

<p>
In package <a href="/pkg/go/token/"><code>go/token</code></a>, the
<a href="/pkg/go/token/#FileSet"><code>token.FileSet</code></a> method <code>Files</code>
(which originally returned a channel of <code>*token.File</code>s) has been replaced
with the iterator <a href="/pkg/go/token/#FileSet.Iterate"><code>Iterate</code></a> that
accepts a function argument instead.
</p>

<p>
<em>Updating</em>:
Code that uses packages in <code>go</code> will have to be updated by hand; the
compiler will reject incorrect uses. Templates used in conjuction with any of the
<code>go/doc</code> types may need manual fixes; the renamed fields will lead
to run-time errors.
</p>

<h3 id="hash">The hash package</h3>

<p>
In Go 1, the definition of <a href="/pkg/hash/#Hash"><code>hash.Hash</code></a> includes
a new method, <code>BlockSize</code>.  This new method is used primarily in the 
cryptographic libraries.
</p>

<p>
The <code>Sum</code> method of the
<a href="/pkg/hash/#Hash"><code>hash.Hash</code></a> interface now takes a
<code>[]byte</code> argument, to which the hash value will be appended.
The previous behavior can be recreated by adding a <code>nil</code> argument to the call.
</p>

<p>
<em>Updating</em>:
Existing implementations of <code>hash.Hash</code> will need to add a
<code>BlockSize</code> method.  Hashes that process the input one byte at
a time can implement <code>BlockSize</code> to return 1.
Running <code>go fix</code> will update calls to the <code>Sum</code> methods of the various
implementations of <code>hash.Hash</code>.
</p>

<p>
<em>Updating</em>:
Since the package's functionality is new, no updating is necessary.
</p>

<h3 id="http">The http package</h3>

<p>
In Go 1 the <a href="/pkg/net/http/"><code>http</code></a> package is refactored,
putting some of the utilities into a
<a href="/pkg/net/httputil/"><code>httputil</code></a> subdirectory.
These pieces are only rarely needed by HTTP clients.
The affected items are:
</p>

<ul>
<li>ClientConn</li>
<li>DumpRequest</li>
<li>DumpRequest</li>
<li>DumpRequestOut</li>
<li>DumpResponse</li>
<li>NewChunkedReader</li>
<li>NewChunkedWriter</li>
<li>NewClientConn</li>
<li>NewProxyClientConn</li>
<li>NewServerConn</li>
<li>NewSingleHostReverseProxy</li>
<li>ReverseProxy</li>
<li>ServerConn</li>
</ul>

<p>
The <code>Request.RawURL</code> field has been removed; it was a
historical artifact.
</p>

<p>
The <code>Handle</code> and <code>HandleFunc</code>
functions, and the similarly-named methods of <code>ServeMux</code>,
now panic if an attempt is made to register the same pattern twice.
</p>

<p>
<em>Updating</em>:
Running <code>go fix</code> will update the few programs that are affected except for
uses of <code>RawURL</code>, which must be fixed by hand.
</p>

<h3 id="image">The image package</h3>

<p>
The <a href="/pkg/image/"><code>image</code></a> package has had a number of
minor changes, rearrangements and renamings.
</p>

<p>
Most of the color handling code has been moved into its own package,
<a href="/pkg/image/color/"><code>image/color</code></a>.
For the elements that moved, a symmetry arises; for instance,
each pixel of an
<a href="/pkg/image/#RGBA"><code>image.RGBA</code></a>
is a
<a href="/pkg/image/color/#RGBA"><code>color.RGBA</code></a>.
</p>

<p>
The old <code>image/ycbcr</code> package has been folded, with some
renamings, into the
<a href="/pkg/image/"><code>image</code></a>
and
<a href="/pkg/image/color/"><code>image/color</code></a>
packages.
</p>

<p>
The old <code>image.ColorImage</code> type is still in the <code>image</code>
package but has been renamed
<a href="/pkg/image/#Uniform"><code>image.Uniform</code></a>,
while <code>image.Tiled</code> has been removed.
</p>

<p>
This table lists the renamings.
</p>

<table class="codetable" frame="border" summary="image renames">
<colgroup align="left" width="50%"></colgroup>
<colgroup align="left" width="50%"></colgroup>
<tr>
<th align="left">Old</th>
<th align="left">New</th>
</tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>image.Color</td> <td>color.Color</td></tr>
<tr><td>image.ColorModel</td> <td>color.Model</td></tr>
<tr><td>image.ColorModelFunc</td> <td>color.ModelFunc</td></tr>
<tr><td>image.PalettedColorModel</td> <td>color.Palette</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>image.RGBAColor</td> <td>color.RGBA</td></tr>
<tr><td>image.RGBA64Color</td> <td>color.RGBA64</td></tr>
<tr><td>image.NRGBAColor</td> <td>color.NRGBA</td></tr>
<tr><td>image.NRGBA64Color</td> <td>color.NRGBA64</td></tr>
<tr><td>image.AlphaColor</td> <td>color.Alpha</td></tr>
<tr><td>image.Alpha16Color</td> <td>color.Alpha16</td></tr>
<tr><td>image.GrayColor</td> <td>color.Gray</td></tr>
<tr><td>image.Gray16Color</td> <td>color.Gray16</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>image.RGBAColorModel</td> <td>color.RGBAModel</td></tr>
<tr><td>image.RGBA64ColorModel</td> <td>color.RGBA64Model</td></tr>
<tr><td>image.NRGBAColorModel</td> <td>color.NRGBAModel</td></tr>
<tr><td>image.NRGBA64ColorModel</td> <td>color.NRGBA64Model</td></tr>
<tr><td>image.AlphaColorModel</td> <td>color.AlphaModel</td></tr>
<tr><td>image.Alpha16ColorModel</td> <td>color.Alpha16Model</td></tr>
<tr><td>image.GrayColorModel</td> <td>color.GrayModel</td></tr>
<tr><td>image.Gray16ColorModel</td> <td>color.Gray16Model</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>ycbcr.RGBToYCbCr</td> <td>color.RGBToYCbCr</td></tr>
<tr><td>ycbcr.YCbCrToRGB</td> <td>color.YCbCrToRGB</td></tr>
<tr><td>ycbcr.YCbCrColorModel</td> <td>color.YCbCrModel</td></tr>
<tr><td>ycbcr.YCbCrColor</td> <td>color.YCbCr</td></tr>
<tr><td>ycbcr.YCbCr</td> <td>image.YCbCr</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>ycbcr.SubsampleRatio444</td> <td>image.YCbCrSubsampleRatio444</td></tr>
<tr><td>ycbcr.SubsampleRatio422</td> <td>image.YCbCrSubsampleRatio422</td></tr>
<tr><td>ycbcr.SubsampleRatio420</td> <td>image.YCbCrSubsampleRatio420</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>image.ColorImage</td> <td>image.Uniform</td></tr>
</table>

<p>
The image package's <code>New</code> functions
(<a href="/pkg/image/#NewRGBA"><code>NewRGBA</code></a>,
<a href="/pkg/image/#NewRGBA64"><code>NewRGBA64</code></a>, etc.)
take an <a href="/pkg/image/#Rectangle"><code>image.Rectangle</code></a> as an argument
instead of four integers.
</p>

<p>
Finally, there are new predefined <code>color.Color</code> variables
<a href="/pkg/image/color/#Black"><code>color.Black</code></a>,
<a href="/pkg/image/color/#White"><code>color.White</code></a>,
<a href="/pkg/image/color/#Opaque"><code>color.Opaque</code></a>
and
<a href="/pkg/image/color/#Transparent"><code>color.Transparent</code></a>.
</p>

<p>
<em>Updating</em>:
Running <code>go fix</code> will update almost all code affected by the change.
</p>

<h3 id="log_syslog">The log/syslog package</h3>

<p>
In Go 1, the <a href="/pkg/log/syslog/#NewLogger"><code>syslog.NewLogger</code></a>     
function returns an error as well as a <code>log.Logger</code>.
</p>

<p>
<em>Updating</em>:
What little code is affected will be caught by the compiler and must be updated by hand.
</p>

<h3 id="mime">The mime package</h3>

<p>
In Go 1, the <a href="/pkg/mime/#FormatMediaType"><code>FormatMediaType</code></a> function
of the <code>mime</code> package has  been simplified to make it
consistent with
<a href="/pkg/mime/#ParseMediaType"><code>ParseMediaType</code></a>. 
It now takes <code>"text/html"</code> rather than <code>"text"</code> and <code>"html"</code>.
</p>

<p>
<em>Updating</em>:
What little code is affected will be caught by the compiler and must be updated by hand.
</p>

<h3 id="net">The net package</h3>

<p>
In Go 1, the various <code>SetTimeout</code>,
<code>SetReadTimeout</code>, and <code>SetWriteTimeout</code> methods
have been replaced with 
<a href="/pkg/net/#IPConn.SetDeadline"><code>SetDeadline</code></a>,
<a href="/pkg/net/#IPConn.SetReadDeadline"><code>SetReadDeadline</code></a>, and 
<a href="/pkg/net/#IPConn.SetWriteDeadline"><code>SetWriteDeadline</code></a>,
respectively.  Rather than taking a timeout value in nanoseconds that
apply to any activity on the connection, the new methods set an
absolute deadline (as a <code>time.Time</code> value) after which
reads and writes will time out and no longer block.
</p>

<p>
There are also new functions
<a href="/pkg/net/#DialTimeout"><code>net.DialTimeout</code></a>
to simplify timing out dialing a network address and
<a href="/pkg/net/#ListenMulticastUDP"><code>net.ListenMulticastUDP</code></a>
to allow multicast UDP to listen concurrently across multiple listeners.
The <code>net.ListenMulticastUDP</code> function replaces the old
<code>JoinGroup</code> and <code>LeaveGroup</code> methods.
</p>

<p>
<em>Updating</em>:
Code that uses the old methods will fail to compile and must be updated by hand.
The semantic change makes it difficult for the fix tool to update automatically.
</p>

<h3 id="os">The os package</h3>

<p>
The <code>Time</code> function has been removed; callers should use
the <a href="/pkg/time/#Time"><code>Time</code></a> type from the
<code>time</code> package.
</p>

<p>
The <code>Exec</code> function has been removed; callers should use
<code>Exec</code> from the <code>syscall</code> package, where available.
</p>

<p>
The <code>ShellExpand</code> function has been renamed to <a
href="/pkg/os/#ExpandEnv"><code>ExpandEnv</code></a>.
</p>

<p>
The <a href="/pkg/os/#NewFile"><code>NewFile</code></a> function
now takes a <code>uintptr</code> fd, instead of an <code>int</code>.
The <a href="/pkg/os/#File.Fd"><code>Fd</code></a> method on files now
also returns a <code>uintptr</code>.
</p>

<p>
There are no longer error constants such as <code>EINVAL</code>
in the <code>os</code> package, since the set of values varied with
the underlying operating system. There are new portable functions like
<a href="/pkg/os/#IsPermission"><code>IsPermission</code></a>
to test common error properties, plus a few new error values
with more Go-like names, such as
<a href="/pkg/os/#ErrPermission"><code>ErrPermission</code></a>
and
<a href="/pkg/os/#ErrNoEnv"><code>ErrNoEnv</code></a>.
</p>

<p>
The <code>Getenverror</code> function has been removed. To distinguish
between a non-existent environment variable and an empty string,
use <a href="/pkg/os/#Environ"><code>os.Environ</code></a> or
<a href="/pkg/syscall/#Getenv"><code>syscall.Getenv</code></a>.
</p>


<p>
The <a href="/pkg/os/#Process.Wait"><code>Process.Wait</code></a> method has
dropped its option argument and the associated constants are gone
from the package.
Also, the function <code>Wait</code> is gone; only the method of
the <code>Process</code> type persists.
</p>

<p>
<em>Updating</em>:
Gofix will rewrite calls to <code>os.Wait</code> with an explicit zero
argument, dropping the argument.
All other changes will be caught by the compiler and must be updated by hand.
</p>

<h4 id="os_fileinfo">The os.FileInfo type</h4>

<p>
Go 1 redefines the <a href="/pkg/os/#FileInfo"><code>os.FileInfo</code></a> type,
changing it from a struct to an interface:
</p>

<pre>
    type FileInfo interface {
        Name() string       // base name of the file
        Size() int64        // length in bytes
        Mode() FileMode     // file mode bits
        ModTime() time.Time // modification time
        IsDir() bool        // abbreviation for Mode().IsDir()
        Sys() interface{}   // underlying data source (can return nil)
    }
</pre>

<p>
The file mode information has been moved into a subtype called
<a href="/pkg/os/#FileMode"><code>os.FileMode</code></a>,
a simple integer type with <code>IsDir</code>, <code>Perm</code>, and <code>String</code>
methods.
</p>

<p>
The system-specific details of file modes and properties such as (on Unix)
i-number have been removed from <code>FileInfo</code> altogether.
Instead, each operating system's <code>os</code> package provides an
implementation of the <code>FileInfo</code> interface, which
has a <code>Sys</code> method that returns the
system-specific representation of file metadata.
For instance, to discover the i-number of a file on a Unix system, unpack
the <code>FileInfo</code> like this:
</p>

<pre>
    fi, err := os.Stat("hello.go")
    if err != nil {
        log.Fatal(err)
    }
    // Check that it's a Unix file.
    unixStat, ok := fi.Sys().(*syscall.Stat_t)
    if !ok {
        log.Fatal("hello.go: not a Unix file")
    }
    fmt.Printf("file i-number: %d\n", unixStat.Ino)
</pre>

<p>
Assuming (which is unwise) that <code>"hello.go"</code> is a Unix file,
the i-number expression could be contracted to
</p>

<pre>
    fi.Sys().(*syscall.Stat_t).Ino
</pre>

<p>
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>

<p>
In Go 1, the <a href="/pkg/path/filepath/#Walk"><code>Walk</code></a> function of the
<code>path/filepath</code> package
has been changed to take a function value of type
<a href="/pkg/path/filepath/#WalkFunc"><code>WalkFunc</code></a>
instead of a <code>Visitor</code> interface value.
<code>WalkFunc</code> unifies the handling of both files and directories.
</p>

<pre>
    type WalkFunc func(path string, info *os.FileInfo, err os.Error) os.Error
</pre>

<p>
The <code>WalkFunc</code> function will be called even for files or directories that could not be opened;
in such cases the error argument will describe the failure.
If a directory's contents are to be skipped,
the function should return the value <a href="/pkg/path/filepath/#variables"><code>filepath.SkipDir</code></a>
</p>

<pre><!--{{code "progs/go1.go" `/STARTWALK/` `/ENDWALK/`}}
-->    markFn := func(path string, info os.FileInfo, err error) error {
        if path == &#34;pictures&#34; { // Will skip walking of directory pictures and its contents.
            return filepath.SkipDir
        }
        if err != nil {
            return err
        }
        log.Println(path)
        return nil
    }
    err := filepath.Walk(&#34;.&#34;, markFn)
    if err != nil {
        log.Fatal(err)
    }</pre>

<p>
<em>Updating</em>:
The change simplifies most code but has subtle consequences, so affected programs
will need to be updated by hand.
The compiler will catch code using the old interface.
</p>

<h3 id="os_signal">The os/signal package</h3>

<p>
The <code>os/signal</code> package in Go 1 replaces the
<code>Incoming</code> function, which returned a channel
that received all incoming signals,
with the selective <code>Notify</code> function, which asks
for delivery of specific signals on an existing channel.
</p>

<p>
<em>Updating</em>:
Code must be updated by hand.
A literal translation of
</p>
<pre>
c := signal.Incoming()
</pre>
<p>
is
</p>
<pre>
c := make(chan os.Signal)
signal.Notify(c) // ask for all signals
</pre>
<p>
but most code should list the specific signals it wants to handle instead:
</p>
<pre>
c := make(chan os.Signal)
signal.Notify(c, syscall.SIGHUP, syscall.SIGQUIT)
</pre>

<h3 id="runtime">The runtime package</h3>

<p>
In Go 1, much of the API exported by package
<code>runtime</code> has been removed in favor of
functionality provided by other packages.
Code using the <code>runtime.Type</code> interface
or its specific concrete type implementations should
now use package <a href="/pkg/reflect/"><code>reflect</code></a>.
Code using <code>runtime.Semacquire</code> or <code>runtime.Semrelease</code>
should use channels or the abstractions in package <a href="/pkg/sync/"><code>sync</code></a>.
The <code>runtime.Alloc</code>, <code>runtime.Free</code>,
and <code>runtime.Lookup</code> functions, an unsafe API created for
debugging the memory allocator, have no replacement.
</p>

<p>
Before, <code>runtime.MemStats</code> was a global variable holding
statistics about memory allocation, and calls to <code>runtime.UpdateMemStats</code>
ensured that it was up to date.
In Go 1, <code>runtime.MemStats</code> is a struct type, and code should use
<a href="/pkg/runtime/#ReadMemStats"><code>runtime.ReadMemStats</code></a>
to obtain the current statistics.
</p>

<p>
The package adds a new function,
<a href="/pkg/runtime/#NumCPU"><code>runtime.NumCPU</code></a>, that returns the number of CPUs available
for parallel execution, as reported by the operating system kernel.
Its value can inform the setting of <code>GOMAXPROCS</code>.
The <code>runtime.Cgocalls</code> and <code>runtime.Goroutines</code> functions
have been renamed to <code>runtime.NumCgoCall</code> and <code>runtime.NumGoroutine</code>.
</p>

<p>
<em>Updating</em>:
Running <code>go fix</code> will update code for the function renamings.
Other code will need to be updated by hand.
</p>

<h3 id="strconv">The strconv package</h3>

<p>
In Go 1, the
<a href="/pkg/strconv/"><code>strconv</code></a>
package has been significantly reworked to make it more Go-like and less C-like,
although <code>Atoi</code> lives on (it's similar to
<code>int(ParseInt(x, 10, 0))</code>, as does
<code>Itoa(x)</code> (<code>FormatInt(int64(x), 10)</code>).
There are also new variants of some of the functions that append to byte slices rather than
return strings, to allow control over allocation.
</p>

<p>
This table summarizes the renamings; see the
<a href="/pkg/strconv/">package documentation</a>
for full details.
</p>

<table class="codetable" frame="border" summary="strconv renames">
<colgroup align="left" width="50%"></colgroup>
<colgroup align="left" width="50%"></colgroup>
<tr>
<th align="left">Old call</th>
<th align="left">New call</th>
</tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>Atob(x)</td> <td>ParseBool(x)</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>Atof32(x)</td> <td>ParseFloat(x, 32)§</td></tr>
<tr><td>Atof64(x)</td> <td>ParseFloat(x, 64)</td></tr>
<tr><td>AtofN(x, n)</td> <td>ParseFloat(x, n)</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>Atoi(x)</td> <td>Atoi(x)</td></tr>
<tr><td>Atoi(x)</td> <td>ParseInt(x, 10, 0)§</td></tr>
<tr><td>Atoi64(x)</td> <td>ParseInt(x, 10, 64)</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>Atoui(x)</td> <td>ParseUint(x, 10, 0)§</td></tr>
<tr><td>Atoi64(x)</td> <td>ParseInt(x, 10, 64)</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>Btoi64(x, b)</td> <td>ParseInt(x, b, 64)</td></tr>
<tr><td>Btoui64(x, b)</td> <td>ParseUint(x, b, 64)</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>Btoa(x)</td> <td>FormatBool(x)</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>Ftoa32(x, f, p)</td> <td>FormatFloat(float64(x), f, p, 32)</td></tr>
<tr><td>Ftoa64(x, f, p)</td> <td>FormatFloat(x, f, p, 64)</td></tr>
<tr><td>FtoaN(x, f, p, n)</td> <td>FormatFloat(x, f, p, n)</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>Itoa(x)</td> <td>Itoa(x)</td></tr>
<tr><td>Itoa(x)</td> <td>FormatInt(int64(x), 10)</td></tr>
<tr><td>Itoa64(x)</td> <td>FormatInt(x, 10)</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>Itob(x, b)</td> <td>FormatInt(int64(x), b)</td></tr>
<tr><td>Itob64(x, b)</td> <td>FormatInt(x, b)</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>Uitoa(x)</td> <td>FormatUint(uint64(x), 10)</td></tr>
<tr><td>Uitoa64(x)</td> <td>FormatUint(x, 10)</td></tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr><td>Uitob(x, b)</td> <td>FormatUint(uint64(x), b)</td></tr>
<tr><td>Uitob64(x, b)</td> <td>FormatUint(x, b)</td></tr>
</table>
		
<p>
<em>Updating</em>:
Running <code>go fix</code> will update almost all code affected by the change.
<br>
§ <code>Atoi</code> persists but <code>Atoui</code> and <code>Atof32</code> do not, so
they may require
a cast that must be added by hand; the <code>go fix</code> tool will warn about it.
</p>


<h3 id="testing">The testing package</h3>

<p>
The testing package has a type, <code>B</code>, passed as an argument to benchmark functions.
In Go 1, <code>B</code> has new methods, analogous to those of <code>T</code>, enabling
logging and failure reporting.
</p>

<pre><!--{{code "progs/go1.go" `/func.*Benchmark/` `/^}/`}}
-->func BenchmarkSprintf(b *testing.B) {
    // Verify correctness before running benchmark.
    b.StopTimer()
    got := fmt.Sprintf(&#34;%x&#34;, 23)
    const expect = &#34;17&#34;
    if expect != got {
        b.Fatalf(&#34;expected %q; got %q&#34;, expect, got)
    }
    b.StartTimer()
    for i := 0; i &lt; b.N; i++ {
        fmt.Sprintf(&#34;%x&#34;, 23)
    }
}</pre>

<p>
<em>Updating</em>:
Existing code is unaffected, although benchmarks that use <code>println</code>
or <code>panic</code> should be updated to use the new methods.
</p>

<h3 id="testing_script">The testing/script package</h3>

<p>
The testing/script package has been deleted. It was a dreg.
</p>

<p>
<em>Updating</em>:
No code is likely to be affected.
</p>

<h3 id="unsafe">The unsafe package</h3>

<p>
In Go 1, the functions
<code>unsafe.Typeof</code>, <code>unsafe.Reflect</code>,
<code>unsafe.Unreflect</code>, <code>unsafe.New</code>, and
<code>unsafe.NewArray</code> have been removed;
they duplicated safer functionality provided by
package <a href="/pkg/reflect/"><code>reflect</code></a>.
</p>

<p>
<em>Updating</em>:
Code using these functions must be rewritten to use
package <a href="/pkg/reflect/"><code>reflect</code></a>.
The changes to <a href="http://code.google.com/p/go/source/detail?r=2646dc956207">encoding/gob</a> and the <a href="http://code.google.com/p/goprotobuf/source/detail?r=5340ad310031">protocol buffer library</a>
may be helpful as examples.
</p>

<h3 id="url">The url package</h3>

<p>
In Go 1 several fields from the <a href="/pkg/net/url/#URL"><code>url.URL</code></a> type
were removed or replaced.
</p>

<p>
The <a href="/pkg/net/url/#URL.String"><code>String</code></a> method now
predictably rebuilds an encoded URL string using all of <code>URL</code>'s
fields as necessary. The resulting string will also no longer have
passwords escaped.
</p>

<p>
The <code>Raw</code> field has been removed. In most cases the <code>String</code>
method may be used in its place.
</p>

<p>
The old <code>RawUserinfo</code> field is replaced by the <code>User</code>
field, of type <a href="/pkg/net/url/#Userinfo"><code>*net.Userinfo</code></a>.
Values of this type may be created using the new <a href="/pkg/net/url/#User"><code>net.User</code></a>
and <a href="/pkg/net/url/#UserPassword"><code>net.UserPassword</code></a>
functions. The <code>EscapeUserinfo</code> and <code>UnescapeUserinfo</code>
functions are also gone.
</p>

<p>
The <code>RawAuthority</code> field has been removed. The same information is
available in the <code>Host</code> and <code>User</code> fields.
</p>

<p>
The <code>RawPath</code> field and the <code>EncodedPath</code> method have
been removed. The path information in rooted URLs (with a slash following the
schema) is now available only in decoded form in the <code>Path</code> field.
Occasionally, the encoded data may be required to obtain information that
was lost in the decoding process. These cases must be handled by accessing
the data the URL was built from.
</p>

<p>
URLs with non-rooted paths, such as <code>"mailto:dev@golang.org?subject=Hi"</code>,
are also handled differently. The <code>OpaquePath</code> boolean field has been
removed and a new <code>Opaque</code> string field introduced to hold the encoded
path for such URLs. In Go 1, the cited URL parses as:
</p>

<pre>
    URL{
        Scheme: "mailto",
        Opaque: "dev@golang.org",
        RawQuery: "subject=Hi",
    }
</pre>

<p>
A new <a href="/pkg/net/url/#URL.RequestURI"><code>RequestURI</code></a> method was
added to <code>URL</code>.
</p>

<p>
The <code>ParseWithReference</code> function has been renamed to <code>ParseWithFragment</code>.
</p>

<p>
<em>Updating</em>:
Code that uses the old fields will fail to compile and must be updated by hand.
The semantic changes make it difficult for the fix tool to update automatically.
</p>

<h2 id="cmd_go">The go command</h2>

<p>
TODO: Write this.
</p>

<h2 id="cmd_cgo">The cgo command</h2>

<p>
In Go 1, the <a href="/cmd/cgo">cgo command</a>
uses a different <code>_cgo_export.h</code>
file, which is generated for packages containing <code>//export</code> lines.
The <code>_cgo_export.h</code> file now begins with the C preamble comment,
so that exported function definitions can use types defined there.
This has the effect of compiling the preamble multiple times, so a
package using <code>//export</code> must not put function definitions
or variable initializations in the C preamble.
</p>

<h2 id="releases">Packaged releases</h2>