go/doc/go1.9.html

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<h2 id="introduction">DRAFT RELEASE NOTES - Introduction to Go 1.9</h2>

<p><strong>
    Go 1.9 is not yet released. These are work-in-progress
    release notes. Go 1.9 is expected to be released in August 2017.
</strong></p>

<p>
  The latest Go release, version 1.9, arrives six months
  after <a href="go1.8">Go 1.8</a> and is the tenth release in
  the <a href="https://golang.org/doc/devel/release.html">Go 1.x
  series</a>.
  There is one <a href="#language">change to the language</a>, adding
  support for type aliases.
  Most of the changes are in the implementation of the toolchain,
  runtime, and libraries.
  As always, the release maintains the Go 1
  <a href="/doc/go1compat.html">promise of compatibility</a>.
  We expect almost all Go programs to continue to compile and run as
  before.
</p>

<p>
  The release
  adds <a href="#monotonic-time">transparent monotonic time support</a>,
  <a href="#parallel-compile">parallelizes compilation of functions</a> within a package,
  better supports <a href="#test-helper">test helper functions</a>,
  includes a new <a href="#math-bits">bit manipulation package</a>,
  and has a new <a href="#sync-map">concurrent map type</a>.
</p>

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

<p>
  There is one change to the language.
  Go now supports type aliases to support gradual code repair while
  moving a type between packages.
  The <a href="https://golang.org/design/18130-type-alias">type alias
  design document</a>
  and <a href="https://talks.golang.org/2016/refactor.article">an
  article on refactoring</a> cover the problem in detail.
  In short, a type alias declaration has the form:
</p>

<pre>
type T1 = T2
</pre>

<p>
  This declaration introduces an alias name <code>T1</code>—an
  alternate spelling—for the type denoted by <code>T2</code>; that is,
  both <code>T1</code> and <code>T2</code> denote the same type.
</p>

<h2 id="ports">Ports</h2>

<p>
  There are no new supported operating systems or processor
  architectures in this release.
</p>

<h3 id="power8">ppc64x requires Power8</h3>

<p> <!-- CL 36725, CL 36832 -->
  Both <code>GOARCH=ppc64</code> and <code>GOARCH=ppc64le</code> now
  require at least Power8 support. In previous releases,
  only <code>GOARCH=ppc64le</code> required Power8 and the big
  endian <code>ppc64</code> architecture supported older
  hardware.
<p>

<h3 id="known_issues">Known Issues</h3>

<p>
  There are some instabilities on FreeBSD that are known but not understood.
  These can lead to program crashes in rare cases.
  See <a href="https://golang.org/issue/15658">issue 15658</a>.
  Any help in solving this FreeBSD-specific issue would be appreciated.
</p>

<h2 id="tools">Tools</h2>

<h3 id="parallel-compile">Parallel Compilation</h3>

<p>
  The Go compiler now supports compiling a package's functions in parallel, taking
  advantage of multiple cores. This is in addition to the <code>go</code> command's
  existing support for parallel compilation of separate packages.
  Parallel compilation is on by default, but can be disabled by setting the
  environment variable <code>GO19CONCURRENTCOMPILATION</code> to <code>0</code>.
</p>

<h3 id="vendor-dotdotdot">Vendor matching with ./...</h3>

<p><!-- CL 38745 -->
  By popular request, <code>./...</code> no longer matches packages
  in <code>vendor</code> directories in tools accepting package names,
  such as <code>go</code> <code>test</code>. To match vendor
  directories, write <code>./vendor/...</code>.
</p>

<h3 id="compiler">Compiler Toolchain</h3>

<p><!-- CL 37441 -->
  Complex division is now C99-compatible. This has always been the
  case in gccgo and is now fixed in the gc toolchain.
</p>

<p> <!-- CL 36983 -->
  The linker will now generate DWARF information for cgo executables on Windows.
</p>

<p> <!-- CL 44210, CL 40095 -->
  The compiler now includes lexical scopes in the generated DWARF, allowing
  debuggers to hide variables that are not in scope. The <code>.debug_info</code>
  section is now DWARF version 4.
</p>

<p> <!-- CL 43855 -->
  The values of <code>GOARM</code> and <code>GO386</code> now affect a
  compiled package's build ID, as used by the <code>go</code> tool's
  dependency caching.
</p>

<h3 id="go-doc">Doc</h3>

<p><!-- CL 36031 -->
  Long lists of arguments are now truncated. This improves the readability
  of <code>go doc</code> on some generated code.
</p>

<p><!-- CL 38438 -->
  Viewing documentation on struct fields is now supported with
  <code>go doc struct.field</code>.
</p>

<h3 id="go-env-json">Env</h3>

<p> <!-- CL 38757 -->
  The new <code>go</code> <code>env</code> <code>-json</code> flag
  enables JSON output, instead of the default OS-specific output
  format.
</p>

<h3 id="go-test-list">Test</h3>

<p> <!-- CL 41195 -->
  The <a href="/cmd/go/#hdr-Description_of_testing_flags"><code>go</code> <code>test</code></a>
  command accepts a new <code>-list</code> flag, which takes a regular
  expression as an argument and prints to stdout the name of any
  tests, benchmarks, or examples that match it, without running them.
</p>


<h3 id="go-tool-pprof-proxy">Pprof</h3>

<p> <!-- CL 38343 -->
  The <code>go</code> <code>tool</code> <code>pprof</code> command now
  uses the HTTP proxy information defined in the environment, using
  <a href="/pkg/net/http/#ProxyFromEnvironment"><code>http.ProxyFromEnvironment</code></a>.
</p>

<h3 id="tools-TODO">TODO</h3>

<p>TODO: finish documenting misc tool changes</p>

<pre>
CL 42028: https://golang.org/cl/42028: cmd/asm: fix operand order of ARM's MULA instruction
CL 40112: https://golang.org/cl/40112: cmd/go: allow full flag processing in go vet
CL 42990: https://golang.org/cl/42990: cmd/internal/obj/x86: add ADDSUBPS/PD
CL 40331: https://golang.org/cl/40331: cmd/link,runtime/cgo: enable PT_TLS generation on OpenBSD
</pre>

<h2 id="performance">Performance</h2>

<p>
  As always, the changes are so general and varied that precise
  statements about performance are difficult to make.  Most programs
  should run a bit faster, due to speedups in the garbage collector,
  better generated code, and optimizations in the core library.
</p>

<h3 id="gc">Garbage Collector</h3>

<p> <!-- CL 37520 -->
  Library functions that used to trigger stop-the-world garbage
  collection now trigger concurrent garbage collection.

  Specifically, <a href="/pkg/runtime/#GC"><code>runtime.GC</code></a>,
  <a href="/pkg/runtime/debug/#SetGCPercent"><code>debug.SetGCPercent</code></a>,
  and
  <a href="/pkg/runtime/debug/#FreeOSMemory"><code>debug.FreeOSMemory</code></a>,
  now trigger concurrent garbage collection, blocking only the calling
  goroutine until the garbage collection is done.
</p>

<p> <!-- CL 34103, CL 39835 -->
  The
  <a href="/pkg/runtime/debug/#SetGCPercent"><code>debug.SetGCPercent</code></a>
  function only triggers a garbage collection if one is immediately
  necessary because of the new GOGC value.
  This makes it possible to adjust GOGC on-the-fly.
</p>

<p> <!-- CL 38732 -->
  Large object allocation performance is significantly improved in
  applications using large (&gt;50GB) heaps containing many large
  objects.
</p>

<p> <!-- CL 34937 -->
  The <a href="/pkg/runtime/#ReadMemStats"><code>runtime.ReadMemStats</code></a>
  function now takes less than 100µs even for very large heaps.
</p>

<h2 id="library">Core library</h2>

<h3 id="monotonic-time">Transparent Monotonic Time support</h3>

<p> <!-- CL 36255 -->
  The <a href="/pkg/time/"><code>time</code></a> package now transparently
  tracks monotonic time in each <a href="/pkg/time/#Time"><code>Time</code></a>
  value, making computing durations between two <code>Time</code> values
  a safe operation in the presence of wall clock adjustments.
  See the <a href="/pkg/time/#hdr-Monotonic_Clocks">package docs</a> and
  <a href="https://golang.org/design/12914-monotonic">design document</a>
  for details.
</p>

<h3 id="math-bits">New bit manipulation package</h3>

<p> <!-- CL 36315 -->
  Go 1.9 includes a new package,
  <a href="/pkg/math/bits/"><code>math/bits</code></a>, with optimized
  implementations for manipulating bits. On most architectures
  functions in this package are additionally recognized by the
  compiler and treated as intrinsics for additional performance.
</p>

<h3 id="test-helper">Test Helper Functions</h3>

<p> <!-- CL 38796 -->
  The
  new <a href="/pkg/testing/#T.Helper"><code>(*T).Helper</code></a>
  an <a href="/pkg/testing/#B.Helper"><code>(*B).Helper</code></a>
  methods mark the calling function as a test helper function.  When
  printing file and line information, that function will be skipped.
  This permits writing test helper functions while still having useful
  line numbers for users.
</p>

<h3 id="sync-map">Concurrent Map</h3>

<p> <!-- CL 36617 -->
  The new <a href="/pkg/sync/#Map"><code>Map</code></a> type
  in the <a href="/pkg/sync/"><code>sync</code></a> package
  is a concurrent map with amortized-constant-time loads, stores, and
  deletes. It is safe for multiple goroutines to call a Map's methods
  concurrently.
</p>

<h3 id="minor_library_changes">Minor changes to the library</h3>

<p>
  As always, there are various minor changes and updates to the library,
  made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
  in mind.
</p>

<dl id="archive/zip"><dt><a href="/pkg/archive/zip/">archive/zip</a></dt>
  <dd>
    <p><!-- CL 39570 -->
      The
      ZIP <a href="/pkg/archive/zip/#Writer"><code>Writer</code></a>
      now sets the UTF-8 bit in
      the <a href="/pkg/archive/zip/#FileHeader.Flags"><code>FileHeader.Flags</code></a>
      when appropriate.
    </p>

</dl><!-- archive/zip -->

<dl id="crypto/rand"><dt><a href="/pkg/crypto/rand/">crypto/rand</a></dt>
  <dd>
    <p><!-- CL 43852 -->
      On Linux, Go now calls the <code>getrandom</code> system call
      without the <code>GRND_NONBLOCK</code> flag; it will now block
      until the kernel has sufficient randomness. On kernels predating
      the <code>getrandom</code> system call, Go continues to read
      from <code>/dev/urandom</code>.
    </p>

</dl><!-- crypto/rand -->

<dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
  <dd>
    <p><!-- CL 36093 -->

      On Unix systems the environment
      variables <code>SSL_CERT_FILE</code>
      and <code>SSL_CERT_DIR</code> can now be used to override the
      system default locations for the SSL certificate file and SSL
      certificate files directory, respectively.
    </p>

    <p>The FreeBSD path <code>/usr/local/etc/ssl/cert.pem</code> is
      now included in the certificate search path.
    </p>

    <p><!-- CL 36900 -->

      The package now supports excluded domains in name constraints.
      In addition to enforcing such constraints,
      <a href="/pkg/crypto/x509/#CreateCertificate"><code>CreateCertificate</code></a>
      will create certificates with excluded name constraints
      if the provided template certificate has the new
      field
      <a href="/pkg/crypto/x509/#Certificate.ExcludedDNSDomains"><code>ExcludedDNSDomains</code></a>
      populated.
    </p>

</dl><!-- crypto/x509 -->

<dl id="database/sql"><dt><a href="/pkg/database/sql/">database/sql</a></dt>
  <dd>
    <p><!-- CL 35476 -->
      The package will now use a cached <a href="/pkg/database/sql/#Stmt"><code>Stmt</code></a> if
      available in <a href="/pkg/database/sql/#Tx.Stmt"><code>Tx.Stmt</code></a>.
      This prevents statements from being re-prepared each time
      <a href="/pkg/database/sql/#Tx.Stmt"><code>Tx.Stmt</code></a> is called.
    </p>

    <p><!-- CL 38533 -->
      The package now allows drivers to implement their own argument checkers by implementing
      <a href="/pkg/database/sql/driver/#NamedValueChecker"><code>driver.NamedValueChecker</code></a>.
      This also allows drivers to support <code>OUTPUT</code> and <code>INOUT</code> parameter types.
      <a href="/pkg/database/sql/#Out"><code>Out</code></a> should be used to return output parameters
      when supported by the driver.
    </p>

    <p><!-- CL 39031 -->
      <a href="/pkg/database/sql/#Rows.Scan"><code>Rows.Scan</code></a> can now scan user-defined string types.
      Previously the package supported scanning into numeric types like <code>type Int int64</code>. It now also supports
      scanning into string types like <code>type String string</code>.
    </p>

    <p><!-- CL 40694 -->
      The new <a href="/pkg/database/sql/#DB.Conn"><code>DB.Conn</code></a> method returns the new
      <a href="/pkg/database/sql/#Conn"><code>Conn</code></a> type representing an
      exclusive connection to the database from the connection pool. All queries run on
      a <a href="/pkg/database/sql/#Conn"><code>Conn</code></a> will use the same underlying
      connection until <a href="/pkg/database/sql/#Conn.Close"><code>Conn.Close</code></a> is called
      to return the connection to the connection pool.
    </p>

</dl><!-- database/sql -->

<dl id="encoding/asn1"><dt><a href="/pkg/encoding/asn1/">encoding/asn1</a></dt>
  <dd>
    <p><!-- CL 38660 -->
	  The new
	  <a href="/pkg/encoding/asn1/#NullBytes"><code>NullBytes</code></a>
	  and
	  <a href="/pkg/encoding/asn1/#NullRawValue"><code>NullRawValue</code></a>
	  represent the <code>ASN.1 NULL</code> type.
    </p>

</dl><!-- encoding/asn1 -->

<dl id="encoding/base32"><dt><a href="/pkg/encoding/base32/">encoding/base32</a></dt>
  <dd>
    <p><!-- CL 38634 --> 
	  The new <a href="/pkg/encoding/base32/#Encoding.WithPadding">Encoding.WithPadding</a>
	  method adds support for custom padding characters and disabling padding.
    </p>

</dl><!-- encoding/base32 -->

<dl id="encoding/csv"><dt><a href="/pkg/encoding/csv/">encoding/csv</a></dt>
  <dd>
    <p><!-- CL 41730 -->
      The new field
      <a href="/pkg/encoding/csv/#Reader.ReuseRecord"><code>Reader.ReuseRecord</code></a>
      controls whether calls to
      <a href="/pkg/encoding/csv/#Reader.Read"><code>Read</code></a>
      may return a slice sharing the backing array of the previous
      call's returned slice for improved performance.
    </p>

</dl><!-- encoding/csv -->

<dl id="fmt"><dt><a href="/pkg/fmt/">fmt</a></dt>
  <dd>
    <p><!-- CL 37051 -->
      The sharp flag ('<code>#</code>') is now supported when printing
      floating point and complex numbers. It will always print a
      decimal point
      for <code>%e</code>, <code>%E</code>, <code>%f</code>, <code>%F</code>, <code>%g</code>
      and <code>%G</code>; it will not remove trailing zeros
      for <code>%g</code> and <code>%G</code>.
    </p>

</dl><!-- fmt -->

<dl id="hash/fnv"><dt><a href="/pkg/hash/fnv/">hash/fnv</a></dt>
  <dd>
    <p><!-- CL 38356 -->
      The package now includes 128-bit FNV-1 and FNV-1a hash support with
      <a href="/pkg/hash/fnv/#New128"><code>New128</code></a> and
      <a href="/pkg/hash/fnv/#New128a"><code>New128a</code></a>, respectively.
    </p>

</dl><!-- hash/fnv -->

<dl id="html/template"><dt><a href="/pkg/html/template/">html/template</a></dt>
  <dd>
    <p><!-- CL 37880, CL 40936 -->
	  The package now reports an error if a predefined escaper (one of
	  "html", "urlquery" and "js") is found in a pipeline and its
	  rewriting by the contextual auto-escaper could potentially lead
	  to security or correctness issues.
    </p>

</dl><!-- html/template -->

<dl id="image"><dt><a href="/pkg/image/">image</a></dt>
  <dd>
    <p><!-- CL 36734 -->
	  The <a href="/pkg/image/#Rectangle.Intersect"><code>Rectangle.Intersect</code></a>
	  method now returns a zero <code>Rectangle</code> when called on
	  adjacent but non-overlapping rectangles, as documented. In
	  earlier releases it would incorrectly return an empty but
	  non-zero <code>Rectangle</code>.
    </p>

</dl><!-- image -->

<dl id="image/color"><dt><a href="/pkg/image/color/">image/color</a></dt>
  <dd>
    <p><!-- CL 36732 -->
	  The YCbCr to RGBA conversion formula has been tweaked to ensure
	  that rounding adjustments span the complete [0, 0xffff] RGBA
	  range.
    </p>

</dl><!-- image/color -->

<dl id="image/png"><dt><a href="/pkg/image/png/">image/png</a></dt>
  <dd>
    <p><!-- CL 34150 -->
	  The new <a href="/pkg/image/png/#Encoder.BufferPool"><code>Encoder.BufferPool</code></a>
	  field allows specifying an <a href="/pkg/image/png/#EncoderBufferPool"><code>EncoderBufferPool</code></a>,
	  that will be used by the encoder to get temporary <code>EncoderBuffer</code>
	  buffers when encoding a PNG image.

	  The use of a <code>BufferPool</code> reduces the number of
	  memory allocations performed while encoding multiple images.
    </p>

    <p><!-- CL 38271 -->
	  The package now supports the decoding of transparent 8-bit
	  grayscale ("Gray8") images.
    </p>

</dl><!-- image/png -->

<dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>
  <dd>
    <p><!-- CL 36487 -->
      The new
      <a href="/pkg/math/big/#Int.IsInt64"><code>IsInt64</code></a>
      and
      <a href="/pkg/math/big/#Int.IsUint64"><code>IsUint64</code></a>
      methods report whether an <code>Int</code>
      may be represented as an <code>int64</code> or <code>uint64</code>
      value.
    </p>

</dl><!-- math/big -->

<dl id="mime/multipart"><dt><a href="/pkg/mime/multipart/">mime/multipart</a></dt>
  <dd>
    <p><!-- CL 39223 -->
      The new
      <a href="/pkg/mime/multipart/#FileHeader.Size"><code>FileHeader.Size</code></a>
      field describes the size of a file in a multipart message.
    </p>

</dl><!-- mime/multipart -->

<dl id="net"><dt><a href="/pkg/net/">net</a></dt>
  <dd>
    <p><!-- CL 32572 -->
      The new
      <a href="/pkg/net/#Resolver.StrictErrors"><code>Resolver.StrictErrors</code></a>
      provides control over how Go's built-in DNS resolver handles
      temporary errors during queries composed of multiple sub-queries,
      such as an A+AAAA address lookup.
    </p>

    <p><!-- CL 37260 -->
      The new
      <a href="/pkg/net/#Resolver.Dial"><code>Resolver.Dial</code></a>
      allows a <code>Resolver</code> to use a custom dial function.
    </p>

    <p><!-- CL 40510 -->
      <a href="/pkg/net/#JoinHostPort"><code>JoinHostPort</code></a> now only places an address in square brackets if the host contains a colon.
      In previous releases it would also wrap addresses in square brackets if they contained a percent ('<code>%</code>') sign.
    </p>

    <p><!-- CL 37913 -->
      The new methods
      <a href="/pkg/net/#TCPConn.SyscallConn"><code>TCPConn.SyscallConn</code></a>,
      <a href="/pkg/net/#IPConn.SyscallConn"><code>IPConn.SyscallConn</code></a>,
      <a href="/pkg/net/#UDPConn.SyscallConn"><code>UDPConn.SyscallConn</code></a>,
      and
      <a href="/pkg/net/#UnixConn.SyscallConn"><code>UnixConn.SyscallConn</code></a>
      provide access to the connections' underlying file descriptors.
    </p>

    <p><!-- 45088 -->
      It is now safe to call <a href="/pkg/net/#Dial"><code>Dial</code></a> with the address obtained from
      <code>(*TCPListener).String()</code> after creating the listener with
      <code><a href="/pkg/net/#Listen">Listen</a>("tcp", ":0")</code>.
      Previously it failed on some machines with half-configured IPv6 stacks.
    </p>

</dl><!-- net -->

<dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
  <dd>

    <p>Server changes:</p>
    <ul>
      <li><!-- CL 38194 -->
        <a href="/pkg/net/http/#ServeMux"><code>ServeMux</code></a> now ignores ports in the host
        header when matching handlers. The host is matched unmodified for <code>CONNECT</code> requests.
      </li>

      <li><!-- CL 34727 -->
        <a href="/pkg/net/http/#Server.WriteTimeout"><code>Server.WriteTimeout</code></a>
        now applies to HTTP/2 connections and is enforced per-stream.
      </li>

      <li><!-- CL 43231 -->
        HTTP/2 now uses the priority write scheduler by default.
        Frames are scheduled by following HTTP/2 priorities as described in
        <a href="https://tools.ietf.org/html/rfc7540#section-5.3">RFC 7540 Section 5.3</a>.
      </li>
    </ul>

    <p>Client &amp; Transport changes:</p>
    <ul>
      <li><!-- CL 35488 -->
        The <a href="/pkg/net/http/#Transport"><code>Transport</code></a>
        now supports making requests via SOCKS5 proxy when the URL returned by
        <a href="/net/http/#Transport.Proxy"><code>Transport.Proxy</code></a>
        has the scheme <code>socks5</code>.
      </li>
    </ul>

</dl><!-- net/http -->

<dl id="net/http/fcgi"><dt><a href="/pkg/net/http/fcgi/">net/http/fcgi</a></dt>
  <dd>
    <p><!-- CL 40012 -->
      The new
      <a href="/pkg/net/http/fcgi/#ProcessEnv"><code>ProcessEnv</code></a>
      function returns FastCGI environment variables associated with an HTTP request
      for which there are no appropriate
      <a href="/pkg/net/http/#Request"><code>http.Request</code></a>
      fields, such as <code>REMOTE_USER</code>.
    </p>

</dl><!-- net/http/fcgi -->

<dl id="net/http/httptest"><dt><a href="/pkg/net/http/httptest/">net/http/httptest</a></dt>
  <dd>
    <p><!-- CL 34639 -->
      The new
      <a href="/pkg/net/http/httptest/#Server.Client"><code>Server.Client</code></a>
      method returns an HTTP client configured for making requests to the test server.
    </p>

    <p>
      The new
      <a href="/pkg/net/http/httptest/#Server.Certificate"><code>Server.Certificate</code></a>
      method returns the test server's TLS certificate, if any.
    </p>

</dl><!-- net/http/httptest -->

<dl id="os"><dt><a href="/pkg/os/">os</a></dt>
  <dd>
    <p><!-- CL 36800 -->
      The <code>os</code> package now uses the internal runtime poller
      for file I/O.
      This reduces the number of threads required for read/write
      operations on pipes, and eliminates races when one goroutine
      closes a file while another using it for I/O.
    </p>

  <dd>
    <p><!-- CL 37915 -->
      On Windows,
      <a href="/pkg/os/#Args"><code>Args</code></a>
      is now populated without <code>shell32.dll</code>, improving process start-up time by 1-7 ms.
      </p>

</dl><!-- os -->

<dl id="os/exec"><dt><a href="/pkg/os/exec/">os/exec</a></dt>
  <dd>
    <p><!-- CL 37586 -->
      The <code>os/exec</code> package now prevents child processes from being created with
      any duplicate environment variables.
      If <a href="/pkg/os/exec/#Cmd.Env"><code>Cmd.Env</code></a>
      contains duplicate environment keys, only the last
      value in the slice for each duplicate key is used.
    </p>

</dl><!-- os/exec -->

<dl id="os/user"><dt><a href="/pkg/os/user/">os/user</a></dt>
  <dd>
    <p><!-- CL 37664 -->
      <a href="/pkg/os/user/#Lookup"><code>Lookup</code></a> and
      <a href="/pkg/os/user/#LookupId"><code>LookupId</code></a> now
      work on Unix systems when <code>CGO_ENABLED=0</code> by reading
      the <code>/etc/passwd</code> file.
    </p>

    <p><!-- CL 33713 -->
      <a href="/pkg/os/user/#LookupGroup"><code>LookupGroup</code></a> and
      <a href="/pkg/os/user/#LookupGroupId"><code>LookupGroupId</code></a> now
      work on Unix systems when <code>CGO_ENABLED=0</code> by reading
      the <code>/etc/group</code> file.
    </p>

</dl><!-- os/user -->

<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
  <dd>
    <p><!-- CL 38335 -->
      The new
      <a href="/pkg/reflect/#MakeMapWithSize"><code>MakeMapWithSize</code></a>
      function creates a map with a capacity hint.
    </p>

</dl><!-- reflect -->

<dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
  <dd>
    <p><!-- CL 37233, CL 37726 -->
      Tracebacks generated by the runtime and recorded in profiles are
      now accurate in the presence of inlining.
      To retrieve tracebacks programmatically, applications should use
      <a href="/pkg/runtime/#CallersFrames"><code>runtime.CallersFrames</code></a>
      rather than directly iterating over the results of
      <a href="/pkg/runtime/#Callers"><code>runtime.Callers</code></a>.
    </p>

    <p><!-- CL 38403 -->
      On Windows, Go no longer forces the system timer to run at high
      resolution when the program is idle.
      This should reduce the impact of Go programs on battery life.
    </p>

    <p><!-- CL 29341 -->
      On FreeBSD, <code>GOMAXPROCS</code> and
      <a href="/pkg/runtime/#NumCPU"><code>runtime.NumCPU</code></a>
      are now based on the process' CPU mask, rather than the total
      number of CPUs.
    </p>

    <p><!-- CL 43641 -->
      The runtime has preliminary support for Android O.
    </p>

</dl><!-- runtime -->

<dl id="runtime/pprof"><dt><a href="/pkg/runtime/pprof/">runtime/pprof</a></dt>
  <dd>
    <p><!-- CL 34198 -->
      TODO: <a href="https://golang.org/cl/34198">https://golang.org/cl/34198</a>: add definitions of profile label types
    </p>

</dl><!-- runtime/pprof -->

<dl id="runtime/debug"><dt><a href="/pkg/runtime/debug/">runtime/debug</a></dt>
  <dd>
    <p><!-- CL 34013 -->
      Calling
      <a href="/pkg/runtime/debug/#SetGCPercent"><code>SetGCPercent</code></a>
      with a negative value no longer runs an immediate garbage collection.
    </p>

</dl><!-- runtime/debug -->

<dl id="runtime/trace"><dt><a href="/pkg/runtime/trace/">runtime/trace</a></dt>
  <dd>
    <p><!-- CL 36015 -->
      The execution trace now displays mark assist events, which
      indicate when an application goroutine is forced to assist
      garbage collection because it is allocating too quickly.
    </p>

    <p><!-- CL 40810 -->
      "Sweep" events now encompass the entire process of finding free
      space for an allocation, rather than recording each individual
      span that is swept.
      This reduces allocation latency when tracing allocation-heavy
      programs.
      The sweep event shows how many bytes were swept and how many
      were reclaimed.
    </p>

</dl><!-- runtime/trace -->

<dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
  <dd>
    <p><!-- CL 34310 -->
      <a href="/pkg/sync/#Mutex"><code>Mutex</code></a> is now more fair.
    </p>

</dl><!-- sync -->

<dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
  <dd>
    <p><!-- CL 36697 -->
      The new field
      <a href="/pkg/syscall/#Credential.NoSetGroups"><code>Credential.NoSetGroups</code></a>
      controls whether Unix systems make a <code>setgroups</code> system call
      to set supplementary groups when starting a new process.
    </p>

    <p><!-- CL 37439 -->
      On 64-bit x86 Linux, process creation latency has been optimized with
      use of <code>CLONE_VFORK</code> and <code>CLONE_VM</code>.
    </p>

    <p><!-- CL 37913 -->
      The new
      <a href="/pkg/syscall/#Conn"><code>Conn</code></a>
      interface describes some types in the
      <a href="/pkg/net/"><code>net</code></a>
      package that can provide access to their underlying file descriptor
      using the new 
      <a href="/pkg/syscall/#RawConn"><code>RawConn</code></a>
      interface.
    </p>

</dl><!-- syscall -->


<dl id="testing/quick"><dt><a href="/pkg/testing/quick/">testing/quick</a></dt>
  <dd>
    <p><!-- CL 39152 -->
	  The package now chooses values in the full range when
	  generating <code>int64</code> and <code>uint64</code> random
	  numbers; in earlier releases generated values were always
	  limited to the [-2<sup>62</sup>, 2<sup>62</sup>) range.
    </p>

</dl><!-- testing/quick -->

<dl id="text/template"><dt><a href="/pkg/text/template/">text/template</a></dt>
  <dd>
    <p><!-- CL 38420 -->
	  The handling of empty blocks, which was broken by a Go 1.8
	  change that made the result dependent on the order of templates,
	  has been fixed, restoring the old Go 1.7 behavior.
    </p>

</dl><!-- text/template -->

<dl id="time"><dt><a href="/pkg/time/">time</a></dt>
  <dd>
    <p><!-- CL 36615 -->
      The new methods
      <a href="/pkg/time/#Duration.Round"><code>Duration.Round</code></a>
      and 
      <a href="/pkg/time/#Duration.Truncate"><code>Duration.Truncate</code></a>
      handle rounding durations away from and towards zero, respectively.
    </p>

    <p><!-- CL 35710 -->
      Retrieving the time and sleeping now work correctly under Wine.
    </p>

    <p>
      If a <code>Time</code> value has a monotonic clock reading, its
      string representation (as returned by <code>String</code>) now includes a
      final field <code>"m=±value"</code>, where <code>value</code> is the
      monotonic clock reading formatted as a decimal number of seconds.
    </p>

    <p><!-- CL 44832 -->
      The included <code>tzdata</code> timezone database has been
      updated to version 2017b. As always, it is only used if the
      system does not already have the database available.
    </p>

</dl><!-- time -->