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    1 <!--{
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   11 
   12 <style>
   13   main ul li { margin: 0.5em 0; }
   14 </style>
   15 
   16 <h2 id="introduction">Introduction to Go 1.6</h2>
   17 
   18 <p>
   19 The latest Go release, version 1.6, arrives six months after 1.5.
   20 Most of its changes are in the implementation of the language, runtime, and libraries.
   21 There are no changes to the language specification.
   22 As always, the release maintains the Go 1 <a href="/doc/go1compat.html">promise of compatibility</a>.
   23 We expect almost all Go programs to continue to compile and run as before.
   24 </p>
   25 
   26 <p>
   27 The release adds new ports to <a href="#ports">Linux on 64-bit MIPS and Android on 32-bit x86</a>;
   28 defined and enforced <a href="#cgo">rules for sharing Go pointers with C</a>;
   29 transparent, automatic <a href="#http2">support for HTTP/2</a>;
   30 and a new mechanism for <a href="#template">template reuse</a>.
   31 </p>
   32 
   33 <h2 id="language">Changes to the language</h2>
   34 
   35 <p>
   36 There are no language changes in this release.
   37 </p>
   38 
   39 <h2 id="ports">Ports</h2>
   40 
   41 <p>
   42 Go 1.6 adds experimental ports to
   43 Linux on 64-bit MIPS (<code>linux/mips64</code> and <code>linux/mips64le</code>).
   44 These ports support <code>cgo</code> but only with internal linking.
   45 </p>
   46 
   47 <p>
   48 Go 1.6 also adds an experimental port to Android on 32-bit x86 (<code>android/386</code>).
   49 </p>
   50 
   51 <p>
   52 On FreeBSD, Go 1.6 defaults to using <code>clang</code>, not <code>gcc</code>, as the external C compiler.
   53 </p>
   54 
   55 <p>
   56 On Linux on little-endian 64-bit PowerPC (<code>linux/ppc64le</code>),
   57 Go 1.6 now supports <code>cgo</code> with external linking and
   58 is roughly feature complete.
   59 </p>
   60 
   61 <p>
   62 On NaCl, Go 1.5 required SDK version pepper-41.
   63 Go 1.6 adds support for later SDK versions.
   64 </p>
   65 
   66 <p>
   67 On 32-bit x86 systems using the <code>-dynlink</code> or <code>-shared</code> compilation modes,
   68 the register CX is now overwritten by certain memory references and should
   69 be avoided in hand-written assembly.
   70 See the <a href="/doc/asm#x86">assembly documentation</a> for details.
   71 </p>
   72 
   73 <h2 id="tools">Tools</h2>
   74 
   75 <h3 id="cgo">Cgo</h3>
   76 
   77 <p>
   78 There is one major change to <a href="/cmd/cgo/"><code>cgo</code></a>, along with one minor change.
   79 </p>
   80 
   81 <p>
   82 The major change is the definition of rules for sharing Go pointers with C code,
   83 to ensure that such C code can coexist with Go's garbage collector.
   84 Briefly, Go and C may share memory allocated by Go
   85 when a pointer to that memory is passed to C as part of a <code>cgo</code> call,
   86 provided that the memory itself contains no pointers to Go-allocated memory,
   87 and provided that C does not retain the pointer after the call returns.
   88 These rules are checked by the runtime during program execution:
   89 if the runtime detects a violation, it prints a diagnosis and crashes the program.
   90 The checks can be disabled by setting the environment variable
   91 <code>GODEBUG=cgocheck=0</code>, but note that the vast majority of
   92 code identified by the checks is subtly incompatible with garbage collection
   93 in one way or another.
   94 Disabling the checks will typically only lead to more mysterious failure modes.
   95 Fixing the code in question should be strongly preferred
   96 over turning off the checks.
   97 See the <a href="/cmd/cgo/#hdr-Passing_pointers"><code>cgo</code> documentation</a> for more details.
   98 </p>
   99 
  100 <p>
  101 The minor change is
  102 the addition of explicit <code>C.complexfloat</code> and <code>C.complexdouble</code> types,
  103 separate from Go's <code>complex64</code> and <code>complex128</code>.
  104 Matching the other numeric types, C's complex types and Go's complex type are
  105 no longer interchangeable.
  106 </p>
  107 
  108 <h3 id="compiler">Compiler Toolchain</h3>
  109 
  110 <p>
  111 The compiler toolchain is mostly unchanged.
  112 Internally, the most significant change is that the parser is now hand-written
  113 instead of generated from <a href="/cmd/yacc/">yacc</a>.
  114 </p>
  115 
  116 <p>
  117 The compiler, linker, and <code>go</code> command have a new flag <code>-msan</code>,
  118 analogous to <code>-race</code> and only available on linux/amd64,
  119 that enables interoperation with the <a href="https://clang.llvm.org/docs/MemorySanitizer.html">Clang MemorySanitizer</a>.
  120 Such interoperation is useful mainly for testing a program containing suspect C or C++ code.
  121 </p>
  122 
  123 <p>
  124 The linker has a new option <code>-libgcc</code> to set the expected location
  125 of the C compiler support library when linking <a href="/cmd/cgo/"><code>cgo</code></a> code.
  126 The option is only consulted when using <code>-linkmode=internal</code>,
  127 and it may be set to <code>none</code> to disable the use of a support library.
  128 </p>
  129 
  130 <p>
  131 The implementation of <a href="/doc/go1.5#link">build modes started in Go 1.5</a> has been expanded to more systems.
  132 This release adds support for the <code>c-shared</code> mode on <code>android/386</code>, <code>android/amd64</code>,
  133 <code>android/arm64</code>, <code>linux/386</code>, and <code>linux/arm64</code>;
  134 for the <code>shared</code> mode on <code>linux/386</code>, <code>linux/arm</code>, <code>linux/amd64</code>, and <code>linux/ppc64le</code>;
  135 and for the new <code>pie</code> mode (generating position-independent executables) on
  136 <code>android/386</code>, <code>android/amd64</code>, <code>android/arm</code>, <code>android/arm64</code>, <code>linux/386</code>,
  137 <code>linux/amd64</code>, <code>linux/arm</code>, <code>linux/arm64</code>, and <code>linux/ppc64le</code>.
  138 See the <a href="https://golang.org/s/execmodes">design document</a> for details.
  139 </p>
  140 
  141 <p>
  142 As a reminder, the linker's <code>-X</code> flag changed in Go 1.5.
  143 In Go 1.4 and earlier, it took two arguments, as in
  144 </p>
  145 
  146 <pre>
  147 -X importpath.name value
  148 </pre>
  149 
  150 <p>
  151 Go 1.5 added an alternative syntax using a single argument
  152 that is itself a <code>name=value</code> pair:
  153 </p>
  154 
  155 <pre>
  156 -X importpath.name=value
  157 </pre>
  158 
  159 <p>
  160 In Go 1.5 the old syntax was still accepted, after printing a warning
  161 suggesting use of the new syntax instead.
  162 Go 1.6 continues to accept the old syntax and print the warning.
  163 Go 1.7 will remove support for the old syntax.
  164 </p>
  165 
  166 <h3 id="gccgo">Gccgo</h3>
  167 
  168 <p>
  169 The release schedules for the GCC and Go projects do not coincide.
  170 GCC release 5 contains the Go 1.4 version of gccgo.
  171 The next release, GCC 6, will have the Go 1.6.1 version of gccgo.
  172 </p>
  173 
  174 <h3 id="go_command">Go command</h3>
  175 
  176 <p>
  177 The <a href="/cmd/go"><code>go</code></a> command's basic operation
  178 is unchanged, but there are a number of changes worth noting.
  179 </p>
  180 
  181 <p>
  182 Go 1.5 introduced experimental support for vendoring,
  183 enabled by setting the <code>GO15VENDOREXPERIMENT</code> environment variable to <code>1</code>.
  184 Go 1.6 keeps the vendoring support, no longer considered experimental,
  185 and enables it by default.
  186 It can be disabled explicitly by setting
  187 the <code>GO15VENDOREXPERIMENT</code> environment variable to <code>0</code>.
  188 Go 1.7 will remove support for the environment variable.
  189 </p>
  190 
  191 <p>
  192 The most likely problem caused by enabling vendoring by default happens
  193 in source trees containing an existing directory named <code>vendor</code> that
  194 does not expect to be interpreted according to new vendoring semantics.
  195 In this case, the simplest fix is to rename the directory to anything other
  196 than <code>vendor</code> and update any affected import paths.
  197 </p>
  198 
  199 <p>
  200 For details about vendoring,
  201 see the documentation for the <a href="/cmd/go/#hdr-Vendor_Directories"><code>go</code> command</a>
  202 and the <a href="https://golang.org/s/go15vendor">design document</a>.
  203 </p>
  204 
  205 <p>
  206 There is a new build flag, <code>-msan</code>,
  207 that compiles Go with support for the LLVM memory sanitizer.
  208 This is intended mainly for use when linking against C or C++ code
  209 that is being checked with the memory sanitizer.
  210 </p>
  211 
  212 <h3 id="doc_command">Go doc command</h3>
  213 
  214 <p>
  215 Go 1.5 introduced the
  216 <a href="/cmd/go/#hdr-Show_documentation_for_package_or_symbol"><code>go doc</code></a> command,
  217 which allows references to packages using only the package name, as in
  218 <code>go</code> <code>doc</code> <code>http</code>.
  219 In the event of ambiguity, the Go 1.5 behavior was to use the package
  220 with the lexicographically earliest import path.
  221 In Go 1.6, ambiguity is resolved by preferring import paths with
  222 fewer elements, breaking ties using lexicographic comparison.
  223 An important effect of this change is that original copies of packages
  224 are now preferred over vendored copies.
  225 Successful searches also tend to run faster.
  226 </p>
  227 
  228 <h3 id="vet_command">Go vet command</h3>
  229 
  230 <p>
  231 The <a href="/cmd/vet"><code>go vet</code></a> command now diagnoses
  232 passing function or method values as arguments to <code>Printf</code>,
  233 such as when passing <code>f</code> where <code>f()</code> was intended.
  234 </p>
  235 
  236 <h2 id="performance">Performance</h2>
  237 
  238 <p>
  239 As always, the changes are so general and varied that precise statements
  240 about performance are difficult to make.
  241 Some programs may run faster, some slower.
  242 On average the programs in the Go 1 benchmark suite run a few percent faster in Go 1.6
  243 than they did in Go 1.5.
  244 The garbage collector's pauses are even lower than in Go 1.5,
  245 especially for programs using
  246 a large amount of memory.
  247 </p>
  248 
  249 <p>
  250 There have been significant optimizations bringing more than 10% improvements
  251 to implementations of the
  252 <a href="/pkg/compress/bzip2/"><code>compress/bzip2</code></a>,
  253 <a href="/pkg/compress/gzip/"><code>compress/gzip</code></a>,
  254 <a href="/pkg/crypto/aes/"><code>crypto/aes</code></a>,
  255 <a href="/pkg/crypto/elliptic/"><code>crypto/elliptic</code></a>,
  256 <a href="/pkg/crypto/ecdsa/"><code>crypto/ecdsa</code></a>, and
  257 <a href="/pkg/sort/"><code>sort</code></a> packages.
  258 </p>
  259 
  260 <h2 id="library">Core library</h2>
  261 
  262 <h3 id="http2">HTTP/2</h3>
  263 
  264 <p>
  265 Go 1.6 adds transparent support in the
  266 <a href="/pkg/net/http/"><code>net/http</code></a> package
  267 for the new <a href="https://http2.github.io/">HTTP/2 protocol</a>.
  268 Go clients and servers will automatically use HTTP/2 as appropriate when using HTTPS.
  269 There is no exported API specific to details of the HTTP/2 protocol handling,
  270 just as there is no exported API specific to HTTP/1.1.
  271 </p>
  272 
  273 <p>
  274 Programs that must disable HTTP/2 can do so by setting
  275 <a href="/pkg/net/http/#Transport"><code>Transport.TLSNextProto</code></a> (for clients)
  276 or
  277 <a href="/pkg/net/http/#Server"><code>Server.TLSNextProto</code></a> (for servers)
  278 to a non-nil, empty map.
  279 </p>
  280 
  281 <p>
  282 Programs that must adjust HTTP/2 protocol-specific details can import and use
  283 <a href="https://golang.org/x/net/http2"><code>golang.org/x/net/http2</code></a>,
  284 in particular its
  285 <a href="https://godoc.org/golang.org/x/net/http2/#ConfigureServer">ConfigureServer</a>
  286 and
  287 <a href="https://godoc.org/golang.org/x/net/http2/#ConfigureTransport">ConfigureTransport</a>
  288 functions.
  289 </p>
  290 
  291 <h3 id="runtime">Runtime</h3>
  292 
  293 <p>
  294 The runtime has added lightweight, best-effort detection of concurrent misuse of maps.
  295 As always, if one goroutine is writing to a map, no other goroutine should be
  296 reading or writing the map concurrently.
  297 If the runtime detects this condition, it prints a diagnosis and crashes the program.
  298 The best way to find out more about the problem is to run the program
  299 under the
  300 <a href="https://blog.golang.org/race-detector">race detector</a>,
  301 which will more reliably identify the race
  302 and give more detail.
  303 </p>
  304 
  305 <p>
  306 For program-ending panics, the runtime now by default
  307 prints only the stack of the running goroutine,
  308 not all existing goroutines.
  309 Usually only the current goroutine is relevant to a panic,
  310 so omitting the others significantly reduces irrelevant output
  311 in a crash message.
  312 To see the stacks from all goroutines in crash messages, set the environment variable
  313 <code>GOTRACEBACK</code> to <code>all</code>
  314 or call
  315 <a href="/pkg/runtime/debug/#SetTraceback"><code>debug.SetTraceback</code></a>
  316 before the crash, and rerun the program.
  317 See the <a href="/pkg/runtime/#hdr-Environment_Variables">runtime documentation</a> for details.
  318 </p>
  319 
  320 <p>
  321 <em>Updating</em>:
  322 Uncaught panics intended to dump the state of the entire program,
  323 such as when a timeout is detected or when explicitly handling a received signal,
  324 should now call <code>debug.SetTraceback("all")</code> before panicking.
  325 Searching for uses of
  326 <a href="/pkg/os/signal/#Notify"><code>signal.Notify</code></a> may help identify such code.
  327 </p>
  328 
  329 <p>
  330 On Windows, Go programs in Go 1.5 and earlier forced
  331 the global Windows timer resolution to 1ms at startup
  332 by calling <code>timeBeginPeriod(1)</code>.
  333 Go no longer needs this for good scheduler performance,
  334 and changing the global timer resolution caused problems on some systems,
  335 so the call has been removed.
  336 </p>
  337 
  338 <p>
  339 When using <code>-buildmode=c-archive</code> or
  340 <code>-buildmode=c-shared</code> to build an archive or a shared
  341 library, the handling of signals has changed.
  342 In Go 1.5 the archive or shared library would install a signal handler
  343 for most signals.
  344 In Go 1.6 it will only install a signal handler for the
  345 synchronous signals needed to handle run-time panics in Go code:
  346 SIGBUS, SIGFPE, SIGSEGV.
  347 See the <a href="/pkg/os/signal">os/signal</a> package for more
  348 details.
  349 </p>
  350 
  351 <h3 id="reflect">Reflect</h3>
  352 
  353 <p>
  354 The
  355 <a href="/pkg/reflect/"><code>reflect</code></a> package has
  356 <a href="https://golang.org/issue/12367">resolved a long-standing incompatibility</a>
  357 between the gc and gccgo toolchains
  358 regarding embedded unexported struct types containing exported fields.
  359 Code that walks data structures using reflection, especially to implement
  360 serialization in the spirit
  361 of the
  362 <a href="/pkg/encoding/json/"><code>encoding/json</code></a> and
  363 <a href="/pkg/encoding/xml/"><code>encoding/xml</code></a> packages,
  364 may need to be updated.
  365 </p>
  366 
  367 <p>
  368 The problem arises when using reflection to walk through
  369 an embedded unexported struct-typed field
  370 into an exported field of that struct.
  371 In this case, <code>reflect</code> had incorrectly reported
  372 the embedded field as exported, by returning an empty <code>Field.PkgPath</code>.
  373 Now it correctly reports the field as unexported
  374 but ignores that fact when evaluating access to exported fields
  375 contained within the struct.
  376 </p>
  377 
  378 <p>
  379 <em>Updating</em>:
  380 Typically, code that previously walked over structs and used
  381 </p>
  382 
  383 <pre>
  384 f.PkgPath != ""
  385 </pre>
  386 
  387 <p>
  388 to exclude inaccessible fields
  389 should now use
  390 </p>
  391 
  392 <pre>
  393 f.PkgPath != "" &amp;&amp; !f.Anonymous
  394 </pre>
  395 
  396 <p>
  397 For example, see the changes to the implementations of
  398 <a href="https://go-review.googlesource.com/#/c/14011/2/src/encoding/json/encode.go"><code>encoding/json</code></a> and
  399 <a href="https://go-review.googlesource.com/#/c/14012/2/src/encoding/xml/typeinfo.go"><code>encoding/xml</code></a>.
  400 </p>
  401 
  402 <h3 id="sort">Sorting</h3>
  403 
  404 <p>
  405 In the
  406 <a href="/pkg/sort/"><code>sort</code></a>
  407 package,
  408 the implementation of
  409 <a href="/pkg/sort/#Sort"><code>Sort</code></a>
  410 has been rewritten to make about 10% fewer calls to the
  411 <a href="/pkg/sort/#Interface"><code>Interface</code></a>'s
  412 <code>Less</code> and <code>Swap</code>
  413 methods, with a corresponding overall time savings.
  414 The new algorithm does choose a different ordering than before
  415 for values that compare equal (those pairs for which <code>Less(i,</code> <code>j)</code> and <code>Less(j,</code> <code>i)</code> are false).
  416 </p>
  417 
  418 <p>
  419 <em>Updating</em>:
  420 The definition of <code>Sort</code> makes no guarantee about the final order of equal values,
  421 but the new behavior may still break programs that expect a specific order.
  422 Such programs should either refine their <code>Less</code> implementations
  423 to report the desired order
  424 or should switch to
  425 <a href="/pkg/sort/#Stable"><code>Stable</code></a>,
  426 which preserves the original input order
  427 of equal values.
  428 </p>
  429 
  430 <h3 id="template">Templates</h3>
  431 
  432 <p>
  433 In the
  434 <a href="/pkg/text/template/">text/template</a> package,
  435 there are two significant new features to make writing templates easier.
  436 </p>
  437 
  438 <p>
  439 First, it is now possible to <a href="/pkg/text/template/#hdr-Text_and_spaces">trim spaces around template actions</a>,
  440 which can make template definitions more readable.
  441 A minus sign at the beginning of an action says to trim space before the action,
  442 and a minus sign at the end of an action says to trim space after the action.
  443 For example, the template
  444 </p>
  445 
  446 <pre>
  447 {{"{{"}}23 -}}
  448    &lt;
  449 {{"{{"}}- 45}}
  450 </pre>
  451 
  452 <p>
  453 formats as <code>23&lt;45</code>.
  454 </p>
  455 
  456 <p>
  457 Second, the new <a href="/pkg/text/template/#hdr-Actions"><code>{{"{{"}}block}}</code> action</a>,
  458 combined with allowing redefinition of named templates,
  459 provides a simple way to define pieces of a template that
  460 can be replaced in different instantiations.
  461 There is <a href="/pkg/text/template/#example_Template_block">an example</a>
  462 in the <code>text/template</code> package that demonstrates this new feature.
  463 </p>
  464 
  465 <h3 id="minor_library_changes">Minor changes to the library</h3>
  466 
  467 <ul>
  468 
  469 <li>
  470 The <a href="/pkg/archive/tar/"><code>archive/tar</code></a> package's
  471 implementation corrects many bugs in rare corner cases of the file format.
  472 One visible change is that the
  473 <a href="/pkg/archive/tar/#Reader"><code>Reader</code></a> type's
  474 <a href="/pkg/archive/tar/#Reader.Read"><code>Read</code></a> method
  475 now presents the content of special file types as being empty,
  476 returning <code>io.EOF</code> immediately.
  477 </li>
  478 
  479 <li>
  480 In the <a href="/pkg/archive/zip/"><code>archive/zip</code></a> package, the
  481 <a href="/pkg/archive/zip/#Reader"><code>Reader</code></a> type now has a
  482 <a href="/pkg/archive/zip/#Reader.RegisterDecompressor"><code>RegisterDecompressor</code></a> method,
  483 and the
  484 <a href="/pkg/archive/zip/#Writer"><code>Writer</code></a> type now has a
  485 <a href="/pkg/archive/zip/#Writer.RegisterCompressor"><code>RegisterCompressor</code></a> method,
  486 enabling control over compression options for individual zip files.
  487 These take precedence over the pre-existing global
  488 <a href="/pkg/archive/zip/#RegisterDecompressor"><code>RegisterDecompressor</code></a> and
  489 <a href="/pkg/archive/zip/#RegisterCompressor"><code>RegisterCompressor</code></a> functions.
  490 </li>
  491 
  492 <li>
  493 The <a href="/pkg/bufio/"><code>bufio</code></a> package's
  494 <a href="/pkg/bufio/#Scanner"><code>Scanner</code></a> type now has a
  495 <a href="/pkg/bufio/#Scanner.Buffer"><code>Buffer</code></a> method,
  496 to specify an initial buffer and maximum buffer size to use during scanning.
  497 This makes it possible, when needed, to scan tokens larger than
  498 <code>MaxScanTokenSize</code>.
  499 Also for the <code>Scanner</code>, the package now defines the
  500 <a href="/pkg/bufio/#ErrFinalToken"><code>ErrFinalToken</code></a> error value, for use by
  501 <a href="/pkg/bufio/#SplitFunc">split functions</a> to abort processing or to return a final empty token.
  502 </li>
  503 
  504 <li>
  505 The <a href="/pkg/compress/flate/"><code>compress/flate</code></a> package
  506 has deprecated its
  507 <a href="/pkg/compress/flate/#ReadError"><code>ReadError</code></a> and
  508 <a href="/pkg/compress/flate/#WriteError"><code>WriteError</code></a> error implementations.
  509 In Go 1.5 they were only rarely returned when an error was encountered;
  510 now they are never returned, although they remain defined for compatibility.
  511 </li>
  512 
  513 <li>
  514 The <a href="/pkg/compress/flate/"><code>compress/flate</code></a>,
  515 <a href="/pkg/compress/gzip/"><code>compress/gzip</code></a>, and
  516 <a href="/pkg/compress/zlib/"><code>compress/zlib</code></a> packages
  517 now report
  518 <a href="/pkg/io/#ErrUnexpectedEOF"><code>io.ErrUnexpectedEOF</code></a> for truncated input streams, instead of
  519 <a href="/pkg/io/#EOF"><code>io.EOF</code></a>.
  520 </li>
  521 
  522 <li>
  523 The <a href="/pkg/crypto/cipher/"><code>crypto/cipher</code></a> package now
  524 overwrites the destination buffer in the event of a GCM decryption failure.
  525 This is to allow the AESNI code to avoid using a temporary buffer.
  526 </li>
  527 
  528 <li>
  529 The <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package
  530 has a variety of minor changes.
  531 It now allows
  532 <a href="/pkg/crypto/tls/#Listen"><code>Listen</code></a>
  533 to succeed when the
  534 <a href="/pkg/crypto/tls/#Config"><code>Config</code></a>
  535 has a nil <code>Certificates</code>, as long as the <code>GetCertificate</code> callback is set,
  536 it adds support for RSA with AES-GCM cipher suites,
  537 and
  538 it adds a
  539 <a href="/pkg/crypto/tls/#RecordHeaderError"><code>RecordHeaderError</code></a>
  540 to allow clients (in particular, the <a href="/pkg/net/http/"><code>net/http</code></a> package)
  541 to report a better error when attempting a TLS connection to a non-TLS server.
  542 </li>
  543 
  544 <li>
  545 The <a href="/pkg/crypto/x509/"><code>crypto/x509</code></a> package
  546 now permits certificates to contain negative serial numbers
  547 (technically an error, but unfortunately common in practice),
  548 and it defines a new
  549 <a href="/pkg/crypto/x509/#InsecureAlgorithmError"><code>InsecureAlgorithmError</code></a>
  550 to give a better error message when rejecting a certificate
  551 signed with an insecure algorithm like MD5.
  552 </li>
  553 
  554 <li>
  555 The <a href="/pkg/debug/dwarf"><code>debug/dwarf</code></a> and
  556 <a href="/pkg/debug/elf/"><code>debug/elf</code></a> packages
  557 together add support for compressed DWARF sections.
  558 User code needs no updating: the sections are decompressed automatically when read.
  559 </li>
  560 
  561 <li>
  562 The <a href="/pkg/debug/elf/"><code>debug/elf</code></a> package
  563 adds support for general compressed ELF sections.
  564 User code needs no updating: the sections are decompressed automatically when read.
  565 However, compressed
  566 <a href="/pkg/debug/elf/#Section"><code>Sections</code></a> do not support random access:
  567 they have a nil <code>ReaderAt</code> field.
  568 </li>
  569 
  570 <li>
  571 The <a href="/pkg/encoding/asn1/"><code>encoding/asn1</code></a> package
  572 now exports
  573 <a href="/pkg/encoding/asn1/#pkg-constants">tag and class constants</a>
  574 useful for advanced parsing of ASN.1 structures.
  575 </li>
  576 
  577 <li>
  578 Also in the <a href="/pkg/encoding/asn1/"><code>encoding/asn1</code></a> package,
  579 <a href="/pkg/encoding/asn1/#Unmarshal"><code>Unmarshal</code></a> now rejects various non-standard integer and length encodings.
  580 </li>
  581 
  582 <li>
  583 The <a href="/pkg/encoding/base64"><code>encoding/base64</code></a> package's
  584 <a href="/pkg/encoding/base64/#Decoder"><code>Decoder</code></a> has been fixed
  585 to process the final bytes of its input. Previously it processed as many four-byte tokens as
  586 possible but ignored the remainder, up to three bytes.
  587 The <code>Decoder</code> therefore now handles inputs in unpadded encodings (like
  588 <a href="/pkg/encoding/base64/#RawURLEncoding">RawURLEncoding</a>) correctly,
  589 but it also rejects inputs in padded encodings that are truncated or end with invalid bytes,
  590 such as trailing spaces.
  591 </li>
  592 
  593 <li>
  594 The <a href="/pkg/encoding/json/"><code>encoding/json</code></a> package
  595 now checks the syntax of a
  596 <a href="/pkg/encoding/json/#Number"><code>Number</code></a>
  597 before marshaling it, requiring that it conforms to the JSON specification for numeric values.
  598 As in previous releases, the zero <code>Number</code> (an empty string) is marshaled as a literal 0 (zero).
  599 </li>
  600 
  601 <li>
  602 The <a href="/pkg/encoding/xml/"><code>encoding/xml</code></a> package's
  603 <a href="/pkg/encoding/xml/#Marshal"><code>Marshal</code></a>
  604 function now supports a <code>cdata</code> attribute, such as <code>chardata</code>
  605 but encoding its argument in one or more <code>&lt;![CDATA[ ... ]]&gt;</code> tags.
  606 </li>
  607 
  608 <li>
  609 Also in the <a href="/pkg/encoding/xml/"><code>encoding/xml</code></a> package,
  610 <a href="/pkg/encoding/xml/#Decoder"><code>Decoder</code></a>'s
  611 <a href="/pkg/encoding/xml/#Decoder.Token"><code>Token</code></a> method
  612 now reports an error when encountering EOF before seeing all open tags closed,
  613 consistent with its general requirement that tags in the input be properly matched.
  614 To avoid that requirement, use
  615 <a href="/pkg/encoding/xml/#Decoder.RawToken"><code>RawToken</code></a>.
  616 </li>
  617 
  618 <li>
  619 The <a href="/pkg/fmt/"><code>fmt</code></a> package now allows
  620 any integer type as an argument to
  621 <a href="/pkg/fmt/#Printf"><code>Printf</code></a>'s <code>*</code> width and precision specification.
  622 In previous releases, the argument to <code>*</code> was required to have type <code>int</code>.
  623 </li>
  624 
  625 <li>
  626 Also in the <a href="/pkg/fmt/"><code>fmt</code></a> package,
  627 <a href="/pkg/fmt/#Scanf"><code>Scanf</code></a> can now scan hexadecimal strings using %X, as an alias for %x.
  628 Both formats accept any mix of upper- and lower-case hexadecimal.
  629 </li>
  630 
  631 <li>
  632 The <a href="/pkg/image/"><code>image</code></a>
  633 and
  634 <a href="/pkg/image/color/"><code>image/color</code></a> packages
  635 add
  636 <a href="/pkg/image/#NYCbCrA"><code>NYCbCrA</code></a>
  637 and
  638 <a href="/pkg/image/color/#NYCbCrA"><code>NYCbCrA</code></a>
  639 types, to support Y'CbCr images with non-premultiplied alpha.
  640 </li>
  641 
  642 <li>
  643 The <a href="/pkg/io/"><code>io</code></a> package's
  644 <a href="/pkg/io/#MultiWriter"><code>MultiWriter</code></a>
  645 implementation now implements a <code>WriteString</code> method,
  646 for use by
  647 <a href="/pkg/io/#WriteString"><code>WriteString</code></a>.
  648 </li>
  649 
  650 <li>
  651 In the <a href="/pkg/math/big/"><code>math/big</code></a> package,
  652 <a href="/pkg/math/big/#Int"><code>Int</code></a> adds
  653 <a href="/pkg/math/big/#Int.Append"><code>Append</code></a>
  654 and
  655 <a href="/pkg/math/big/#Int.Text"><code>Text</code></a>
  656 methods to give more control over printing.
  657 </li>
  658 
  659 <li>
  660 Also in the <a href="/pkg/math/big/"><code>math/big</code></a> package,
  661 <a href="/pkg/math/big/#Float"><code>Float</code></a> now implements
  662 <a href="/pkg/encoding/#TextMarshaler"><code>encoding.TextMarshaler</code></a> and
  663 <a href="/pkg/encoding/#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>,
  664 allowing it to be serialized in a natural form by the
  665 <a href="/pkg/encoding/json/"><code>encoding/json</code></a> and
  666 <a href="/pkg/encoding/xml/"><code>encoding/xml</code></a> packages.
  667 </li>
  668 
  669 <li>
  670 Also in the <a href="/pkg/math/big/"><code>math/big</code></a> package,
  671 <a href="/pkg/math/big/#Float"><code>Float</code></a>'s
  672 <a href="/pkg/math/big/#Float.Append"><code>Append</code></a> method now supports the special precision argument -1.
  673 As in
  674 <a href="/pkg/strconv/#ParseFloat"><code>strconv.ParseFloat</code></a>,
  675 precision -1 means to use the smallest number of digits necessary such that
  676 <a href="/pkg/math/big/#Float.Parse"><code>Parse</code></a>
  677 reading the result into a <code>Float</code> of the same precision
  678 will yield the original value.
  679 </li>
  680 
  681 <li>
  682 The <a href="/pkg/math/rand/"><code>math/rand</code></a> package
  683 adds a
  684 <a href="/pkg/math/rand/#Read"><code>Read</code></a>
  685 function, and likewise
  686 <a href="/pkg/math/rand/#Rand"><code>Rand</code></a> adds a
  687 <a href="/pkg/math/rand/#Rand.Read"><code>Read</code></a> method.
  688 These make it easier to generate pseudorandom test data.
  689 Note that, like the rest of the package,
  690 these should not be used in cryptographic settings;
  691 for such purposes, use the <a href="/pkg/crypto/rand/"><code>crypto/rand</code></a> package instead.
  692 </li>
  693 
  694 <li>
  695 The <a href="/pkg/net/"><code>net</code></a> package's
  696 <a href="/pkg/net/#ParseMAC"><code>ParseMAC</code></a> function now accepts 20-byte IP-over-InfiniBand (IPoIB) link-layer addresses.
  697 </li>
  698 
  699 
  700 <li>
  701 Also in the <a href="/pkg/net/"><code>net</code></a> package,
  702 there have been a few changes to DNS lookups.
  703 First, the
  704 <a href="/pkg/net/#DNSError"><code>DNSError</code></a> error implementation now implements
  705 <a href="/pkg/net/#Error"><code>Error</code></a>,
  706 and in particular its new
  707 <a href="/pkg/net/#DNSError.IsTemporary"><code>IsTemporary</code></a>
  708 method returns true for DNS server errors.
  709 Second, DNS lookup functions such as
  710 <a href="/pkg/net/#LookupAddr"><code>LookupAddr</code></a>
  711 now return rooted domain names (with a trailing dot)
  712 on Plan 9 and Windows, to match the behavior of Go on Unix systems.
  713 </li>
  714 
  715 <li>
  716 The <a href="/pkg/net/http/"><code>net/http</code></a> package has
  717 a number of minor additions beyond the HTTP/2 support already discussed.
  718 First, the
  719 <a href="/pkg/net/http/#FileServer"><code>FileServer</code></a> now sorts its generated directory listings by file name.
  720 Second, the
  721 <a href="/pkg/net/http/#ServeFile"><code>ServeFile</code></a> function now refuses to serve a result
  722 if the request's URL path contains &ldquo;..&rdquo; (dot-dot) as a path element.
  723 Programs should typically use <code>FileServer</code> and 
  724 <a href="/pkg/net/http/#Dir"><code>Dir</code></a>
  725 instead of calling <code>ServeFile</code> directly.
  726 Programs that need to serve file content in response to requests for URLs containing dot-dot can 
  727 still call <a href="/pkg/net/http/#ServeContent"><code>ServeContent</code></a>.
  728 Third, the
  729 <a href="/pkg/net/http/#Client"><code>Client</code></a> now allows user code to set the
  730 <code>Expect:</code> <code>100-continue</code> header (see
  731 <a href="/pkg/net/http/#Transport"><code>Transport.ExpectContinueTimeout</code></a>).
  732 Fourth, there are
  733 <a href="/pkg/net/http/#pkg-constants">five new error codes</a>:
  734 <code>StatusPreconditionRequired</code> (428),
  735 <code>StatusTooManyRequests</code> (429),
  736 <code>StatusRequestHeaderFieldsTooLarge</code> (431), and
  737 <code>StatusNetworkAuthenticationRequired</code> (511) from RFC 6585,
  738 as well as the recently-approved
  739 <code>StatusUnavailableForLegalReasons</code> (451).
  740 Fifth, the implementation and documentation of
  741 <a href="/pkg/net/http/#CloseNotifier"><code>CloseNotifier</code></a>
  742 has been substantially changed.
  743 The <a href="/pkg/net/http/#Hijacker"><code>Hijacker</code></a>
  744 interface now works correctly on connections that have previously
  745 been used with <code>CloseNotifier</code>.
  746 The documentation now describes when <code>CloseNotifier</code>
  747 is expected to work.
  748 </li>
  749 
  750 <li>
  751 Also in the <a href="/pkg/net/http/"><code>net/http</code></a> package,
  752 there are a few changes related to the handling of a
  753 <a href="/pkg/net/http/#Request"><code>Request</code></a> data structure with its <code>Method</code> field set to the empty string.
  754 An empty <code>Method</code> field has always been documented as an alias for <code>"GET"</code>
  755 and it remains so.
  756 However, Go 1.6 fixes a few routines that did not treat an empty
  757 <code>Method</code> the same as an explicit <code>"GET"</code>.
  758 Most notably, in previous releases
  759 <a href="/pkg/net/http/#Client"><code>Client</code></a> followed redirects only with
  760 <code>Method</code> set explicitly to <code>"GET"</code>;
  761 in Go 1.6 <code>Client</code> also follows redirects for the empty <code>Method</code>.
  762 Finally,
  763 <a href="/pkg/net/http/#NewRequest"><code>NewRequest</code></a> accepts a <code>method</code> argument that has not been
  764 documented as allowed to be empty.
  765 In past releases, passing an empty <code>method</code> argument resulted
  766 in a <code>Request</code> with an empty <code>Method</code> field.
  767 In Go 1.6, the resulting <code>Request</code> always has an initialized
  768 <code>Method</code> field: if its argument is an empty string, <code>NewRequest</code>
  769 sets the <code>Method</code> field in the returned <code>Request</code> to <code>"GET"</code>.
  770 </li>
  771 
  772 <li>
  773 The <a href="/pkg/net/http/httptest/"><code>net/http/httptest</code></a> package's
  774 <a href="/pkg/net/http/httptest/#ResponseRecorder"><code>ResponseRecorder</code></a> now initializes a default Content-Type header
  775 using the same content-sniffing algorithm as in
  776 <a href="/pkg/net/http/#Server"><code>http.Server</code></a>.
  777 </li>
  778 
  779 <li>
  780 The <a href="/pkg/net/url/"><code>net/url</code></a> package's
  781 <a href="/pkg/net/url/#Parse"><code>Parse</code></a> is now stricter and more spec-compliant regarding the parsing
  782 of host names.
  783 For example, spaces in the host name are no longer accepted.
  784 </li>
  785 
  786 <li>
  787 Also in the <a href="/pkg/net/url/"><code>net/url</code></a> package,
  788 the <a href="/pkg/net/url/#Error"><code>Error</code></a> type now implements
  789 <a href="/pkg/net/#Error"><code>net.Error</code></a>.
  790 </li>
  791 
  792 <li>
  793 The <a href="/pkg/os/"><code>os</code></a> package's
  794 <a href="/pkg/os/#IsExist"><code>IsExist</code></a>,
  795 <a href="/pkg/os/#IsNotExist"><code>IsNotExist</code></a>,
  796 and
  797 <a href="/pkg/os/#IsPermission"><code>IsPermission</code></a>
  798 now return correct results when inquiring about an
  799 <a href="/pkg/os/#SyscallError"><code>SyscallError</code></a>.
  800 </li>
  801 
  802 <li>
  803 On Unix-like systems, when a write
  804 to <a href="/pkg/os/#pkg-variables"><code>os.Stdout</code>
  805 or <code>os.Stderr</code></a> (more precisely, an <code>os.File</code>
  806 opened for file descriptor 1 or 2) fails due to a broken pipe error,
  807 the program will raise a <code>SIGPIPE</code> signal.
  808 By default this will cause the program to exit; this may be changed by
  809 calling the
  810 <a href="/pkg/os/signal"><code>os/signal</code></a>
  811 <a href="/pkg/os/signal/#Notify"><code>Notify</code></a> function
  812 for <code>syscall.SIGPIPE</code>.
  813 A write to a broken pipe on a file descriptor other 1 or 2 will simply
  814 return <code>syscall.EPIPE</code> (possibly wrapped in
  815 <a href="/pkg/os#PathError"><code>os.PathError</code></a>
  816 and/or <a href="/pkg/os#SyscallError"><code>os.SyscallError</code></a>)
  817 to the caller.
  818 The old behavior of raising an uncatchable <code>SIGPIPE</code> signal
  819 after 10 consecutive writes to a broken pipe no longer occurs.
  820 </li>
  821 
  822 <li>
  823 In the <a href="/pkg/os/exec/"><code>os/exec</code></a> package,
  824 <a href="/pkg/os/exec/#Cmd"><code>Cmd</code></a>'s
  825 <a href="/pkg/os/exec/#Cmd.Output"><code>Output</code></a> method continues to return an
  826 <a href="/pkg/os/exec/#ExitError"><code>ExitError</code></a> when a command exits with an unsuccessful status.
  827 If standard error would otherwise have been discarded,
  828 the returned <code>ExitError</code> now holds a prefix and suffix
  829 (currently 32 kB) of the failed command's standard error output,
  830 for debugging or for inclusion in error messages.
  831 The <code>ExitError</code>'s
  832 <a href="/pkg/os/exec/#ExitError.String"><code>String</code></a>
  833 method does not show the captured standard error;
  834 programs must retrieve it from the data structure
  835 separately.
  836 </li>
  837 
  838 <li>
  839 On Windows, the <a href="/pkg/path/filepath/"><code>path/filepath</code></a> package's
  840 <a href="/pkg/path/filepath/#Join"><code>Join</code></a> function now correctly handles the case when the base is a relative drive path.
  841 For example, <code>Join(`c:`,</code> <code>`a`)</code> now
  842 returns <code>`c:a`</code> instead of <code>`c:\a`</code> as in past releases.
  843 This may affect code that expects the incorrect result.
  844 </li>
  845 
  846 <li>
  847 In the <a href="/pkg/regexp/"><code>regexp</code></a> package,
  848 the
  849 <a href="/pkg/regexp/#Regexp"><code>Regexp</code></a> type has always been safe for use by
  850 concurrent goroutines.
  851 It uses a <a href="/pkg/sync/#Mutex"><code>sync.Mutex</code></a> to protect
  852 a cache of scratch spaces used during regular expression searches.
  853 Some high-concurrency servers using the same <code>Regexp</code> from many goroutines
  854 have seen degraded performance due to contention on that mutex.
  855 To help such servers, <code>Regexp</code> now has a
  856 <a href="/pkg/regexp/#Regexp.Copy"><code>Copy</code></a> method,
  857 which makes a copy of a <code>Regexp</code> that shares most of the structure
  858 of the original but has its own scratch space cache.
  859 Two goroutines can use different copies of a <code>Regexp</code>
  860 without mutex contention.
  861 A copy does have additional space overhead, so <code>Copy</code>
  862 should only be used when contention has been observed.
  863 </li>
  864 
  865 <li>
  866 The <a href="/pkg/strconv/"><code>strconv</code></a> package adds
  867 <a href="/pkg/strconv/#IsGraphic"><code>IsGraphic</code></a>,
  868 similar to <a href="/pkg/strconv/#IsPrint"><code>IsPrint</code></a>.
  869 It also adds
  870 <a href="/pkg/strconv/#QuoteToGraphic"><code>QuoteToGraphic</code></a>,
  871 <a href="/pkg/strconv/#QuoteRuneToGraphic"><code>QuoteRuneToGraphic</code></a>,
  872 <a href="/pkg/strconv/#AppendQuoteToGraphic"><code>AppendQuoteToGraphic</code></a>,
  873 and
  874 <a href="/pkg/strconv/#AppendQuoteRuneToGraphic"><code>AppendQuoteRuneToGraphic</code></a>,
  875 analogous to
  876 <a href="/pkg/strconv/#QuoteToASCII"><code>QuoteToASCII</code></a>,
  877 <a href="/pkg/strconv/#QuoteRuneToASCII"><code>QuoteRuneToASCII</code></a>,
  878 and so on.
  879 The <code>ASCII</code> family escapes all space characters except ASCII space (U+0020).
  880 In contrast, the <code>Graphic</code> family does not escape any Unicode space characters (category Zs).
  881 </li>
  882 
  883 <li>
  884 In the <a href="/pkg/testing/"><code>testing</code></a> package,
  885 when a test calls
  886 <a href="/pkg/testing/#T.Parallel">t.Parallel</a>,
  887 that test is paused until all non-parallel tests complete, and then
  888 that test continues execution with all other parallel tests.
  889 Go 1.6 changes the time reported for such a test:
  890 previously the time counted only the parallel execution,
  891 but now it also counts the time from the start of testing
  892 until the call to <code>t.Parallel</code>.
  893 </li>
  894 
  895 <li>
  896 The <a href="/pkg/text/template/"><code>text/template</code></a> package
  897 contains two minor changes, in addition to the <a href="#template">major changes</a>
  898 described above.
  899 First, it adds a new
  900 <a href="/pkg/text/template/#ExecError"><code>ExecError</code></a> type
  901 returned for any error during
  902 <a href="/pkg/text/template/#Template.Execute"><code>Execute</code></a>
  903 that does not originate in a <code>Write</code> to the underlying writer.
  904 Callers can distinguish template usage errors from I/O errors by checking for
  905 <code>ExecError</code>.
  906 Second, the
  907 <a href="/pkg/text/template/#Template.Funcs"><code>Funcs</code></a> method
  908 now checks that the names used as keys in the
  909 <a href="/pkg/text/template/#FuncMap"><code>FuncMap</code></a>
  910 are identifiers that can appear in a template function invocation.
  911 If not, <code>Funcs</code> panics.
  912 </li>
  913 
  914 <li>
  915 The <a href="/pkg/time/"><code>time</code></a> package's
  916 <a href="/pkg/time/#Parse"><code>Parse</code></a> function has always rejected any day of month larger than 31,
  917 such as January 32.
  918 In Go 1.6, <code>Parse</code> now also rejects February 29 in non-leap years,
  919 February 30, February 31, April 31, June 31, September 31, and November 31.
  920 </li>
  921 
  922 </ul>
  923