## Introduction to Go 1.1

The release of Go version 1 (Go 1 or Go 1.0 for short) in March of 2012 introduced a new period of stability in the Go language and libraries. That stability has helped nourish a growing community of Go users and systems around the world. Several "point" releases since then—1.0.1, 1.0.2, and 1.0.3—have been issued. These point releases fixed known bugs but made no non-critical changes to the implementation.

This new release, Go 1.1, keeps the promise of compatibility but adds a couple of significant (backwards-compatible, of course) language changes, has a long list of (again, compatible) library changes, and includes major work on the implementation of the compilers, libraries, and run-time. The focus is on performance. Benchmarking is an inexact science at best, but we see significant, sometimes dramatic speedups for many of our test programs. We trust that many of our users' programs will also see improvements just by updating their Go installation and recompiling.

This document summarizes the changes between Go 1 and Go 1.1. Very little if any code will need modification to run with Go 1.1, although a couple of rare error cases surface with this release and need to be addressed if they arise. Details appear below; see the discussion of 64-bit ints and Unicode literals in particular.

## Changes to the language

The Go compatibility document promises that programs written to the Go 1 language specification will continue to operate, and those promises are maintained. In the interest of firming up the specification, though, there are details about some error cases that have been clarified. There are also some new language features.

### Integer division by zero

In Go 1, integer division by a constant zero produced a run-time panic:

    func f(x int) int {
    	return x/0
    }

In Go 1.1, an integer division by constant zero is not a legal program, so it is a compile-time error.

### Surrogates in Unicode literals

The definition of string and rune literals has been refined to exclude surrogate halves from the set of valid Unicode code points. See the Unicode section for more information.

### Method values

Go 1.1 now implements method values, which are functions that have been bound to a specific receiver value. For instance, given a `Writer` value `w`, the expression `w.Write`, a method value, is a function that will always write to `w`; it is equivalent to a function literal closing over `w`:

    func (p []byte) (n int, err error) {
    	return w.Write(p)
    }

Method values are distinct from method expressions, which generate functions from methods of a given type; the method expression `(*bufio.Writer).Write` is equivalent to a function with an extra first argument, a receiver of type `(*bufio.Writer)`:

    func (w *bufio.Writer, p []byte) (n int, err error) {
    	return w.Write(p)
    }

Updating: No existing code is affected; the change is strictly backward-compatible.

### Return requirements

Before Go 1.1, a function that returned a value needed an explicit "return" or call to `panic` at the end of the function; this was a simple way to make the programmer be explicit about the meaning of the function. But there are many cases where a final "return" is clearly unnecessary, such as a function with only an infinite "for" loop.

In Go 1.1, the rule about final "return" statements is more permissive. It introduces the concept of a terminating statement, a statement that is guaranteed to be the last one a function executes. Examples include "for" loops with no condition and "if-else" statements in which each half ends in a "return". If the final statement of a function can be shown syntactically to be a terminating statement, no final "return" statement is needed.

Note that the rule is purely syntactic: it pays no attention to the values in the code and therefore requires no complex analysis.

Updating: The change is backward-compatible, but existing code with superfluous "return" statements and calls to `panic` may be simplified manually. Such code can be identified by `go vet`.

## Changes to the implementations and tools

### Status of gccgo

The GCC release schedule does not coincide with the Go release schedule, so some skew is inevitable in `gccgo`'s releases. The 4.8.0 version of GCC shipped in March, 2013 and includes a nearly-Go 1.1 version of `gccgo`. Its library is a little behind the release, but the biggest difference is that method values are not implemented. Sometime around July 2013, we expect 4.8.2 of GCC to ship with a `gccgo` providing a complete Go 1.1 implementation.

### Command-line flag parsing

In the gc toolchain, the compilers and linkers now use the same command-line flag parsing rules as the Go flag package, a departure from the traditional Unix flag parsing. This may affect scripts that invoke the tool directly. For example, `go tool 6c -Fw -Dfoo` must now be written `go tool 6c -F -w -D foo`.

### Size of int on 64-bit platforms

The language allows the implementation to choose whether the `int` type and `uint` types are 32 or 64 bits. Previous Go implementations made `int` and `uint` 32 bits on all systems. Both the gc and gccgo implementations now make `int` and `uint` 64 bits on 64-bit platforms such as AMD64/x86-64. Among other things, this enables the allocation of slices with more than 2 billion elements on 64-bit platforms.

Updating: Most programs will be unaffected by this change. Because Go does not allow implicit conversions between distinct numeric types, no programs will stop compiling due to this change. However, programs that contain implicit assumptions that `int` is only 32 bits may change behavior. For example, this code prints a positive number on 64-bit systems and a negative one on 32-bit systems:

    x := ^uint32(0) // x is 0xffffffff
    i := int(x)     // i is -1 on 32-bit systems, 0xffffffff on 64-bit
    fmt.Println(i)

Portable code intending 32-bit sign extension (yielding `-1` on all systems) would instead say:

    i := int(int32(x))

### Heap size on 64-bit architectures

On 64-bit architectures, the maximum heap size has been enlarged substantially, from a few gigabytes to several tens of gigabytes. (The exact details depend on the system and may change.)

On 32-bit architectures, the heap size has not changed.

Updating: This change should have no effect on existing programs beyond allowing them to run with larger heaps.

### Unicode

To make it possible to represent code points greater than 65535 in UTF-16, Unicode defines surrogate halves, a range of code points to be used only in the assembly of large values, and only in UTF-16. The code points in that surrogate range are illegal for any other purpose. In Go 1.1, this constraint is honored by the compiler, libraries, and run-time: a surrogate half is illegal as a rune value, when encoded as UTF-8, or when encoded in isolation as UTF-16. When encountered, for example in converting from a rune to UTF-8, it is treated as an encoding error and will yield the replacement rune, `utf8.RuneError`, U+FFFD.

This program,

    import "fmt"

    func main() {
        fmt.Printf("%+q\n", string(0xD800))
    }

printed `"\ud800"` in Go 1.0, but prints `"\ufffd"` in Go 1.1.

Surrogate-half Unicode values are now illegal in rune and string constants, so constants such as `'\ud800'` and `"\ud800"` are now rejected by the compilers. When written explicitly as UTF-8 encoded bytes, such strings can still be created, as in `"\xed\xa0\x80"`. However, when such a string is decoded as a sequence of runes, as in a range loop, it will yield only `utf8.RuneError` values.

The Unicode byte order mark U+FEFF, encoded in UTF-8, is now permitted as the first character of a Go source file. Even though its appearance in the byte-order-free UTF-8 encoding is clearly unnecessary, some editors add the mark as a kind of "magic number" identifying a UTF-8 encoded file.

Updating: Most programs will be unaffected by the surrogate change. Programs that depend on the old behavior should be modified to avoid the issue. The byte-order-mark change is strictly backward-compatible.

### Race detector

A major addition to the tools is a race detector, a way to find bugs in programs caused by concurrent access of the same variable, where at least one of the accesses is a write. This new facility is built into the `go` tool. For now, it is only available on Linux, Mac OS X, and Windows systems with 64-bit x86 processors. To enable it, set the `-race` flag when building or testing your program (for instance, `go test -race`). The race detector is documented in a separate article.

### The gc assemblers

Due to the change of the `int` to 64 bits and a new internal representation of functions, the arrangement of function arguments on the stack has changed in the gc toolchain. Functions written in assembly will need to be revised at least to adjust frame pointer offsets.

Updating: The `go vet` command now checks that functions implemented in assembly match the Go function prototypes they implement.

### Changes to the go command

The `go` command has acquired several changes intended to improve the experience for new Go users.

First, when compiling, testing, or running Go code, the `go` command will now give more detailed error messages, including a list of paths searched, when a package cannot be located.

    $ go build foo/quxx
    can't load package: package foo/quxx: cannot find package "foo/quxx" in any of:
            /home/you/go/src/pkg/foo/quxx (from $GOROOT)
            /home/you/src/foo/quxx (from $GOPATH)

Second, the `go get` command no longer allows `$GOROOT` as the default destination when downloading package source. To use the `go get` command, a valid `$GOPATH` is now required.

    $ GOPATH= go get code.google.com/p/foo/quxx
    package code.google.com/p/foo/quxx: cannot download, $GOPATH not set. For more details see: go help gopath

Finally, as a result of the previous change, the `go get` command will also fail when `$GOPATH` and `$GOROOT` are set to the same value.

    $ GOPATH=$GOROOT go get code.google.com/p/foo/quxx
    warning: GOPATH set to GOROOT (/home/you/go) has no effect
    package code.google.com/p/foo/quxx: cannot download, $GOPATH must not be set to $GOROOT. For more details see: go help gopath

### Changes to the go test command

The `go test` command no longer deletes the binary when run with profiling enabled, to make it easier to analyze the profile. The implementation sets the `-c` flag automatically, so after running,

    $ go test -cpuprofile cpuprof.out mypackage

the file `mypackage.test` will be left in the directory where `go test` was run.

The `go test` command can now generate profiling information that reports where goroutines are blocked, that is, where they tend to stall waiting for an event such as a channel communication. The information is presented as a blocking profile enabled with the `-blockprofile` option of `go test`. Run `go help test` for more information.

### Changes to the go fix command

The `fix` command, usually run as `go fix`, no longer applies fixes to update code from before Go 1 to use Go 1 APIs. To update pre-Go 1 code to Go 1.1, use a Go 1.0 toolchain to convert the code to Go 1.0 first.

### Build constraints

The "`go1.1`" tag has been added to the list of default build constraints. This permits packages to take advantage of the new features in Go 1.1 while remaining compatible with earlier versions of Go.

To build a file only with Go 1.1 and above, add this build constraint:

    // +build go1.1

To build a file only with Go 1.0.x, use the converse constraint:

    // +build !go1.1

### Additional platforms

The Go 1.1 toolchain adds experimental support for `freebsd/arm`, `netbsd/386`, `netbsd/amd64`, `netbsd/arm`, `openbsd/386` and `openbsd/amd64` platforms.

An ARMv6 or later processor is required for `freebsd/arm` or `netbsd/arm`.

Go 1.1 adds experimental support for `cgo` on `linux/arm`.

### Cross compilation

When cross-compiling, the `go` tool will disable `cgo` support by default.

To explicitly enable `cgo`, set `CGO_ENABLED=1`.

## Performance

The performance of code compiled with the Go 1.1 gc tool suite should be noticeably better for most Go programs. Typical improvements relative to Go 1.0 seem to be about 30%-40%, sometimes much more, but occasionally less or even non-existent. There are too many small performance-driven tweaks through the tools and libraries to list them all here, but the following major changes are worth noting:

- The gc compilers generate better code in many cases, most noticeably for floating point on the 32-bit Intel architecture.
- The gc compilers do more in-lining, including for some operations in the run-time such as `append` and interface conversions.
- There is a new implementation of Go maps with significant reduction in memory footprint and CPU time.
- The garbage collector has been made more parallel, which can reduce latencies for programs running on multiple CPUs.
- The garbage collector is also more precise, which costs a small amount of CPU time but can reduce the size of the heap significantly, especially on 32-bit architectures.
- Due to tighter coupling of the run-time and network libraries, fewer context switches are required on network operations.

## Changes to the standard library

### bufio.Scanner

The various routines to scan textual input in the `bufio` package, `ReadBytes`, `ReadString` and particularly `ReadLine`, are needlessly complex to use for simple purposes. In Go 1.1, a new type, `Scanner`, has been added to make it easier to do simple tasks such as read the input as a sequence of lines or space-delimited words. It simplifies the problem by terminating the scan on problematic input such as pathologically long lines, and having a simple default: line-oriented input, with each line stripped of its terminator. Here is code to reproduce the input a line at a time:

    scanner := bufio.NewScanner(os.Stdin)
    for scanner.Scan() {
        fmt.Println(scanner.Text()) // Println will add back the final '\n'
    }
    if err := scanner.Err(); err != nil {
        fmt.Fprintln(os.Stderr, "reading standard input:", err)
    }

Scanning behavior can be adjusted through a function to control subdividing the input (see the documentation for `SplitFunc`), but for tough problems or the need to continue past errors, the older interface may still be required.

### net

The protocol-specific resolvers in the `net` package were formerly lax about the network name passed in. Although the documentation was clear that the only valid networks for `ResolveTCPAddr` are `"tcp"`, `"tcp4"`, and `"tcp6"`, the Go 1.0 implementation silently accepted any string. The Go 1.1 implementation returns an error if the network is not one of those strings. The same is true of the other protocol-specific resolvers `ResolveIPAddr`, `ResolveUDPAddr`, and `ResolveUnixAddr`.

The previous implementation of `ListenUnixgram` returned a `UDPConn` as a representation of the connection endpoint. The Go 1.1 implementation instead returns a `UnixConn` to allow reading and writing with its `ReadFrom` and `WriteTo` methods.

The data structures `IPAddr`, `TCPAddr`, and `UDPAddr` add a new string field called `Zone`. Code using untagged composite literals (e.g. `net.TCPAddr{ip, port}`) instead of tagged literals (`net.TCPAddr{IP: ip, Port: port}`) will break due to the new field. The Go 1 compatibility rules allow this change: client code must use tagged literals to avoid such breakages.

Updating: To correct breakage caused by the new struct field, `go fix` will rewrite code to add tags for these types. More generally, `go vet` will identify composite literals that should be revised to use field tags.

### reflect

The `reflect` package has several significant additions.

It is now possible to run a "select" statement using the `reflect` package; see the description of `Select` and `SelectCase` for details.

The new method `Value.Convert` (or `Type.ConvertibleTo`) provides functionality to execute a Go conversion or type assertion operation on a `Value` (or test for its possibility).

The new function `MakeFunc` creates a wrapper function to make it easier to call a function with existing `Values`, doing the standard Go conversions among the arguments, for instance to pass an actual `int` to a formal `interface{}`.

Finally, the new functions `ChanOf`, `MapOf` and `SliceOf` construct new `Types` from existing types, for example to construct the type `[]T` given only `T`.

### time

On FreeBSD, Linux, NetBSD, OS X and OpenBSD, previous versions of the `time` package returned times with microsecond precision. The Go 1.1 implementation on these systems now returns times with nanosecond precision. Programs that write to an external format with microsecond precision and read it back, expecting to recover the original value, will be affected by the loss of precision. There are two new methods of `Time`, `Round` and `Truncate`, that can be used to remove precision from a time before passing it to external storage.

The new method `YearDay` returns the one-indexed integral day number of the year specified by the time value.

The `Timer` type has a new method `Reset` that modifies the timer to expire after a specified duration.

Finally, the new function `ParseInLocation` is like the existing `Parse` but parses the time in the context of a location (time zone), ignoring time zone information in the parsed string. This function addresses a common source of confusion in the time API.

Updating: Code that needs to read and write times using an external format with lower precision should be modified to use the new methods.

### Exp and old subtrees moved to go.exp and go.text subrepositories

To make it easier for binary distributions to access them if desired, the `exp` and `old` source subtrees, which are not included in binary distributions, have been moved to the new `go.exp` subrepository at `code.google.com/p/go.exp`. To access the `ssa` package, for example, run

    $ go get code.google.com/p/go.exp/ssa

and then in Go source,

    import "code.google.com/p/go.exp/ssa"

The old package `exp/norm` has also been moved, but to a new repository `go.text`, where the Unicode APIs and other text-related packages will be developed.

### New packages

There are three new packages.

- The `go/format` package provides a convenient way for a program to access the formatting capabilities of the `go fmt` command. It has two functions, `Node` to format a Go parser `Node`, and `Source` to reformat arbitrary Go source code into the standard format as provided by the `go fmt` command.
- The `net/http/cookiejar` package provides the basics for managing HTTP cookies.
- The `runtime/race` package provides low-level facilities for data race detection. It is internal to the race detector and does not otherwise export any user-visible functionality.

### Minor changes to the library

The following list summarizes a number of minor changes to the library, mostly additions. See the relevant package documentation for more information about each change.

- The `bytes` package has two new functions, `TrimPrefix` and `TrimSuffix`, with self-evident properties. Also, the `Buffer` type has a new method `Grow` that provides some control over memory allocation inside the buffer. Finally, the `Reader` type now has a `WriteTo` method so it implements the `io.WriterTo` interface.
- The `compress/gzip` package has a new `Flush` method for its `Writer` type that flushes its underlying `flate.Writer`.
- The `crypto/hmac` package has a new function, `Equal`, to compare two MACs.
- The `crypto/x509` package now supports PEM blocks (see `DecryptPEMBlock` for instance), and a new function `ParseECPrivateKey` to parse elliptic curve private keys.
- The `database/sql` package has a new `Ping` method for its `DB` type that tests the health of the connection.
- The `database/sql/driver` package has a new `Queryer` interface that a `Conn` may implement to improve performance.
- The `encoding/json` package's `Decoder` has a new method `Buffered` to provide access to the remaining data in its buffer, as well as a new method `UseNumber` to unmarshal a value into the new type `Number`, a string, rather than a float64.
- The `encoding/xml` package has a new function, `EscapeText`, which writes escaped XML output, and a method on `Encoder`, `Indent`, to specify indented output.
- In the `go/ast` package, a new type `CommentMap` and associated methods makes it easier to extract and process comments in Go programs.
- In the `go/doc` package, the parser now keeps better track of stylized annotations such as `TODO(joe)` throughout the code, information that the `godoc` command can filter or present according to the value of the `-notes` flag.
- The undocumented and only partially implemented "noescape" feature of the `html/template` package has been removed; programs that depend on it will break.
- The `image/jpeg` package now reads progressive JPEG files and handles a few more subsampling configurations.
- The `io` package now exports the `io.ByteWriter` interface to capture the common functionality of writing a byte at a time. It also exports a new error, `ErrNoProgress`, used to indicate a `Read` implementation is looping without delivering data.
- The `log/syslog` package now provides better support for OS-specific logging features.
- The `math/big` package's `Int` type now has methods `MarshalJSON` and `UnmarshalJSON` to convert to and from a JSON representation. Also, `Int` can now convert directly to and from a `uint64` using `Uint64` and `SetUint64`, while `Rat` can do the same with `float64` using `Float64` and `SetFloat64`.
- The `mime/multipart` package has a new method for its `Writer`, `SetBoundary`, to define the boundary separator used to package the output. The `Reader` also now transparently decodes any `quoted-printable` parts and removes the `Content-Transfer-Encoding` header when doing so.
- The `net` package's `ListenUnixgram` function has changed return types: it now returns a `UnixConn` rather than a `UDPConn`, which was clearly a mistake in Go 1.0. Since this API change fixes a bug, it is permitted by the Go 1 compatibility rules.
- The `net` package includes a new type, `Dialer`, to supply options to `Dial`.
- The `net` package adds support for link-local IPv6 addresses with zone qualifiers, such as `fe80::1%lo0`. The address structures `IPAddr`, `UDPAddr`, and `TCPAddr` record the zone in a new field, and functions that expect string forms of these addresses, such as `Dial`, `ResolveIPAddr`, `ResolveUDPAddr`, and `ResolveTCPAddr`, now accept the zone-qualified form.
- The `net` package adds `LookupNS` to its suite of resolving functions. `LookupNS` returns the NS records for a host name.
- The `net` package adds protocol-specific packet reading and writing methods to `IPConn` (`ReadMsgIP` and `WriteMsgIP`) and `UDPConn` (`ReadMsgUDP` and `WriteMsgUDP`). These are specialized versions of `PacketConn`'s `ReadFrom` and `WriteTo` methods that provide access to out-of-band data associated with the packets.
- The `net` package adds methods to `UnixConn` to allow closing half of the connection (`CloseRead` and `CloseWrite`), matching the existing methods of `TCPConn`.
- The `net/http` package includes several new additions. `ParseTime` parses a time string, trying several common HTTP time formats. The `PostFormValue` method of `Request` is like `FormValue` but ignores URL parameters. The `CloseNotifier` interface provides a mechanism for a server handler to discover when a client has disconnected. The `ServeMux` type now has a `Handler` method to access a path's `Handler` without executing it. The `Transport` can now cancel an in-flight request with `CancelRequest`. Finally, the Transport is now more aggressive at closing TCP connections when a `Response.Body` is closed before being fully consumed.
- The `net/mail` package has two new functions, `ParseAddress` and `ParseAddressList`, to parse RFC 5322-formatted mail addresses into `Address` structures.
- The `net/smtp` package's `Client` type has a new method, `Hello`, which transmits a `HELO` or `EHLO` message to the server.
- The `net/textproto` package has two new functions, `TrimBytes` and `TrimString`, which do ASCII-only trimming of leading and trailing spaces.
- The new method `os.FileMode.IsRegular` makes it easy to ask if a file is a plain file.
- The `os/signal` package has a new function, `Stop`, which stops the package delivering any further signals to the channel.
- The `regexp` package now supports Unix-original leftmost-longest matches through the `Regexp.Longest` method, while `Regexp.Split` slices strings into pieces based on separators defined by the regular expression.
- The `runtime/debug` package has three new functions regarding memory usage. The `FreeOSMemory` function triggers a run of the garbage collector and then attempts to return unused memory to the operating system; the `ReadGCStats` function retrieves statistics about the collector; and `SetGCPercent` provides a programmatic way to control how often the collector runs, including disabling it altogether.
- The `sort` package has a new function, `Reverse`. Wrapping the argument of a call to `sort.Sort` with a call to `Reverse` causes the sort order to be reversed.
- The `strings` package has two new functions, `TrimPrefix` and `TrimSuffix` with self-evident properties, and the new method `Reader.WriteTo` so the `Reader` type now implements the `io.WriterTo` interface.
- The `syscall` package's `Fchflags` function on various BSDs (including Darwin) has changed signature. It now takes an int as the first parameter instead of a string. Since this API change fixes a bug, it is permitted by the Go 1 compatibility rules.
- The `syscall` package also has received many updates to make it more inclusive of constants and system calls for each supported operating system.
- The `testing` package now automates the generation of allocation statistics in tests and benchmarks using the new `AllocsPerRun` function. And the `ReportAllocs` method on `testing.B` will enable printing of memory allocation statistics for the calling benchmark. It also introduces the `AllocsPerOp` method of `BenchmarkResult`. There is also a new `Verbose` function to test the state of the `-v` command-line flag, and a new `Skip` method of `testing.B` and `testing.T` to simplify skipping an inappropriate test.
- In the `text/template` and `html/template` packages, templates can now use parentheses to group the elements of pipelines, simplifying the construction of complex pipelines. Also, as part of the new parser, the `Node` interface got two new methods to provide better error reporting. Although this violates the Go 1 compatibility rules, no existing code should be affected because this interface is explicitly intended only to be used by the `text/template` and `html/template` packages and there are safeguards to guarantee that.
- The implementation of the `unicode` package has been updated to Unicode version 6.2.0.
- In the `unicode/utf8` package, the new function `ValidRune` reports whether the rune is a valid Unicode code point. To be valid, a rune must be in range and not be a surrogate half.