Go 1.7 Release Notes

Introduction to Go 1.7

The latest Go release, version 1.7, arrives six months after 1.6. Most of its changes are in the implementation of the toolchain, runtime, and libraries. There is one minor change to the language specification. As always, the release maintains the Go 1 promise of compatibility. We expect almost all Go programs to continue to compile and run as before.

The release adds a port to IBM LinuxOne; updates the x86-64 compiler back end to generate more efficient code; includes the context package, promoted from the x/net subrepository and now used in the standard library; and adds support in the testing package for creating hierarchies of tests and benchmarks. The release also finalizes the vendoring support started in Go 1.5, making it a standard feature.

Changes to the language

There is one tiny language change in this release. The section on terminating statements clarifies that to determine whether a statement list ends in a terminating statement, the “final non-empty statement” is considered the end, matching the existing behavior of the gc and gccgo compiler toolchains. In earlier releases the definition referred only to the “final statement,” leaving the effect of trailing empty statements at the least unclear. The go/types package has been updated to match the gc and gccgo compiler toolchains in this respect. This change has no effect on the correctness of existing programs.


Go 1.7 adds support for macOS 10.12 Sierra. Binaries built with versions of Go before 1.7 will not work correctly on Sierra.

Go 1.7 adds an experimental port to Linux on z Systems (linux/s390x) and the beginning of a port to Plan 9 on ARM (plan9/arm).

The experimental ports to Linux on 64-bit MIPS (linux/mips64 and linux/mips64le) added in Go 1.6 now have full support for cgo and external linking.

The experimental port to Linux on little-endian 64-bit PowerPC (linux/ppc64le) now requires the POWER8 architecture or later. Big-endian 64-bit PowerPC (linux/ppc64) only requires the POWER5 architecture.

The OpenBSD port now requires OpenBSD 5.6 or later, for access to the getentropy(2) system call.

Known Issues

There are some instabilities on FreeBSD that are known but not understood. These can lead to program crashes in rare cases. See issue 16136, issue 15658, and issue 16396. Any help in solving these FreeBSD-specific issues would be appreciated.



For 64-bit ARM systems, the vector register names have been corrected to V0 through V31; previous releases incorrectly referred to them as V32 through V63.


Compiler Toolchain

This release includes a new code generation back end for 64-bit x86 systems, following a proposal from 2015 that has been under development since then. The new back end, based on SSA, generates more compact, more efficient code and provides a better platform for optimizations such as bounds check elimination. The new back end reduces the CPU time required by our benchmark programs by 5-35%.

For this release, the new back end can be disabled by passing -ssa=0 to the compiler. If you find that your program compiles or runs successfully only with the new back end disabled, please file a bug report.

The format of exported metadata written by the compiler in package archives has changed: the old textual format has been replaced by a more compact binary format. This results in somewhat smaller package archives and fixes a few long-standing corner case bugs.

For this release, the new export format can be disabled by passing -newexport=0 to the compiler. If you find that your program compiles or runs successfully only with the new export format disabled, please file a bug report.

The linker’s -X option no longer supports the unusual two-argument form -X name value, as announced in the Go 1.6 release and in warnings printed by the linker. Use -X name=value instead.

The compiler and linker have been optimized and run significantly faster in this release than in Go 1.6, although they are still slower than we would like and will continue to be optimized in future releases.

Due to changes across the compiler toolchain and standard library, binaries built with this release should typically be smaller than binaries built with Go 1.6, sometimes by as much as 20-30%.

On x86-64 systems, Go programs now maintain stack frame pointers as expected by profiling tools like Linux’s perf and Intel’s VTune, making it easier to analyze and optimize Go programs using these tools. The frame pointer maintenance has a small run-time overhead that varies but averages around 2%. We hope to reduce this cost in future releases. To build a toolchain that does not use frame pointers, set GOEXPERIMENT=noframepointer when running make.bash, make.bat, or make.rc.


Packages using cgo may now include Fortran source files (in addition to C, C++, Objective C, and SWIG), although the Go bindings must still use C language APIs.

Go bindings may now use a new helper function C.CBytes. In contrast to C.CString, which takes a Go string and returns a *C.byte (a C char*), C.CBytes takes a Go []byte and returns an unsafe.Pointer (a C void*).

Packages and binaries built using cgo have in past releases produced different output on each build, due to the embedding of temporary directory names. When using this release with new enough versions of GCC or Clang (those that support the -fdebug-prefix-map option), those builds should finally be deterministic.


Due to the alignment of Go’s semiannual release schedule with GCC’s annual release schedule, GCC release 6 contains the Go 1.6.1 version of gccgo. The next release, GCC 7, will likely have the Go 1.8 version of gccgo.

Go command

The go command’s basic operation is unchanged, but there are a number of changes worth noting.

This release removes support for the GO15VENDOREXPERIMENT environment variable, as announced in the Go 1.6 release. Vendoring support is now a standard feature of the go command and toolchain.

The Package data structure made available to “go list” now includes a StaleReason field explaining why a particular package is or is not considered stale (in need of rebuilding). This field is available to the -f or -json options and is useful for understanding why a target is being rebuilt.

The “go get” command now supports import paths referring to git.openstack.org.

This release adds experimental, minimal support for building programs using binary-only packages, packages distributed in binary form without the corresponding source code. This feature is needed in some commercial settings but is not intended to be fully integrated into the rest of the toolchain. For example, tools that assume access to complete source code will not work with such packages, and there are no plans to support such packages in the “go get” command.

Go doc

The “go doc” command now groups constructors with the type they construct, following godoc.

Go vet

The “go vet” command has more accurate analysis in its -copylock and -printf checks, and a new -tests check that checks the name and signature of likely test functions. To avoid confusion with the new -tests check, the old, unadvertised -test option has been removed; it was equivalent to -all -shadow.

The vet command also has a new check, -lostcancel, which detects failure to call the cancellation function returned by the WithCancel, WithTimeout, and WithDeadline functions in Go 1.7’s new context package (see below). Failure to call the function prevents the new Context from being reclaimed until its parent is canceled. (The background context is never canceled.)

Go tool dist

The new subcommand “go tool dist list” prints all supported operating system/architecture pairs.

Go tool trace

The “go tool trace” command, introduced in Go 1.5, has been refined in various ways.

First, collecting traces is significantly more efficient than in past releases. In this release, the typical execution-time overhead of collecting a trace is about 25%; in past releases it was at least 400%. Second, trace files now include file and line number information, making them more self-contained and making the original executable optional when running the trace tool. Third, the trace tool now breaks up large traces to avoid limits in the browser-based viewer.

Although the trace file format has changed in this release, the Go 1.7 tools can still read traces from earlier releases.


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 and optimizations in the core library. On x86-64 systems, many programs will run significantly faster, due to improvements in generated code brought by the new compiler back end. As noted above, in our own benchmarks, the code generation changes alone typically reduce program CPU time by 5-35%.

There have been significant optimizations bringing more than 10% improvements to implementations in the crypto/sha1, crypto/sha256, encoding/binary, fmt, hash/adler32, hash/crc32, hash/crc64, image/color, math/big, strconv, strings, unicode, and unicode/utf16 packages.

Garbage collection pauses should be significantly shorter than they were in Go 1.6 for programs with large numbers of idle goroutines, substantial stack size fluctuation, or large package-level variables.

Core library


Go 1.7 moves the golang.org/x/net/context package into the standard library as context. This allows the use of contexts for cancellation, timeouts, and passing request-scoped data in other standard library packages, including net, net/http, and os/exec, as noted below.

For more information about contexts, see the package documentation and the Go blog post “Go Concurrent Patterns: Context.”

HTTP Tracing

Go 1.7 introduces net/http/httptrace, a package that provides mechanisms for tracing events within HTTP requests.


The testing package now supports the definition of tests with subtests and benchmarks with sub-benchmarks. This support makes it easy to write table-driven benchmarks and to create hierarchical tests. It also provides a way to share common setup and tear-down code. See the package documentation for details.


All panics started by the runtime now use panic values that implement both the builtin error, and runtime.Error, as required by the language specification.

During panics, if a signal’s name is known, it will be printed in the stack trace. Otherwise, the signal’s number will be used, as it was before Go1.7.

The new function KeepAlive provides an explicit mechanism for declaring that an allocated object must be considered reachable at a particular point in a program, typically to delay the execution of an associated finalizer.

The new function CallersFrames translates a PC slice obtained from Callers into a sequence of frames corresponding to the call stack. This new API should be preferred instead of direct use of FuncForPC, because the frame sequence can more accurately describe call stacks with inlined function calls.

The new function SetCgoTraceback facilitates tighter integration between Go and C code executing in the same process called using cgo.

On 32-bit systems, the runtime can now use memory allocated by the operating system anywhere in the address space, eliminating the “memory allocated by OS not in usable range” failure common in some environments.

The runtime can now return unused memory to the operating system on all architectures. In Go 1.6 and earlier, the runtime could not release memory on ARM64, 64-bit PowerPC, or MIPS.

On Windows, Go programs in Go 1.5 and earlier forced the global Windows timer resolution to 1ms at startup by calling timeBeginPeriod(1). Changing the global timer resolution caused problems on some systems, and testing suggested that the call was not needed for good scheduler performance, so Go 1.6 removed the call. Go 1.7 brings the call back: under some workloads the call is still needed for good scheduler performance.

Minor changes to the library

As always, there are various minor changes and updates to the library, made with the Go 1 promise of compatibility in mind.


In previous releases of Go, if Reader’s Peek method were asked for more bytes than fit in the underlying buffer, it would return an empty slice and the error ErrBufferFull. Now it returns the entire underlying buffer, still accompanied by the error ErrBufferFull.


The new functions ContainsAny and ContainsRune have been added for symmetry with the strings package.

In previous releases of Go, if Reader’s Read method were asked for zero bytes with no data remaining, it would return a count of 0 and no error. Now it returns a count of 0 and the error io.EOF.

The Reader type has a new method Reset to allow reuse of a Reader.


There are many performance optimizations throughout the package. Decompression speed is improved by about 10%, while compression for DefaultCompression is twice as fast.

In addition to those general improvements, the BestSpeed compressor has been replaced entirely and uses an algorithm similar to Snappy, resulting in about a 2.5X speed increase, although the output can be 5-10% larger than with the previous algorithm.

There is also a new compression level HuffmanOnly that applies Huffman but not Lempel-Ziv encoding. Forgoing Lempel-Ziv encoding means that HuffmanOnly runs about 3X faster than the new BestSpeed but at the cost of producing compressed outputs that are 20-40% larger than those generated by the new BestSpeed.

It is important to note that both BestSpeed and HuffmanOnly produce a compressed output that is RFC 1951 compliant. In other words, any valid DEFLATE decompressor will continue to be able to decompress these outputs.

Lastly, there is a minor change to the decompressor’s implementation of io.Reader. In previous versions, the decompressor deferred reporting io.EOF until exactly no more bytes could be read. Now, it reports io.EOF more eagerly when reading the last set of bytes.


The TLS implementation sends the first few data packets on each connection using small record sizes, gradually increasing to the TLS maximum record size. This heuristic reduces the amount of data that must be received before the first packet can be decrypted, improving communication latency over low-bandwidth networks. Setting Config’s DynamicRecordSizingDisabled field to true forces the behavior of Go 1.6 and earlier, where packets are as large as possible from the start of the connection.

The TLS client now has optional, limited support for server-initiated renegotiation, enabled by setting the Config’s Renegotiation field. This is needed for connecting to many Microsoft Azure servers.

The errors returned by the package now consistently begin with a tls: prefix. In past releases, some errors used a crypto/tls: prefix, some used a tls: prefix, and some had no prefix at all.

When generating self-signed certificates, the package no longer sets the “Authority Key Identifier” field by default.


The new function SystemCertPool provides access to the entire system certificate pool if available. There is also a new associated error type SystemRootsError.


The Reader type’s new SeekPC method and the Data type’s new Ranges method help to find the compilation unit to pass to a LineReader and to identify the specific function for a given program counter.


The new R_390 relocation type and its many predefined constants support the S390 port.


The ASN.1 decoder now rejects non-minimal integer encodings. This may cause the package to reject some invalid but formerly accepted ASN.1 data.


The Encoder’s new SetIndent method sets the indentation parameters for JSON encoding, like in the top-level Indent function.

The Encoder’s new SetEscapeHTML method controls whether the &, <, and > characters in quoted strings should be escaped as \u0026, \u003c, and \u003e, respectively. As in previous releases, the encoder defaults to applying this escaping, to avoid certain problems that can arise when embedding JSON in HTML.

In earlier versions of Go, this package only supported encoding and decoding maps using keys with string types. Go 1.7 adds support for maps using keys with integer types: the encoding uses a quoted decimal representation as the JSON key. Go 1.7 also adds support for encoding maps using non-string keys that implement the MarshalText (see encoding.TextMarshaler) method, as well as support for decoding maps using non-string keys that implement the UnmarshalText (see encoding.TextUnmarshaler) method. These methods are ignored for keys with string types in order to preserve the encoding and decoding used in earlier versions of Go.

When encoding a slice of typed bytes, Marshal now generates an array of elements encoded using that byte type’s MarshalJSON or MarshalText method if present, only falling back to the default base64-encoded string data if neither method is available. Earlier versions of Go accept both the original base64-encoded string encoding and the array encoding (assuming the byte type also implements UnmarshalJSON or UnmarshalText as appropriate), so this change should be semantically backwards compatible with earlier versions of Go, even though it does change the chosen encoding.


To implement the go command’s new support for binary-only packages and for Fortran code in cgo-based packages, the Package type adds new fields BinaryOnly, CgoFFLAGS, and FFiles.


To support the corresponding change in go test described above, Example struct adds an Unordered field indicating whether the example may generate its output lines in any order.


The package adds new constants SeekStart, SeekCurrent, and SeekEnd, for use with Seeker implementations. These constants are preferred over os.SEEK_SET, os.SEEK_CUR, and os.SEEK_END, but the latter will be preserved for compatibility.


The Float type adds GobEncode and GobDecode methods, so that values of type Float can now be encoded and decoded using the encoding/gob package.


The Read function and Rand’s Read method now produce a pseudo-random stream of bytes that is consistent and not dependent on the size of the input buffer.

The documentation clarifies that Rand’s Seed and Read methods are not safe to call concurrently, though the global functions Seed and Read are (and have always been) safe.


The Writer implementation now emits each multipart section’s header sorted by key. Previously, iteration over a map caused the section header to use a non-deterministic order.


As part of the introduction of context, the Dialer type has a new method DialContext, like Dial but adding the context.Context for the dial operation. The context is intended to obsolete the Dialer’s Cancel and Deadline fields, but the implementation continues to respect them, for backwards compatibility.

The IP type’s String method has changed its result for invalid IP addresses. In past releases, if an IP byte slice had length other than 0, 4, or 16, String returned "?". Go 1.7 adds the hexadecimal encoding of the bytes, as in "?12ab".

The pure Go name resolution implementation now respects nsswitch.conf’s stated preference for the priority of DNS lookups compared to local file (that is, /etc/hosts) lookups.


ResponseWriter’s documentation now makes clear that beginning to write the response may prevent future reads on the request body. For maximal compatibility, implementations are encouraged to read the request body completely before writing any part of the response.

As part of the introduction of context, the Request has a new methods Context, to retrieve the associated context, and WithContext, to construct a copy of Request with a modified context.

In the Server implementation, Serve records in the request context both the underlying *Server using the key ServerContextKey and the local address on which the request was received (a Addr) using the key LocalAddrContextKey. For example, the address on which a request received is req.Context().Value(http.LocalAddrContextKey).(net.Addr).

The server’s Serve method now only enables HTTP/2 support if the Server.TLSConfig field is nil or includes "h2" in its TLSConfig.NextProtos.

The server implementation now pads response codes less than 100 to three digits as required by the protocol, so that w.WriteHeader(5) uses the HTTP response status 005, not just 5.

The server implementation now correctly sends only one “Transfer-Encoding” header when “chunked” is set explicitly, following RFC 7230.

The server implementation is now stricter about rejecting requests with invalid HTTP versions. Invalid requests claiming to be HTTP/0.x are now rejected (HTTP/0.9 was never fully supported), and plaintext HTTP/2 requests other than the “PRI * HTTP/2.0” upgrade request are now rejected as well. The server continues to handle encrypted HTTP/2 requests.

In the server, a 200 status code is sent back by the timeout handler on an empty response body, instead of sending back 0 as the status code.

In the client, the Transport implementation passes the request context to any dial operation connecting to the remote server. If a custom dialer is needed, the new Transport field DialContext is preferred over the existing Dial field, to allow the transport to supply a context.

The Transport also adds fields IdleConnTimeout, MaxIdleConns, and MaxResponseHeaderBytes to help control client resources consumed by idle or chatty servers.

A Client’s configured CheckRedirect function can now return ErrUseLastResponse to indicate that the most recent redirect response should be returned as the result of the HTTP request. That response is now available to the CheckRedirect function as req.Response.

Since Go 1, the default behavior of the HTTP client is to request server-side compression using the Accept-Encoding request header and then to decompress the response body transparently, and this behavior is adjustable using the Transport’s DisableCompression field. In Go 1.7, to aid the implementation of HTTP proxies, the Response’s new Uncompressed field reports whether this transparent decompression took place.

DetectContentType adds support for a few new audio and video content types.


The Handler adds a new field Stderr that allows redirection of the child process’s standard error away from the host process’s standard error.


The new function NewRequest prepares a new http.Request suitable for passing to an http.Handler during a test.

The ResponseRecorder’s new Result method returns the recorded http.Response. Tests that need to check the response’s headers or trailers should call Result and inspect the response fields instead of accessing ResponseRecorder’s HeaderMap directly.


The ReverseProxy implementation now responds with “502 Bad Gateway” when it cannot reach a back end; in earlier releases it responded with “500 Internal Server Error.”

Both ClientConn and ServerConn have been documented as deprecated. They are low-level, old, and unused by Go’s current HTTP stack and will no longer be updated. Programs should use http.Client, http.Transport, and http.Server instead.


The runtime trace HTTP handler, installed to handle the path /debug/pprof/trace, now accepts a fractional number in its seconds query parameter, allowing collection of traces for intervals smaller than one second. This is especially useful on busy servers.


The address parser now allows unescaped UTF-8 text in addresses following RFC 6532, but it does not apply any normalization to the result. For compatibility with older mail parsers, the address encoder, namely Address’s String method, continues to escape all UTF-8 text following RFC 5322.

The ParseAddress function and the AddressParser.Parse method are stricter. They used to ignore any characters following an e-mail address, but will now return an error for anything other than whitespace.


The URL’s new ForceQuery field records whether the URL must have a query string, in order to distinguish URLs without query strings (like /search) from URLs with empty query strings (like /search?).


IsExist now returns true for syscall.ENOTEMPTY, on systems where that error exists.

On Windows, Remove now removes read-only files when possible, making the implementation behave as on non-Windows systems.


As part of the introduction of context, the new constructor CommandContext is like Command but includes a context that can be used to cancel the command execution.


The Current function is now implemented even when cgo is not available.

The new Group type, along with the lookup functions LookupGroup and LookupGroupId and the new field GroupIds in the User struct, provides access to system-specific user group information.


Although Value’s Field method has always been documented to panic if the given field number i is out of range, it has instead silently returned a zero Value. Go 1.7 changes the method to behave as documented.

The new StructOf function constructs a struct type at run time. It completes the set of type constructors, joining ArrayOf, ChanOf, FuncOf, MapOf, PtrTo, and SliceOf.

StructTag’s new method Lookup is like Get but distinguishes the tag not containing the given key from the tag associating an empty string with the given key.

The Method and NumMethod methods of Type and Value no longer return or count unexported methods.


In previous releases of Go, if Reader’s Read method were asked for zero bytes with no data remaining, it would return a count of 0 and no error. Now it returns a count of 0 and the error io.EOF.

The Reader type has a new method Reset to allow reuse of a Reader.


Duration’s time.Duration.String method now reports the zero duration as "0s", not "0". ParseDuration continues to accept both forms.

The method call time.Local.String() now returns "Local" on all systems; in earlier releases, it returned an empty string on Windows.

The time zone database in $GOROOT/lib/time has been updated to IANA release 2016d. This fallback database is only used when the system time zone database cannot be found, for example on Windows. The Windows time zone abbreviation list has also been updated.


On Linux, the SysProcAttr struct (as used in os/exec.Cmd’s SysProcAttr field) has a new Unshareflags field. If the field is nonzero, the child process created by ForkExec (as used in exec.Cmd’s Run method) will call the unshare(2) system call before executing the new program.


The unicode package and associated support throughout the system has been upgraded from version 8.0 to Unicode 9.0.