Go 1.12 Release Notes

Introduction to Go 1.12

The latest Go release, version 1.12, arrives six months after Go 1.11. Most of its changes are in the implementation of the toolchain, runtime, and libraries. 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.

Changes to the language

There are no changes to the language specification.


The race detector is now supported on linux/arm64.

Go 1.12 is the last release that is supported on FreeBSD 10.x, which has already reached end-of-life. Go 1.13 will require FreeBSD 11.2+ or FreeBSD 12.0+. FreeBSD 12.0+ requires a kernel with the COMPAT_FREEBSD11 option set (this is the default).

cgo is now supported on linux/ppc64.

hurd is now a recognized value for GOOS, reserved for the GNU/Hurd system for use with gccgo.


Go’s new windows/arm port supports running Go on Windows 10 IoT Core on 32-bit ARM chips such as the Raspberry Pi 3.


Go now supports AIX 7.2 and later on POWER8 architectures (aix/ppc64). External linking, cgo, pprof and the race detector aren’t yet supported.


Go 1.12 is the last release that will run on macOS 10.10 Yosemite. Go 1.13 will require macOS 10.11 El Capitan or later.

libSystem is now used when making syscalls on Darwin, ensuring forward-compatibility with future versions of macOS and iOS. The switch to libSystem triggered additional App Store checks for private API usage. Since it is considered private, syscall.Getdirentries now always fails with ENOSYS on iOS. Additionally, syscall.Setrlimit reports invalid argument in places where it historically succeeded. These consequences are not specific to Go and users should expect behavioral parity with libSystem’s implementation going forward.


go tool vet no longer supported

The go vet command has been rewritten to serve as the base for a range of different source code analysis tools. See the golang.org/x/tools/go/analysis package for details. A side-effect is that go tool vet is no longer supported. External tools that use go tool vet must be changed to use go vet. Using go vet instead of go tool vet should work with all supported versions of Go.

As part of this change, the experimental -shadow option is no longer available with go vet. Checking for variable shadowing may now be done using

go get -u golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
go vet -vettool=$(which shadow)


The Go tour is no longer included in the main binary distribution. To run the tour locally, instead of running go tool tour, manually install it:

go get -u golang.org/x/tour

Build cache requirement

The build cache is now required as a step toward eliminating $GOPATH/pkg. Setting the environment variable GOCACHE=off will cause go commands that write to the cache to fail.

Binary-only packages

Go 1.12 is the last release that will support binary-only packages.


Go 1.12 will translate the C type EGLDisplay to the Go type uintptr. This change is similar to how Go 1.10 and newer treats Darwin’s CoreFoundation and Java’s JNI types. See the cgo documentation for more information.

Mangled C names are no longer accepted in packages that use Cgo. Use the Cgo names instead. For example, use the documented cgo name C.char rather than the mangled name _Ctype_char that cgo generates.


When GO111MODULE is set to on, the go command now supports module-aware operations outside of a module directory, provided that those operations do not need to resolve import paths relative to the current directory or explicitly edit the go.mod file. Commands such as go get, go list, and go mod download behave as if in a module with initially-empty requirements. In this mode, go env GOMOD reports the system’s null device (/dev/null or NUL).

go commands that download and extract modules are now safe to invoke concurrently. The module cache (GOPATH/pkg/mod) must reside in a filesystem that supports file locking.

The go directive in a go.mod file now indicates the version of the language used by the files within that module. It will be set to the current release (go 1.12) if no existing version is present. If the go directive for a module specifies a version newer than the toolchain in use, the go command will attempt to build the packages regardless, and will note the mismatch only if that build fails.

This changed use of the go directive means that if you use Go 1.12 to build a module, thus recording go 1.12 in the go.mod file, you will get an error when attempting to build the same module with Go 1.11 through Go 1.11.3. Go 1.11.4 or later will work fine, as will releases older than Go 1.11. If you must use Go 1.11 through 1.11.3, you can avoid the problem by setting the language version to 1.11, using the Go 1.12 go tool, via go mod edit -go=1.11.

When an import cannot be resolved using the active modules, the go command will now try to use the modules mentioned in the main module’s replace directives before consulting the module cache and the usual network sources. If a matching replacement is found but the replace directive does not specify a version, the go command uses a pseudo-version derived from the zero time.Time (such as v0.0.0-00010101000000-000000000000).

Compiler toolchain

The compiler’s live variable analysis has improved. This may mean that finalizers will be executed sooner in this release than in previous releases. If that is a problem, consider the appropriate addition of a runtime.KeepAlive call.

More functions are now eligible for inlining by default, including functions that do nothing but call another function. This extra inlining makes it additionally important to use runtime.CallersFrames instead of iterating over the result of runtime.Callers directly.

// Old code which no longer works correctly (it will miss inlined call frames).
var pcs [10]uintptr
n := runtime.Callers(1, pcs[:])
for _, pc := range pcs[:n] {
    f := runtime.FuncForPC(pc)
    if f != nil {
// New code which will work correctly.
var pcs [10]uintptr
n := runtime.Callers(1, pcs[:])
frames := runtime.CallersFrames(pcs[:n])
for {
    frame, more := frames.Next()
    if !more {

Wrappers generated by the compiler to implement method expressions are no longer reported by runtime.CallersFrames and runtime.Stack. They are also not printed in panic stack traces. This change aligns the gc toolchain to match the gccgo toolchain, which already elided such wrappers from stack traces. Clients of these APIs might need to adjust for the missing frames. For code that must interoperate between 1.11 and 1.12 releases, you can replace the method expression x.M with the function literal func (...) { x.M(...) } .

The compiler now accepts a -lang flag to set the Go language version to use. For example, -lang=go1.8 causes the compiler to emit an error if the program uses type aliases, which were added in Go 1.9. Language changes made before Go 1.12 are not consistently enforced.

The compiler toolchain now uses different conventions to call Go functions and assembly functions. This should be invisible to users, except for calls that simultaneously cross between Go and assembly and cross a package boundary. If linking results in an error like “relocation target not defined for ABIInternal (but is defined for ABI0)”, please refer to the compatibility section of the ABI design document.

There have been many improvements to the DWARF debug information produced by the compiler, including improvements to argument printing and variable location information.

Go programs now also maintain stack frame pointers on linux/arm64 for the benefit of profiling tools like perf. The frame pointer maintenance has a small run-time overhead that varies but averages around 3%. To build a toolchain that does not use frame pointers, set GOEXPERIMENT=noframepointer when running make.bash.

The obsolete “safe” compiler mode (enabled by the -u gcflag) has been removed.

godoc and go doc

In Go 1.12, godoc no longer has a command-line interface and is only a web server. Users should use go doc for command-line help output instead. Go 1.12 is the last release that will include the godoc webserver; in Go 1.13 it will be available via go get.

go doc now supports the -all flag, which will cause it to print all exported APIs and their documentation, as the godoc command line used to do.

go doc also now includes the -src flag, which will show the target’s source code.


The trace tool now supports plotting mutator utilization curves, including cross-references to the execution trace. These are useful for analyzing the impact of the garbage collector on application latency and throughput.


On arm64, the platform register was renamed from R18 to R18_PLATFORM to prevent accidental use, as the OS could choose to reserve this register.


Go 1.12 significantly improves the performance of sweeping when a large fraction of the heap remains live. This reduces allocation latency immediately following a garbage collection.

The Go runtime now releases memory back to the operating system more aggressively, particularly in response to large allocations that can’t reuse existing heap space.

The Go runtime’s timer and deadline code is faster and scales better with higher numbers of CPUs. In particular, this improves the performance of manipulating network connection deadlines.

On Linux, the runtime now uses MADV_FREE to release unused memory. This is more efficient but may result in higher reported RSS. The kernel will reclaim the unused data when it is needed. To revert to the Go 1.11 behavior (MADV_DONTNEED), set the environment variable GODEBUG=madvdontneed=1.

Adding cpu.extension=off to the GODEBUG environment variable now disables the use of optional CPU instruction set extensions in the standard library and runtime. This is not yet supported on Windows.

Go 1.12 improves the accuracy of memory profiles by fixing overcounting of large heap allocations.

Tracebacks, runtime.Caller, and runtime.Callers no longer include compiler-generated initialization functions. Doing a traceback during the initialization of a global variable will now show a function named PKG.init.ializers.

Standard library

TLS 1.3

Go 1.12 adds opt-in support for TLS 1.3 in the crypto/tls package as specified by RFC 8446. It can be enabled by adding the value tls13=1 to the GODEBUG environment variable. It will be enabled by default in Go 1.13.

To negotiate TLS 1.3, make sure you do not set an explicit MaxVersion in Config and run your program with the environment variable GODEBUG=tls13=1 set.

All TLS 1.2 features except TLSUnique in ConnectionState and renegotiation are available in TLS 1.3 and provide equivalent or better security and performance. Note that even though TLS 1.3 is backwards compatible with previous versions, certain legacy systems might not work correctly when attempting to negotiate it. RSA certificate keys too small to be secure (including 512-bit keys) will not work with TLS 1.3.

TLS 1.3 cipher suites are not configurable. All supported cipher suites are safe, and if PreferServerCipherSuites is set in Config the preference order is based on the available hardware.

Early data (also called “0-RTT mode”) is not currently supported as a client or server. Additionally, a Go 1.12 server does not support skipping unexpected early data if a client sends it. Since TLS 1.3 0-RTT mode involves clients keeping state regarding which servers support 0-RTT, a Go 1.12 server cannot be part of a load-balancing pool where some other servers do support 0-RTT. If switching a domain from a server that supported 0-RTT to a Go 1.12 server, 0-RTT would have to be disabled for at least the lifetime of the issued session tickets before the switch to ensure uninterrupted operation.

In TLS 1.3 the client is the last one to speak in the handshake, so if it causes an error to occur on the server, it will be returned on the client by the first Read, not by Handshake. For example, that will be the case if the server rejects the client certificate. Similarly, session tickets are now post-handshake messages, so are only received by the client upon its first Read.

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.


Reader’s UnreadRune and UnreadByte methods will now return an error if they are called after Peek.


The new function ReplaceAll returns a copy of a byte slice with all non-overlapping instances of a value replaced by another.

A pointer to a zero-value Reader is now functionally equivalent to NewReader(nil). Prior to Go 1.12, the former could not be used as a substitute for the latter in all cases.


A warning will now be printed to standard error the first time Reader.Read is blocked for more than 60 seconds waiting to read entropy from the kernel.

On FreeBSD, Reader now uses the getrandom system call if available, /dev/urandom otherwise.


This release removes the assembly implementations, leaving only the pure Go version. The Go compiler generates code that is either slightly better or slightly worse, depending on the exact CPU. RC4 is insecure and should only be used for compatibility with legacy systems.


If a client sends an initial message that does not look like TLS, the server will no longer reply with an alert, and it will expose the underlying net.Conn in the new field Conn of RecordHeaderError.


A query cursor can now be obtained by passing a *Rows value to the Row.Scan method.


The new Delete method allows for deletion of key/value pairs from a Map.


Maps are now printed in key-sorted order to ease testing. The ordering rules are:

When printing maps, non-reflexive key values like NaN were previously displayed as <nil>. As of this release, the correct values are printed.


To address some outstanding issues in cmd/doc, this package has a new Mode bit, PreserveAST, which controls whether AST data is cleared.


The File type has a new LineStart field, which returns the position of the start of a given line. This is especially useful in programs that occasionally handle non-Go files, such as assembly, but wish to use the token.Pos mechanism to identify file positions.


The RegisterFormat function is now safe for concurrent use.


Paletted images with fewer than 16 colors now encode to smaller outputs.


The new StringWriter interface wraps the WriteString function.


The functions Sin, Cos, Tan, and Sincos now apply Payne-Hanek range reduction to huge arguments. This produces more accurate answers, but they will not be bit-for-bit identical with the results in earlier releases.


New extended precision operations Add, Sub, Mul, and Div are available in uint, uint32, and uint64 versions.


The Dialer.DualStack setting is now ignored and deprecated; RFC 6555 Fast Fallback (“Happy Eyeballs”) is now enabled by default. To disable, set Dialer.FallbackDelay to a negative value.

Similarly, TCP keep-alives are now enabled by default if Dialer.KeepAlive is zero. To disable, set it to a negative value.

On Linux, the splice system call is now used when copying from a UnixConn to a TCPConn.


The HTTP server now rejects misdirected HTTP requests to HTTPS servers with a plaintext “400 Bad Request” response.

The new Client.CloseIdleConnections method calls the Client’s underlying Transport’s CloseIdleConnections if it has one.

The Transport no longer rejects HTTP responses which declare HTTP Trailers but don’t use chunked encoding. Instead, the declared trailers are now just ignored.

The Transport no longer handles MAX_CONCURRENT_STREAMS values advertised from HTTP/2 servers as strictly as it did during Go 1.10 and Go 1.11. The default behavior is now back to how it was in Go 1.9: each connection to a server can have up to MAX_CONCURRENT_STREAMS requests active and then new TCP connections are created as needed. In Go 1.10 and Go 1.11 the http2 package would block and wait for requests to finish instead of creating new connections. To get the stricter behavior back, import the golang.org/x/net/http2 package directly and set Transport.StrictMaxConcurrentStreams to true.


Parse, ParseRequestURI, and URL.Parse now return an error for URLs containing ASCII control characters, which includes NULL, tab, and newlines.


The ReverseProxy now automatically proxies WebSocket requests.


The new ProcessState.ExitCode method returns the process’s exit code.

ModeCharDevice has been added to the ModeType bitmask, allowing for ModeDevice | ModeCharDevice to be recovered when masking a FileMode with ModeType.

The new function UserHomeDir returns the current user’s home directory.

RemoveAll now supports paths longer than 4096 characters on most Unix systems.

File.Sync now uses F_FULLFSYNC on macOS to correctly flush the file contents to permanent storage. This may cause the method to run more slowly than in previous releases.

File now supports a SyscallConn method returning a syscall.RawConn interface value. This may be used to invoke system-specific operations on the underlying file descriptor.


The IsAbs function now returns true when passed a reserved filename on Windows such as NUL. List of reserved names.


A new MapIter type is an iterator for ranging over a map. This type is exposed through the Value type’s new MapRange method. This follows the same iteration semantics as a range statement, with Next to advance the iterator, and Key/Value to access each entry.


Copy is no longer necessary to avoid lock contention, so it has been given a partial deprecation comment. Copy may still be appropriate if the reason for its use is to make two copies with different Longest settings.


A new BuildInfo type exposes the build information read from the running binary, available only in binaries built with module support. This includes the main package path, main module information, and the module dependencies. This type is given through the ReadBuildInfo function on BuildInfo.


The new function ReplaceAll returns a copy of a string with all non-overlapping instances of a value replaced by another.

A pointer to a zero-value Reader is now functionally equivalent to NewReader(nil). Prior to Go 1.12, the former could not be used as a substitute for the latter in all cases.

The new Builder.Cap method returns the capacity of the builder’s underlying byte slice.

The character mapping functions Map, Title, ToLower, ToLowerSpecial, ToTitle, ToTitleSpecial, ToUpper, and ToUpperSpecial now always guarantee to return valid UTF-8. In earlier releases, if the input was invalid UTF-8 but no character replacements needed to be applied, these routines incorrectly returned the invalid UTF-8 unmodified.


64-bit inodes are now supported on FreeBSD 12. Some types have been adjusted accordingly.

The Unix socket (AF_UNIX) address family is now supported for compatible versions of Windows.

The new function Syscall18 has been introduced for Windows, allowing for calls with up to 18 arguments.


The Callback type and NewCallback function have been renamed; they are now called Func and FuncOf, respectively. This is a breaking change, but WebAssembly support is still experimental and not yet subject to the Go 1 compatibility promise. Any code using the old names will need to be updated.

If a type implements the new Wrapper interface, ValueOf will use it to return the JavaScript value for that type.

The meaning of the zero Value has changed. It now represents the JavaScript undefined value instead of the number zero. This is a breaking change, but WebAssembly support is still experimental and not yet subject to the Go 1 compatibility promise. Any code relying on the zero Value to mean the number zero will need to be updated.

The new Value.Truthy method reports the JavaScript “truthiness” of a given value.


The -benchtime flag now supports setting an explicit iteration count instead of a time when the value ends with an “x”. For example, -benchtime=100x runs the benchmark 100 times.


When executing a template, long context values are no longer truncated in errors.

executing "tmpl" at <.very.deep.context.v...>: map has no entry for key "notpresent"

is now

executing "tmpl" at <.very.deep.context.value.notpresent>: map has no entry for key "notpresent"

If a user-defined function called by a template panics, the panic is now caught and returned as an error by the Execute or ExecuteTemplate method.


The time zone database in $GOROOT/lib/time/zoneinfo.zip has been updated to version 2018i. Note that this ZIP file is only used if a time zone database is not provided by the operating system.


It is invalid to convert a nil unsafe.Pointer to uintptr and back with arithmetic. (This was already invalid, but will now cause the compiler to misbehave.)