Go 1.3 Release Notes
Introduction to Go 1.3
The latest Go release, version 1.3, arrives six months after 1.2, and contains no language changes. It focuses primarily on implementation work, providing precise garbage collection, a major refactoring of the compiler toolchain that results in faster builds, especially for large projects, significant performance improvements across the board, and support for DragonFly BSD, Solaris, Plan 9 and Google’s Native Client architecture (NaCl). It also has an important refinement to the memory model regarding synchronization. As always, Go 1.3 keeps the promise of compatibility, and almost everything will continue to compile and run without change when moved to 1.3.
Changes to the supported operating systems and architectures
Removal of support for Windows 2000
Microsoft stopped supporting Windows 2000 in 2010. Since it has implementation difficulties regarding exception handling (signals in Unix terminology), as of Go 1.3 it is not supported by Go either.
Support for DragonFly BSD
Go 1.3 now includes experimental support for DragonFly BSD on the
amd64 (64-bit x86) and
386 (32-bit x86) architectures.
It uses DragonFly BSD 3.6 or above.
Support for FreeBSD
It was not announced at the time, but since the release of Go 1.2, support for Go on FreeBSD requires FreeBSD 8 or above.
As of Go 1.3, support for Go on FreeBSD requires that the kernel be compiled with the
COMPAT_FREEBSD32 flag configured.
In concert with the switch to EABI syscalls for ARM platforms, Go 1.3 will run only on FreeBSD 10. The x86 platforms, 386 and amd64, are unaffected.
Support for Native Client
Support for the Native Client virtual machine architecture has returned to Go with the 1.3 release.
It runs on the 32-bit Intel architectures (
GOARCH=386) and also on 64-bit Intel, but using
32-bit pointers (
There is not yet support for Native Client on ARM.
Note that this is Native Client (NaCl), not Portable Native Client (PNaCl).
Details about Native Client are here;
how to set up the Go version is described here.
Support for NetBSD
As of Go 1.3, support for Go on NetBSD requires NetBSD 6.0 or above.
Support for OpenBSD
As of Go 1.3, support for Go on OpenBSD requires OpenBSD 5.5 or above.
Support for Plan 9
Go 1.3 now includes experimental support for Plan 9 on the
386 (32-bit x86) architecture.
It requires the
Tsemacquire syscall, which has been in Plan 9 since June, 2012.
Support for Solaris
Go 1.3 now includes experimental support for Solaris on the
amd64 (64-bit x86) architecture.
It requires illumos, Solaris 11 or above.
Changes to the memory model
The Go 1.3 memory model adds a new rule concerning sending and receiving on buffered channels, to make explicit that a buffered channel can be used as a simple semaphore, using a send into the channel to acquire and a receive from the channel to release. This is not a language change, just a clarification about an expected property of communication.
Changes to the implementations and tools
Go 1.3 has changed the implementation of goroutine stacks away from the old, “segmented” model to a contiguous model. When a goroutine needs more stack than is available, its stack is transferred to a larger single block of memory. The overhead of this transfer operation amortizes well and eliminates the old “hot spot” problem when a calculation repeatedly steps across a segment boundary. Details including performance numbers are in this design document.
Changes to the garbage collector
For a while now, the garbage collector has been precise when examining values in the heap; the Go 1.3 release adds equivalent precision to values on the stack. This means that a non-pointer Go value such as an integer will never be mistaken for a pointer and prevent unused memory from being reclaimed.
Starting with Go 1.3, the runtime assumes that values with pointer type contain pointers and other values do not. This assumption is fundamental to the precise behavior of both stack expansion and garbage collection. Programs that use package unsafe to store integers in pointer-typed values are illegal and will crash if the runtime detects the behavior. Programs that use package unsafe to store pointers in integer-typed values are also illegal but more difficult to diagnose during execution. Because the pointers are hidden from the runtime, a stack expansion or garbage collection may reclaim the memory they point at, creating dangling pointers.
Updating: Code that uses
unsafe.Pointer to convert
an integer-typed value held in memory into a pointer is illegal and must be rewritten.
Such code can be identified by
Iterations over small maps no longer happen in a consistent order. Go 1 defines that “The iteration order over maps is not specified and is not guaranteed to be the same from one iteration to the next.” To keep code from depending on map iteration order, Go 1.0 started each map iteration at a random index in the map. A new map implementation introduced in Go 1.1 neglected to randomize iteration for maps with eight or fewer entries, although the iteration order can still vary from system to system. This has allowed people to write Go 1.1 and Go 1.2 programs that depend on small map iteration order and therefore only work reliably on certain systems. Go 1.3 reintroduces random iteration for small maps in order to flush out these bugs.
Updating: If code assumes a fixed iteration order for small maps, it will break and must be rewritten not to make that assumption. Because only small maps are affected, the problem arises most often in tests.
As part of the general overhaul to
the Go linker, the compilers and linkers have been refactored.
The linker is still a C program, but now the instruction selection phase that
was part of the linker has been moved to the compiler through the creation of a new
By doing instruction selection only once, when the package is first compiled,
this can speed up compilation of large projects significantly.
Updating: Although this is a major internal change, it should have no effect on programs.
Status of gccgo
GCC release 4.9 will contain the Go 1.2 (not 1.3) version of gccgo. The release schedules for the GCC and Go projects do not coincide, which means that 1.3 will be available in the development branch but that the next GCC release, 4.10, will likely have the Go 1.4 version of gccgo.
Changes to the go command
cmd/go command has several new
go run and
go test subcommands
support a new
-exec option to specify an alternate
way to run the resulting binary.
Its immediate purpose is to support NaCl.
The test coverage support of the
subcommand now automatically sets the coverage mode to
when the race detector is enabled, to eliminate false reports about unsafe
access to coverage counters.
go test subcommand
now always builds the package, even if it has no test files.
Previously, it would do nothing if no test files were present.
go build subcommand
supports a new
-i option to install dependencies
of the specified target, but not the target itself.
Cross compiling with
is now supported.
The CC_FOR_TARGET and CXX_FOR_TARGET environment
variables are used when running all.bash to specify the cross compilers
for C and C++ code, respectively.
Finally, the go command now supports packages that import Objective-C
.m) through cgo.
Changes to cgo
import "C" declarations in Go packages,
has corrected a serious bug that may cause some packages to stop compiling.
Previously, all pointers to incomplete struct types translated to the Go type
with the effect that the Go compiler could not diagnose passing one kind of struct pointer
to a function expecting another.
Go 1.3 corrects this mistake by translating each different
incomplete struct to a different named type.
Given the C declaration
typedef struct S T for an incomplete
some Go code used this bug to refer to the types
Cgo now explicitly allows this use, even for completed struct types.
However, some Go code also used this bug to pass (for example) a
from one package to another.
This is not legal and no longer works: in general Go packages
should avoid exposing C types and names in their APIs.
Updating: Code confusing pointers to incomplete types or
passing them across package boundaries will no longer compile
and must be rewritten.
If the conversion is correct and must be preserved,
use an explicit conversion via
SWIG 3.0 required for programs that use SWIG
For Go programs that use SWIG, SWIG version 3.0 is now required.
cmd/go command will now link the
SWIG generated object files directly into the binary, rather than
building and linking with a shared library.
Command-line flag parsing
In the gc toolchain, the assemblers 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.
go tool 6a -SDfoo must now be written
go tool 6a -S -D foo.
(The same change was made to the compilers and linkers in Go 1.1.)
Changes to godoc
When invoked with the
now performs sophisticated static
analysis of the code it indexes.
The results of analysis are presented in both the source view and the
package documentation view, and include the call graph of each package
and the relationships between
definitions and references,
types and their methods,
interfaces and their implementations,
send and receive operations on channels,
functions and their callers, and
call sites and their callees.
misc/benchcmp that compares
performance across benchmarking runs has been rewritten.
Once a shell and awk script in the main repository, it is now a Go program in the
Documentation is here.
For the few of us that build Go distributions, the tool
misc/dist has been
moved and renamed; it now lives in
misc/makerelease, still in the main repository.
The performance of Go binaries for this release has improved in many cases due to changes in the runtime and garbage collection, plus some changes to libraries. Significant instances include:
- The runtime handles defers more efficiently, reducing the memory footprint by about two kilobytes per goroutine that calls defer.
- The garbage collector has been sped up, using a concurrent sweep algorithm, better parallelization, and larger pages. The cumulative effect can be a 50-70% reduction in collector pause time.
- The race detector (see this guide) is now about 40% faster.
- The regular expression package
regexpis now significantly faster for certain simple expressions due to the implementation of a second, one-pass execution engine. The choice of which engine to use is automatic; the details are hidden from the user.
Also, the runtime now includes in stack dumps how long a goroutine has been blocked, which can be useful information when debugging deadlocks or performance issues.
Changes to the standard library
A new package
debug/plan9obj was added to the standard library.
It implements access to Plan 9 a.out object files.
Major changes to the library
A previous bug in
made it possible to skip verification in TLS inadvertently.
In Go 1.3, the bug is fixed: one must specify either ServerName or
InsecureSkipVerify, and if ServerName is specified it is enforced.
This may break existing code that incorrectly depended on insecure
There is an important new type added to the standard library:
It provides an efficient mechanism for implementing certain types of caches whose memory
can be reclaimed automatically by the system.
testing package’s benchmarking helper,
B, now has a
to make it easier to run benchmarks that exercise multiple CPUs.
Updating: The crypto/tls fix may break existing code, but such code was erroneous and should be updated.
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.
- In the
crypto/tlspackage, a new
DialWithDialerfunction lets one establish a TLS connection using an existing dialer, making it easier to control dial options such as timeouts. The package also now reports the TLS version used by the connection in the
CreateCertificatefunction of the
crypto/tlspackage now supports parsing (and elsewhere, serialization) of PKCS #10 certificate signature requests.
- The formatted print functions of the
fmtpackage now define
%Fas a synonym for
%fwhen printing floating-point values.
Rattypes now implement
- The complex power function,
Pow, now specifies the behavior when the first argument is zero. It was undefined before. The details are in the documentation for the function.
net/httppackage now exposes the properties of a TLS connection used to make a client request in the new
net/httppackage now allows setting an optional server error logger with
Server.ErrorLog. The default is still that all errors go to stderr.
net/httppackage now supports disabling HTTP keep-alive connections on the server with
Server.SetKeepAlivesEnabled. The default continues to be that the server does keep-alive (reuses connections for multiple requests) by default. Only resource-constrained servers or those in the process of graceful shutdown will want to disable them.
net/httppackage adds an optional
Transport.TLSHandshakeTimeoutsetting to cap the amount of time HTTP client requests will wait for TLS handshakes to complete. It’s now also set by default on
DefaultTransport, used by the HTTP client code, now enables TCP keep-alives by default. Other
Transportvalues with a nil
Dialfield continue to function the same as before: no TCP keep-alives are used.
net/httppackage now enables TCP keep-alives for incoming server requests when
ListenAndServeTLSare used. When a server is started otherwise, TCP keep-alives are not enabled.
net/httppackage now provides an optional
Server.ConnStatecallback to hook various phases of a server connection’s lifecycle (see
ConnState). This can be used to implement rate limiting or graceful shutdown.
net/httppackage’s HTTP client now has an optional
Client.Timeoutfield to specify an end-to-end timeout on requests made using the client.
Request.ParseMultipartFormmethod will now return an error if the body’s
multipart/form-data. Prior to Go 1.3 it would silently fail and return
nil. Code that relies on the previous behavior should be updated.
- In the
Dialerstruct now has a
KeepAliveoption to specify a keep-alive period for the connection.
Request.Bodyconsistently, even on error.
os/execpackage now implements what the documentation has always said with regard to relative paths for the binary. In particular, it only calls
LookPathwhen the binary’s file name contains no path separators.
SetMapIndexfunction in the
reflectpackage no longer panics when deleting from a
- If the main goroutine calls
runtime.Goexitand all other goroutines finish execution, the program now always crashes, reporting a detected deadlock. Earlier versions of Go handled this situation inconsistently: most instances were reported as deadlocks, but some trivial cases exited cleanly instead.
- The runtime/debug package now has a new function
debug.WriteHeapDumpthat writes out a description of the heap.
CanBackquotefunction in the
strconvpackage now considers the
U+007F, to be non-printing.
syscallpackage now provides
SendmsgNas an alternate version of
Sendmsgthat returns the number of bytes written.
- On Windows, the
syscallpackage now supports the cdecl calling convention through the addition of a new function
NewCallbackCDeclalongside the existing function
testingpackage now diagnoses tests that call
panic(nil), which are almost always erroneous. Also, tests now write profiles (if invoked with profiling flags) even on failure.
unicodepackage and associated support throughout the system has been upgraded from Unicode 6.2.0 to Unicode 6.3.0.