Introduction to Go 1.1

Changes to the language

Integer division by zero

In Go 1, integer division by a constant zero produced a runtime 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.

Changes to the implementations and tools

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 (TODO: check that gccgo does) 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))

Assembler

Due to the int and TODO: OTHER changes, the placement of function arguments on the stack has changed. Functions written in assembly will need to be revised at least to adjust frame pointer offsets.

Changes to the go tool

The go tool has acquired several improvements which are intended to improve the experience for new Go users.

Firstly, when compiling, testing, or running Go code, the go tool will now give more detailed errors 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/User/go/src/pkg/foo/quxx (from $GOROOT)
        /home/User/src/foo/quxx (from $GOPATH) 

Secondly, the go get command no longer allows $GOROOT as the default destination when downloading package source. To use 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/User/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 go fix

The go fix command 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.

Changes to the standard library

debug/elf

Previous versions of the debug/elf package intentionally skipped over the first symbol in the ELF symbol table, since it is always an empty symbol. This symbol is no longer skipped since indexes into the symbol table returned by debug/elf, will be different to indexes into the original ELF symbol table. Any code that calls the debug/elf functions Symbols or ImportedSymbols may need to be adjusted to account for the additional symbol and the change in symbol offsets.

html/template

Templates using the undocumented and only partially implemented "noescape" feature will break: that feature was removed.

net

The protocol-specific resolvers were formerly lax about the network name passed in. For example, 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 ListenUnixgram returned UDPConn as arepresentation of the connection endpoint. The Go 1.1 implementation returns UnixConn to allow reading and writing with ReadFrom and WriteTo methods on the UnixConn.

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 of time on these systems now returns times with nanosecond precision. Code may exist that expects to be able to store such a time in an external format with only microsecond precision, read it back, and recover exactly the same time instant. In Go 1.1 the same time will not be recovered, since the external storage will have discarded nanoseconds. To address this case, there are two new methods of time.Time, Round and Truncate, that can be used to remove precision from a time before passing it to external storage.