"PK\x01\x02\x14\x00\x14\x00\b\b\b\x004\x9d3?Ļ\x14\xe3\xd8\x01\x00\x00\xd8\x03\x00\x00 \x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00:E\x02\x00res/layout/wallpaper_cropper.xml",

---
title: "Go, Backwards Compatibility, and GODEBUG"
layout: article
---

<!--
This document is kept in the Go repo, not x/website,
because it documents the full list of known GODEBUG settings,
which are tied to a specific release.
-->

## Introduction {#intro}

Go's emphasis on backwards compatibility is one of its key strengths.
There are, however, times when we cannot maintain complete compatibility.

	<!-- ul below only for regular layout -->
	<ul> </ul>
</li>

<li>
	A call to the built-in function
	<a href="#Handling_panics"><code>panic</code></a>.
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	<ul> </ul>
</li>

<li>
	A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement.
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	<ul> </ul>
</li>

<li>

together. The directive can include a list of build constraints limiting its
effect to systems satisfying one of the constraints
(see https://golang.org/pkg/go/build/#hdr-Build_Constraints for details about the constraint syntax).
For example:

	// #cgo CFLAGS: -DPNG_DEBUG=1
	// #cgo amd64 386 CFLAGS: -DX86=1
	// #cgo LDFLAGS: -lpng
	// #include <png.h>
	import "C"

	// Make sure that the input format is GNU.
	// Unfortunately, the STAR format also has a sparse header format that uses
	// the same type flag but has a completely different layout.
	if blk.getFormat() != FormatGNU {
		return nil, ErrHeader
	}
	hdr.Format.mayOnlyBe(FormatGNU)

	var p parser
	hdr.Size = p.parseNumeric(blk.toGNU().realSize())
	if p.err != nil {
		return nil, p.err

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	<ul> </ul>
</li>

<li>
	A call to the built-in function
	<a href="#Handling_panics"><code>panic</code></a>.
	<!-- ul below only for regular layout -->
	<ul> </ul>
</li>

<li>
	A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement.
	<!-- ul below only for regular layout -->
	<ul> </ul>
</li>

<li>

// allow the I/O-intensive phases of 'go list' to overlap) instead of scaling
// all the way up to GOMAXPROCS.
var listSem = make(chan semToken, 2)

type semToken struct{}

// loadImports populates w with information about the packages in the standard
// library and the packages they themselves import in w's build context.
//
// The source import path and expanded import path are identical except for vendored packages.
// For example, on return: