MADV_WILLNEED                   = 3  // for Linux compatibility -- no zos semantics
	MADV_REMOVE                     = 4  // for Linux compatibility -- no zos semantics
	MADV_DONTFORK                   = 5  // for Linux compatibility -- no zos semantics
	MADV_DOFORK                     = 6  // for Linux compatibility -- no zos semantics
	MADV_HWPOISON                   = 7  // for Linux compatibility -- no zos semantics

	form                     Form
	composing, compatibility bool // form type
	info                     lookupFunc
	nextMain                 iterFunc
}

var formTable = []*formInfo{{
	form:          NFC,
	composing:     true,
	compatibility: false,
	info:          lookupInfoNFC,
	nextMain:      nextComposed,
}, {
	form:          NFD,
	composing:     false,
	compatibility: false,
	info:          lookupInfoNFC,

			}
		}
	}

	// Pass 2: fields and methods.
	//
	// We allow fields and methods if their associated type is
	// disallowed, as otherwise we would report false positives
	// for compatibility shims. Consider:
	//
	//   //go:build go1.22
	//   type T struct { F std.Real } // correct new API
	//
	//   //go:build !go1.22
	//   type T struct { F fake } // shim
	//   type fake struct { ... }

internal symbols in the standard library (including the runtime) that
are not marked with `//go:linkname` on their definitions.
Similarly, the linker disallows references to such symbols from assembly
code.
For backward compatibility, existing usages of `//go:linkname` found in
a large open-source code corpus remain supported.
Any new references to standard library internal symbols will be disallowed.

the Go team, including tools in the Go distribution and auxiliary tools like
[gopls](https://pkg.go.dev/golang.org/x/tools/gopls) or
[govulncheck](https://pkg.go.dev/golang.org/x/vuln/cmd/govulncheck). There are
no compatibility guarantees for any of the packages here: public APIs will
change in breaking ways as the telemetry integration is refined.

## Notable Packages

---
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.

go1.txt (and similarly named files) are frozen once a version has been
shipped. Each file adds new lines but does not remove any.

except.txt lists features that may disappear without breaking true
compatibility.

Starting with go1.19.txt, each API feature line must end in "#nnnnn"
giving the GitHub issue number of the proposal issue that accepted
the new API. This helps with our end-of-cycle audit of new APIs.

	}
	data, err := os.ReadFile(d.modefile)
	if err != nil {
		return "local", time.Time{} // default
	}
	mode := string(data)
	mode = strings.TrimSpace(mode)

	// Forward compatibility for https://go.dev/issue/63142#issuecomment-1734025130
	//
	// If the modefile contains a date, return it.
	if idx := strings.Index(mode, " "); idx >= 0 {
		d, err := time.Parse("2006-01-02", mode[idx+1:])

	"fmt"
	"log"
	"os"
	"sort"

	"cmd/internal/objfile"
	"cmd/internal/telemetry"
)

const helpText = `usage: go tool nm [options] file...
  -n
      an alias for -sort address (numeric),
      for compatibility with other nm commands
  -size
      print symbol size in decimal between address and type
  -sort {address,name,none,size}
      sort output in the given order (default name)
      size orders from largest to smallest

[exec:patch] cmp go.mod go.mod.tidyResult
[exec:patch] cmp go.sum go.sum.tidyResult

go list -m all
cmp stdout m_all.txt

go mod edit -go=1.16
go list -m all
cmp stdout m_all.txt


# If we explicitly drop compatibility with 1.16, we retain fewer checksums,
# which gives a cleaner go.sum file but causes 1.16 to fail in readonly mode.

cp go.mod.orig go.mod
go mod tidy -compat=1.17
cmp go.mod go.mod.orig