}
		if cfg.BuildN || cfg.BuildX {
			sh.ShowCmd("", "ar d %s _cgo_flags", newArchive)
			if cfg.BuildN {
				// TODO(rsc): We could do better about showing the right _cgo_flags even in -n mode.
				// Either the archive is already built and we can read them out,
				// or we're printing commands to build the archive and can
				// forward the _cgo_flags directly to this step.
				return "", nil
			}
		}

			cgoCFLAGS = append(cgoCFLAGS, "-fsplit-stack")
		}
		cgoflags = append(cgoflags, "-gccgo")
		if pkgpath := gccgoPkgpath(p); pkgpath != "" {
			cgoflags = append(cgoflags, "-gccgopkgpath="+pkgpath)
		}
		if !BuildToolchain.(gccgoToolchain).supportsCgoIncomplete(b, a) {
			cgoflags = append(cgoflags, "-gccgo_define_cgoincomplete")
		}
	}

'.', that will not be interpreted in unexpected ways. Attempts to use
forbidden characters will get a "malformed #cgo argument" error.

When building, the CGO_CFLAGS, CGO_CPPFLAGS, CGO_CXXFLAGS, CGO_FFLAGS and
CGO_LDFLAGS environment variables are added to the flags derived from
these directives. Package-specific flags should be set using the
directives, not the environment variables, so that builds work in

	var gccgoInit strings.Builder

	if !*gccgo {
		for _, arg := range p.LdFlags {
			fmt.Fprintf(fgo2, "//go:cgo_ldflag %q\n", arg)
		}
	} else {
		fflg := creat(*objDir + "_cgo_flags")
		for _, arg := range p.LdFlags {
			fmt.Fprintf(fflg, "_CGO_LDFLAGS=%s\n", arg)
		}
		fflg.Close()
	}

	// Write C main file for using gcc to resolve imports.
	fmt.Fprintf(fm, "#include <stddef.h>\n") // For size_t below.

// of the Go command.
const KnownEnv = `
	AR
	CC
	CGO_CFLAGS
	CGO_CFLAGS_ALLOW
	CGO_CFLAGS_DISALLOW
	CGO_CPPFLAGS
	CGO_CPPFLAGS_ALLOW
	CGO_CPPFLAGS_DISALLOW
	CGO_CXXFLAGS
	CGO_CXXFLAGS_ALLOW
	CGO_CXXFLAGS_DISALLOW
	CGO_ENABLED
	CGO_FFLAGS
	CGO_FFLAGS_ALLOW
	CGO_FFLAGS_DISALLOW
	CGO_LDFLAGS
	CGO_LDFLAGS_ALLOW
	CGO_LDFLAGS_DISALLOW
	CXX
	FC

		}
		return q
	}

	ret := []cfg.EnvVar{
		// Note: Update the switch in runEnv below when adding to this list.
		{Name: "CGO_CFLAGS", Value: join(cflags)},
		{Name: "CGO_CPPFLAGS", Value: join(cppflags)},
		{Name: "CGO_CXXFLAGS", Value: join(cxxflags)},
		{Name: "CGO_FFLAGS", Value: join(fflags)},
		{Name: "CGO_LDFLAGS", Value: join(ldflags)},
		{Name: "PKG_CONFIG", Value: b.PkgconfigCmd()},

env GOSUMDB=nodefault
env GOPROXY=nodefault
env GO111MODULE=auto
env CGO_CFLAGS=nodefault
env CGO_CPPFLAGS=nodefault

go env -changed
# linux output like GOTOOLCHAIN='local'
# windows output like GOTOOLCHAIN=local
stdout 'GOTOOLCHAIN=''?local''?'
stdout 'GOSUMDB=''?nodefault''?'
stdout 'GOPROXY=''?nodefault''?'
stdout 'GO111MODULE=''?auto''?'
stdout 'CGO_CFLAGS=''?nodefault''?'
stdout 'CGO_CPPFLAGS=''?nodefault''?'

# Explicitly rebuild it to ensure that it is cached.
# (See https://go.dev/issue/50892.)
#
# If running in non-short mode, explicitly vary CGO_CFLAGS
# as a control case (to ensure that our regexps do catch rebuilds).

[!short] env GOCACHE=$WORK/cache
[!short] env CGO_CFLAGS=-DTestScript_cgo_stale_precompiled=true
go build -x runtime/cgo
[!short] stderr '[/\\]cgo'$GOEXE'["]? .* -importpath runtime/cgo'

		Does not apply to the CGO_CFLAGS environment variable.
	CGO_CPPFLAGS, CGO_CPPFLAGS_ALLOW, CGO_CPPFLAGS_DISALLOW
		Like CGO_CFLAGS, CGO_CFLAGS_ALLOW, and CGO_CFLAGS_DISALLOW,
		but for the C preprocessor.
	CGO_CXXFLAGS, CGO_CXXFLAGS_ALLOW, CGO_CXXFLAGS_DISALLOW
		Like CGO_CFLAGS, CGO_CFLAGS_ALLOW, and CGO_CFLAGS_DISALLOW,
		but for the C++ compiler.
	CGO_FFLAGS, CGO_FFLAGS_ALLOW, CGO_FFLAGS_DISALLOW

 */
struct cgoTracebackArg {
	uintptr_t  Context;
	uintptr_t  SigContext;
	uintptr_t* Buf;
	uintptr_t  Max;
};

/*
 * TSAN support.  This is only useful when building with
 *   CGO_CFLAGS="-fsanitize=thread" CGO_LDFLAGS="-fsanitize=thread" go install
 */
#undef CGO_TSAN
#if defined(__has_feature)
# if __has_feature(thread_sanitizer)
#  define CGO_TSAN
# endif
#elif defined(__SANITIZE_THREAD__)