}
		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

	default:
		t.Fatalf("bad case %d", test)
	}

	cmd := testenv.Command(t, testenv.GoToolPath(t), "build")
	cmd.Dir = dir
	cgoCflags += " -flto"
	cmd.Env = append(cmd.Environ(), "CGO_CFLAGS="+cgoCflags)

	t.Logf("CGO_CFLAGS=%q %v", cgoCflags, cmd)
	out, err := cmd.CombinedOutput()
	t.Logf("%s", out)

	if err != nil {
		t.Logf("go build failed: %v", err)

	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.

# Other flags are added if CGO_ENABLED is true.
env CGO_ENABLED=0
go build
go version -m m$GOEXE
stdout '^\tbuild\tCGO_ENABLED=0$'
! stdout CGO_CPPFLAGS|CGO_CFLAGS|CGO_CXXFLAGS|CGO_LDFLAGS

[cgo] env CGO_ENABLED=1
[cgo] env CGO_CPPFLAGS=-DFROM_CPPFLAGS=1
[cgo] env CGO_CFLAGS=-DFROM_CFLAGS=1
[cgo] env CGO_CXXFLAGS=-DFROM_CXXFLAGS=1
[cgo] env CGO_LDFLAGS=-L/extra/dir/does/not/exist

// 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

        GCCGO="gccgo"
        AR="ar"
        CC="clang"
        CXX="clang++"
        CGO_ENABLED="1"
        GOMOD="/dev/null"
        GOWORK=""
        CGO_CFLAGS="-O2 -g"
        CGO_CPPFLAGS=""
        CGO_CXXFLAGS="-O2 -g"
        CGO_FFLAGS="-O2 -g"
        CGO_LDFLAGS="-O2 -g"
        PKG_CONFIG="pkg-config"

		if err != nil {
			t.Fatalf("%v: %v\n%s", cmd, err, cmd.Stderr)
		}
		out := string(bytes.TrimSpace(line))
		t.Logf("%v: %q", cmd, out)
		return out
	}

	cc := goEnv("CC")
	cgoCflags := goEnv("CGO_CFLAGS")

	t.Parallel()

	tmpdir := t.TempDir()
	dllfile := filepath.Join(tmpdir, "test.dll")
	exefile := filepath.Join(tmpdir, "gotest.exe")

	// build go dll

env CGO_CFLAGS=-fplugin
go build -x -n -o dummy.exe ./usesInternalCgo
stderr preferlinkext
env CGO_CFLAGS=-fprofile-instr-generate
go build -x -n -o dummy.exe ./usesInternalCgo
stderr preferlinkext

# The -fdebug-prefix-map=path is permitted for internal linking.
env CGO_CFLAGS=-fdebug-prefix-map=/some/sandbox/execroot/workspace=/tmp/new
go build -x -n -o dummy.exe ./usesInternalCgo
! stderr preferlinkext