golang/go · error

unknown compiler %q

Error message

unknown compiler %q

What it means

buildCompiler.Set is the flag.Value for -compiler. It accepts only 'gc' (the standard Go compiler) and 'gccgo' (the GCC-based Go frontend). Any other value returns this error, and flag parsing aborts before any build work begins.

Source

Thrown at src/cmd/go/internal/work/build.go:283

var (
	BuildToolchain toolchain = noToolchain{}
	ldBuildmode    string
)

// buildCompiler implements flag.Var.
// It implements Set by updating both
// BuildToolchain and buildContext.Compiler.
type buildCompiler struct{}

func (c buildCompiler) Set(value string) error {
	switch value {
	case "gc":
		BuildToolchain = gcToolchain{}
	case "gccgo":
		BuildToolchain = gccgoToolchain{}
	default:
		return fmt.Errorf("unknown compiler %q", value)
	}
	cfg.BuildToolchainName = value
	cfg.BuildContext.Compiler = value
	return nil
}

func (c buildCompiler) String() string {
	return cfg.BuildContext.Compiler
}

func init() {
	switch build.Default.Compiler {
	case "gc", "gccgo":
		buildCompiler{}.Set(build.Default.Compiler)
	}
}

type BuildFlagMask int

View on GitHub (pinned to b6b368adc5)

Solutions

1. Use '-compiler=gccgo' for the GCC-based Go frontend (install gccgo first).
2. Omit -compiler entirely to use the default 'gc'.
3. Check GOFLAGS in the environment: 'go env GOFLAGS' and unset the bad value.
4. For C compiler selection in cgo, use CC= not -compiler.

Example fix

// before
$ go build -compiler=gcc ./...
// error: unknown compiler "gcc"

// after
$ go build -compiler=gccgo ./...   # if you meant gccgo
# or
$ go build ./...                   # default gc compiler
# for cgo C-compiler selection:
$ CC=gcc go build ./...

var validCompilers = map[string]bool{"gc": true, "gccgo": true}

func validateCompilerFlag(v string) error {
    if !validCompilers[v] {
        return fmt.Errorf("-compiler must be 'gc' or 'gccgo', got %q", v)
    }
    return nil
}

When it happens

Trigger: Invoking 'go build -compiler=<x>' (or GOFLAGS=-compiler=<x>, or setting the compiler via -gcflags with a bad toolchain name) where <x> is not 'gc' or 'gccgo'. Commonly a typo like 'gcc' or 'llvm' or 'go'.

Common situations: Users coming from C ecosystems who try '-compiler=gcc'; tooling scripts that interpolate an unknown compiler name; copied GOFLAGS from another project.

Related errors

• failed to locate cmd/compile for target platform
• CC not set and no default found
• local imports disallowed
• file not found
• cannot import "main"

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/2abfb9b86424dd38. Report an issue: GitHub.