What causes "malformed env var %q from input" in fyne-io/fyne?

Fix the offending entry to KEY=VALUE form - the panic message prints the exact malformed string

malformed env var %q from input

Error message

malformed env var %q from input

What it means

environ(kv) merges the process environment with caller-supplied KEY=VALUE strings used for the gomobile build env (CC, CGO_CFLAGS, GOARM, ...). Entries coming from os.Environ of unusual form are passed through untouched, but every kv entry is treated as trusted input and must contain '=' with a non-empty key; anything else panics so it cannot silently corrupt the child build environment.

Source

Thrown at cmd/fyne/internal/mobile/env.go:315

	envs := make(map[string]string, len(cur))
	for _, ev := range cur {
		elem := strings.SplitN(ev, "=", 2)
		if len(elem) != 2 || elem[0] == "" {
			// pass the env var of unusual form untouched.
			// e.g. Windows may have env var names starting with "=".
			new = append(new, ev)
			continue
		}
		if goos == "windows" {
			elem[0] = strings.ToUpper(elem[0])
		}
		envs[elem[0]] = elem[1]
	}
	for _, ev := range kv {
		elem := strings.SplitN(ev, "=", 2)
		if len(elem) != 2 || elem[0] == "" {
			panic(fmt.Sprintf("malformed env var %q from input", ev))
		}
		if goos == "windows" {
			elem[0] = strings.ToUpper(elem[0])
		}
		envs[elem[0]] = elem[1]
	}
	for k, v := range envs {
		new = append(new, k+"="+v)
	}
	return new
}

func archNDK() string {
	if runtime.GOOS == "windows" && runtime.GOARCH == "386" {
		return "windows"
	}

	var arch string

View on GitHub (pinned to 8860ee95c3)

Solutions

Fix the offending entry to KEY=VALUE form - the panic message prints the exact malformed string
Filter the slice before passing it: drop empty strings and entries without '=' or with an empty key
Audit env/CI files for stray newlines or quotes feeding the build script

Example fix

// before
kv := []string{"CGO_CFLAGS", "CGO_ENABLED=1"}

// after
kv := []string{"CGO_CFLAGS=-O2", "CGO_ENABLED=1"}

Defensive patterns

Strategy: validation

Validate before calling

// Pre-flight: sanitize env pairs before passing them to the build environment.
func sanitizeEnv(kv []string) ([]string, error) {
    out := make([]string, 0, len(kv))
    for _, ev := range kv {
        if ev == "" { continue }
        k, v, ok := strings.Cut(ev, "=")
        if !ok || k == "" { return nil, fmt.Errorf("malformed env var %q", ev) }
        out = append(out, k+"="+v)
    }
    return out, nil
}

Prevention

Build env lists with explicit key=value constants, not string concatenation
Trim whitespace/newlines when parsing env files into slices
Unit-test your env assembly helper for entries missing '='

When it happens

Trigger: Passing an env slice containing an entry without '=' (e.g. 'CGO_CFLAGS') or with an empty name (e.g. '=foo') into environ - usually via custom build scripts or CI that assembles the variable list by string concatenation.

Common situations: Env lists built by concatenation where a value or '=' was dropped; env files with trailing newlines splitting 'A=b' into 'A=b' plus a fragment; CI matrix variables with missing separators.

Understand the failure class

Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

AI-assisted analysis of fyne-io/fyne@8860ee95c3 (2026-08-15). Data as JSON: /api/errors/b95a5c3517319a22. Report an issue: GitHub.