What causes "failed to write config: %s" in bcicen/ctop?

Check free disk space on the filesystem holding the config

failed to write config: %s

Error message

failed to write config: %s

What it means

After the config file opens successfully, Write encodes exportConfig() as TOML via toml.NewEncoder(file).Encode. If encoding/writing to the file fails, the error is wrapped as 'failed to write config'.

Source

Thrown at config/file.go:109

		}
	}

	// remove prior to writing new file
	if err := os.Remove(path); err != nil {
		if !os.IsNotExist(err) {
			return path, err
		}
	}

	file, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0644)
	if err != nil {
		return path, fmt.Errorf("failed to open config for writing: %s", err)
	}

	writer := toml.NewEncoder(file)
	err = writer.Encode(exportConfig())
	if err != nil {
		return path, fmt.Errorf("failed to write config: %s", err)
	}

	return path, nil
}

// determine config path from environment
func getConfigPath() (path string, err error) {
	homeDir, ok := os.LookupEnv("HOME")
	if !ok {
		return path, fmt.Errorf("$HOME not set")
	}

	// use xdg config home if possible
	if xdgSupport() {
		xdgHome, ok := os.LookupEnv("XDG_CONFIG_HOME")
		if !ok {
			xdgHome = fmt.Sprintf("%s/.config", homeDir)
		}

View on GitHub (pinned to 59f00dd6aa)

Solutions

Check free disk space on the filesystem holding the config
Audit the exported config struct for TOML-unsupported types (channels, funcs, complex numbers, non-string map keys)
Re-run write after removing any special file at the config path

Example fix

// before
type Config struct { Callbacks map[int]func() } // unsupported by TOML
// after
type Config struct { Callbacks map[string]string }

Defensive patterns

Strategy: try-catch

Validate before calling

// keep config struct TOML-encodable: no funcs/channels/non-string map keys
toml.Marshal(exportConfig()) // dry-run encode to memory first

Try / catch

_, err := config.Write()
if err != nil && strings.HasPrefix(err.Error(), "failed to write config") {
    // free disk space / fix unsupported field types
}

Prevention

Monitor free disk space on the config volume
Dry-run TOML encode in tests to catch unsupported types
Avoid special files (fifo/socket) at the config path

When it happens

Trigger: toml Encoder.Encode returns an error — typically an I/O write failure (disk full, closed handle) or a value in the config struct that the TOML encoder cannot serialize (e.g. unsupported type like a map with non-string keys, channel, func).

Common situations: Disk full on the volume holding $HOME; config struct gained a field of an unsupported type after an upgrade; file replaced by a pipe/fifo or special file mid-write.

Related errors

AI-assisted analysis of bcicen/ctop@59f00dd6aa (2026-09-02). Data as JSON: /api/errors/12a45ef65c43afe8. Report an issue: GitHub.