What causes "Failed to print help output" in ReFirmLabs/binwalk?

Check that stdout is writable and the downstream pipe consumer is alive

Failed to print help output

Error message

Failed to print help output

What it means

In parse(), if the process was invoked with no arguments, the code tries to print the clap command's help text and exits. This expect() fires only if clap's print_help() fails while writing help output (an I/O error on stdout/stderr).

Source

Thrown at src/cliparser.rs:76

    /// Only scan for these signatures
    #[arg(short = 'y', long, value_delimiter = ',', num_args = 1.., conflicts_with = "exclude")]
    pub include: Option<Vec<String>>,

    /// Extract files/folders to a custom directory
    #[arg(short, long, default_value = "extractions")]
    pub directory: String,

    /// Path to the file to analyze
    pub file_name: Option<String>,
}

pub fn parse() -> CliArgs {
    let args = CliArgs::parse();

    if std::env::args().len() == 1 {
        CliArgs::command()
            .print_help()
            .expect("Failed to print help output");
        std::process::exit(0);
    }

    args
}

View on GitHub (pinned to 26713972e3)

Solutions

Check that stdout is writable and the downstream pipe consumer is alive
Run the binary normally (with arguments) so this no-args help path isn't taken
Replace expect with graceful handling: print a note to stderr and exit with a proper code if help can't be printed
Verify clap version behavior — in newer clap, use `Cli::command().print_help()` result explicitly rather than expect

Example fix

// before
CliArgs::command()
    .print_help()
    .expect("Failed to print help output");
// after
if CliArgs::command().print_help().is_err() {
    eprintln!("Failed to print help output");
    std::process::exit(1);
}

Defensive patterns

Strategy: try-catch

Validate before calling

if std::env::args().len() == 1 && std::io::stdout().write(&[]).is_err() {
    eprintln!("stdout is not writable");
    std::process::exit(1);
}

Try / catch

if let Err(e) = CliArgs::command().print_help() {
    eprintln!("Failed to print help: {e}");
    std::process::exit(1);
}

Prevention

Handle print_help()'s Result instead of expect()ing it
Avoid invoking with a closed stdout pipe (e.g. `| head` with no reads)
Test no-argument startup in CI with normal stdio
Consider clap's built-in arg_required_else_help(true) instead of manual parsing

When it happens

Trigger: Calling the binary with zero arguments AND the write of help text to stdout fails (closed/broken stdout pipe, e.g. `binary | head -0`, or a full/unavailable output stream).

Common situations: Piping output into a closed consumer; running in a stripped environment where stdout is not writable; embedding parse() in a test harness with redirected/closed stdio.

Understand the failure class

Background: "failed to write file", "Could not save figure", "Error saving remote file" — file write failed: causes and fixes across languages and libraries — this error's family across 38 libraries.

Related errors

AI-assisted analysis of ReFirmLabs/binwalk@26713972e3 (2026-09-06). Data as JSON: /api/errors/1c162d1d86e9e174. Report an issue: GitHub.