fwtool

fwtool is a command-line utility for creating, updating, inspecting, and verifying a custom firmware metadata header prepended to a binary image.

It is intended for firmware packaging workflows where a raw application binary is post-processed into a single flashable image:

• a fixed-size header is placed at the beginning that contains version, firmware size, and CRC information
• the firmware firmware follows immediately after

This is useful for bootloaders or firmware update logic that need to validate and identify an application image before booting or programming it.

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What it does

fwtool supports four subcommands:

1. attach — prepend a new header to a raw firmware binary
2. edit — replace the header of an already packaged binary
3. inspect — print the parsed contents of an existing header
4. verify — verify that an existing header matches the firmware

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Header format

The tool generates a header with the following layout:

Offset	Size	Field	Description
0x00..0x03	4	magic	ASCII string XLAB
0x04..0x07	4	version	Little-endian bytes: [0x00, patch, minor, major]
0x08..0x0B	4	size	firmware size in bytes, little-endian uint32
0x0C..0x0F	4	crc32	CRC-32/MPEG-2 of firmware, little-endian uint32
0x10..end	—	padding	Filled with 0xFF

The total header size defaults to 512 bytes and is configurable via --header-size. It must be a power of two (minimum 16 bytes) so that the application vector table following it in flash is correctly aligned.

With the default 512-byte header, the padding region spans 0x10..0x1FF (496 bytes of 0xFF).

Notes

• The CRC is calculated over the firmware only, not over the header.
• The size stored in the header is the firmware size only.
• The version is stored as:
  • byte 0: 0x00
  • byte 1: patch
  • byte 2: minor
  • byte 3: major

For example, version 1.2.3 is stored as:

0x00030201

Typical use case

A common workflow looks like this:

• Link the firmware application so that it expects to execute after the reserved header space.
• Build the raw application binary.
• Use fwtool to prepend the metadata header.
• Program the resulting combined binary into flash.

For example, with the default 512-byte header:

Flash address 0x08004000:
    [512-byte metadata header]
    [firmware payload]

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Installation

Install using pip

pip install fwtool

Install using pipx

pipx install fwtool

Install using uv

uv tool install fwtool

Check installation

fwtool --version

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Usage

fwtool <command> [arguments] [options]
  Subcommands
    Command Description
    attach Attach a new header to a raw firmware binary
    edit Replace the header of a packaged binary
    inspect Print header fields from a packaged binary
    verify Verify header of a packaged binary

  Common options
    --version Show program version and exit
    --help Show help message and exit

attach:

fwtool attach <binary> <version> <output> [options]
fwtool attach <binary> <version> --in-place [options]
  Argument / Option Description
  binary Path to raw input binary file
  version Firmware version string (e.g. 1, 1.2, 1.2.3)
  output Path to output file
  --in-place Modify the input file directly instead of writing output
  --header-size N Total header size in bytes; must be a power of 2 (default: 512)

edit:

fwtool edit <binary> <version> <output> [options]
fwtool edit <binary> <version> --in-place [options]
  Argument / Option Description
  binary Path to packaged binary file with existing header
  version New firmware version string
  output Path to output file
  --in-place Modify the input file directly instead of writing output
  --header-size N Total header size in bytes; must be a power of 2 (default: inferred from file)

inspect:

fwtool inspect <binary> [options]
  Argument / Option Description
  binary Path to packaged binary file
  --json Emit machine-readable JSON output

verify:

fwtool verify <binary> [options]
  Argument / Option Description
  binary Path to packaged binary file
  --json Emit machine-readable JSON output
  --quiet Suppress output; use exit code only
  --header-size N Total header size in bytes; must be a power of 2 (default: inferred from file)

Examples

Attach a new header to a raw binary

fwtool attach firmware.bin 1.2.3 packaged.bin

This creates:

packaged.bin = [512-byte header][firmware.bin firmware]

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Attach with a custom header size

fwtool attach firmware.bin 1.2.3 packaged.bin --header-size 1024

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Replace the header of an existing packaged binary

fwtool edit packaged.bin 1.2.4 updated.bin

This keeps the firmware but replaces the header with updated metadata.

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Replace the header in place

fwtool edit packaged.bin 1.2.4 --in-place

This modifies packaged.bin directly.

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Attach a header in place

fwtool attach firmware.bin 1.2.3 --in-place

This replaces the raw input file with a packaged binary containing the header.

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Print header contents

fwtool inspect packaged.bin

Example output:

magic:   b'XLAB'
version: 1.2.3
size:    123456 bytes
crc32:   0x1a2b3c4d

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Print header contents as JSON

fwtool inspect packaged.bin --json

Example output:

{
  "magic_ascii": "XLAB",
  "magic_hex": "584c4142",
  "version": {
    "major": 1,
    "minor": 2,
    "patch": 3,
    "string": "1.2.3"
  },
  "size": 123456,
  "crc": {
    "int": 439041101,
    "hex": "0x1a2b3c4d"
  }
}

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Verify a packaged binary

fwtool verify packaged.bin

Example output:

magic:         OK
version:       1.2.3
header size:   512 bytes
size:          OK (header=123456, actual=123456)
crc32:         OK (header=0x1a2b3c4d, actual=0x1a2b3c4d)
verification:  OK

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Verify quietly using only the exit code

fwtool verify packaged.bin --quiet
echo $?

Exit code meanings:

0: verification passed 1: verification failed

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Verify with JSON output

fwtool verify packaged.bin --json

Example output:

{
  "ok": true,
  "magic_ok": true,
  "size_ok": true,
  "crc_ok": true,
  "header_size": 512,
  "header": {
    "magic_ascii": "XLAB",
    "magic_hex": "584c4142",
    "version": {
      "major": 1,
      "minor": 2,
      "patch": 3,
      "string": "1.2.3"
    },
    "size": 123456,
    "crc": {
      "int": 439041101,
      "hex": "0x1a2b3c4d"
    }
  },
  "firmware": {
    "size": 123456,
    "crc": {
      "int": 439041101,
      "hex": "0x1a2b3c4d"
    }
  }
}

Header size

The header size is configurable and defaults to 512 bytes.

Rules:

• must be a power of two (16, 32, 64, 128, 256, 512, 1024, …)
• must be at least 16 bytes (the minimum to hold the metadata fields)
• ensures the application vector table following the header is correctly aligned
• For edit and verify, if --header-size is not specified, the tool infers the header size from the stored firmware size field:

header_size = file_size - stored_firmware_size

If inference fails (e.g. corrupted size field), use --header-size to specify the size explicitly.

Version handling

Accepted version formats:

• 1 -> 1.0.0
• 1.2 -> 1.2.0
• 1.2.3 -> 1.2.3

Rules:

• missing components are filled with zero
• extra components are ignored
• each component must be in the range 0..255

Verification behavior

When fwtool verify is used, the tool checks:

• the magic field is XLAB
• the firmware size matches the size stored in the header
• the firmware CRC matches the CRC stored in the header
• The header itself is not included in the size or CRC calculation.

Exit codes

General operations

• 0 on success
• non-zero on failure

verify

• 0 if verification succeeds
• 1 if verification fails

Common workflow example

• Build a raw firmware image:

arm-none-eabi-objcopy -O binary app.elf app.bin

• Attach metadata header:

fwtool attach app.bin 1.2.3 app_packed.bin

• Verify the packaged image:

fwtool verify app_packed.bin

• Program the combined image to flash:

st-flash write app_packed.bin 0x08004000

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Notes for embedded use

If your firmware image is packaged with a prepended header, the application must typically be linked to execute after the reserved header region.

Example with the default 512-byte (0x200) header:

• metadata region starts at 0x08004000
• header size is 0x200 (512 bytes)
• application is linked to start at 0x08004200
• Then the combined image can be programmed at 0x08004000, and the application
• firmware will land at the correct runtime address.

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Development

Run tests

pytest -q

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License

MIT License