# ULog File Format

ULog is the file format used for logging messages. The format is self-describing, i.e. it contains the format and [uORB](../middleware/uorb.md) message types that are logged.
This document is meant to be the ULog File Format Spec Documentation.
It is intended especially for anyone who is interested in writing a ULog parser / serializer and needs to decode / encode files.

PX4 uses ULog to log uORB topics as messages related to (but not limited to) the following sources:

- **Device inputs:** Sensors, RC input, etc.
- **Internal states:** CPU load, attitude, EKF state, etc.
- **String messages:** `printf` statements, including `PX4_INFO()` and `PX4_ERR()`.

The format uses [little endian](https://en.wikipedia.org/wiki/Endianness) memory layout for all binary types (the least significant byte (LSB) of data type is placed at the lowest memory address).

## Data types

The following binary types are used for logging. They all correspond to the types in C.

| Type              | Size in Bytes |
| ----------------- | ------------- |
| int8_t, uint8_t   | 1             |
| int16_t, uint16_t | 2             |
| int32_t, uint32_t | 4             |
| int64_t, uint64_t | 8             |
| float             | 4             |
| double            | 8             |
| bool, char        | 1             |

Additionally the types can be used as a fixed-size array: e.g. `float[5]`.

Strings (`char[length]`) do not contain the termination NULL character `'\0'` at the end.

::: info
String comparisons are case sensitive, which should be taken into account when comparing message names when [adding subscriptions](#a-subscription-message).
:::

## ULog File Structure

ULog files have the following three sections:

```
----------------------
|       Header       |
----------------------
|    Definitions     |
----------------------
|        Data        |
----------------------
```

A description of each section is provided below.

### Header Section

The header is a fixed-size section and has the following format (16 bytes):

```plain
----------------------------------------------------------------------
| 0x55 0x4c 0x6f 0x67 0x01 0x12 0x35 | 0x01         | uint64_t       |
| File magic (7B)                    | Version (1B) | Timestamp (8B) |
----------------------------------------------------------------------
```

- **File Magic (7 Bytes):** File type indicator that reads "ULogXYZ where XYZ is the magic bytes sequence `0x01 0x12 0x35`"
- **Version (1 Byte):** File format version (currently 1)
- **Timestamp (8 Bytes):** `uint64_t` integer that denotes when the logging started in microseconds.

### Definition & Data Section Message Header

The _Definitions and Data_ sections contain a number of **messages**. Each message is preceded by this header:

```c
struct message_header_s {
  uint16_t msg_size;
  uint8_t msg_type;
};
```

- `msg_size` is the size of the message in bytes without the header.
- `msg_type` defines the content, and is a single byte.

::: info
Message sections below are prefixed with the character that corresponds to it's `msg_type`.
:::

### Definitions Section

The definitions section contains basic information such as software version, message format, initial parameter values, and so on.

The message types in this section are:

1. [Flag Bits](#b-flag-bits-message)
2. [Format Definition](#f-format-message)
3. [Information](#i-information-message)
4. [Multi Information](#m-multi-information-message)
5. [Parameter](#p-parameter-message)
6. [Default Parameter](#q-default-parameter-message)

#### 'B': Flag Bits Message

::: info
This message must be the **first message** right after the header section, so that it has a fixed constant offset from the start of the file!
:::

This message provides information to the log parser whether the log is parsable or not.

```c
struct ulog_message_flag_bits_s {
  struct message_header_s header; // msg_type = 'B'
  uint8_t compat_flags[8];
  uint8_t incompat_flags[8];
  uint64_t appended_offsets[3]; // file offset(s) for appended data if appending bit is set
};
```

- `compat_flags`: compatible flag bits
  - These flags indicate the presence of features in the log file that are compatible with any ULog parser.
  - `compat_flags[0]`: _DEFAULT_PARAMETERS_ (Bit 0): if set, the log contains [default parameters message](#q-default-parameter-message)

  The rest of the bits are currently not defined and must be set to 0.
  These bits can be used for future ULog changes that are compatible with existing parsers.
  For example, adding a new message type can be indicated by defining a new bit in the standard, and existing parsers will ignore the new message type.
  It means parsers can just ignore the bits if one of the unknown bits is set.

- `incompat_flags`: incompatible flag bits.
  - `incompat_flags[0]`: _DATA_APPENDED_ (Bit 0): if set, the log contains appended data and at least one of the `appended_offsets` is non-zero.

  The rest of the bits are currently not defined and must be set to 0.
  This can be used to introduce breaking changes that existing parsers cannot handle. For example, when an old ULog parser that didn't have the concept of _DATA_APPENDED_ reads the newer ULog, it would stop parsing the log as the log will contain out-of-spec messages / concepts.
  If a parser finds any of these bits set that isn't specified, it must refuse to parse the log.

- `appended_offsets`: File offset (0-based) for appended data.
  If no data is appended, all offsets must be zero.
  This can be used to reliably append data for logs that may stop in the middle of a message.
  For example, crash dumps.

  A process appending data should do:
  - set the relevant `incompat_flags` bit
  - set the first `appended_offsets` that is currently 0 to the length of the log file without the appended data, as that is where the new data will start
  - append any type of messages that are valid for the Data section.

It is possible that there are more fields appended at the end of this message in future ULog specifications.
This means a parser must not assume a fixed length of this message.
If the `msg_size` is bigger than expected (currently 40), any additional bytes must be ignored/discarded.

#### 'F': Format Message

Format message defines a single message name and its inner fields in a single string.

```c
struct message_format_s {
  struct message_header_s header; // msg_type = 'F'
  char format[header.msg_size];
};
```

- `format` is a plain-text string with the following format: `message_name:field0;field1;`
  - There can be an arbitrary amount of fields (minimum 1), separated by `;`.
  - `message_name`: an arbitrary non-empty string with these allowed characters: `a-zA-Z0-9_-/` (and different from any of the [basic types](#data-types)).

A `field` has the format: `type field_name`, or for an array: `type[array_length] field_name` is used (only fixed size arrays are supported).
`field_name` must consist of the characters in the set `a-zA-Z0-9_`.

A `type` is one of the [basic binary types](#data-types) or a `message_name` of another format definition (nested usage).

- A type can be used before it's defined.
  - e.g. The message `MessageA:MessageB[2] msg_b` can come before the `MessageB:uint_8[3] data`
- There can be arbitrary nesting but **no circular dependencies**
  - e.g. `MessageA:MessageB[2] msg_b` & `MessageB:MessageA[4] msg_a`

Some field names are special:

- `timestamp`: every message format with a [Subscription Message](#a-subscription-message) must include a timestamp field (for example a message format only used as part of a nested definition by another format may not include a timestamp field)
  - Its type must be `uint64_t`.
  - The unit is microseconds.
  - The timestamp must always be monotonic increasing for a message series with the same `msg_id` (same subscription).
- `_padding{}`: field names that start with `_padding` (e.g. `_padding[3]`) should not be displayed and their data must be ignored by a reader.
  - These fields can be inserted by a writer to ensure correct alignment.
  - If the padding field is the last field, then this field may not be logged, to avoid writing unnecessary data.
  - This means the `message_data_s.data` will be shorter by the size of the padding.
  - However the padding is still needed when the message is used in a nested definition.
- In general, message fields are not necessarily aligned (i.e. the field offset within the message is not necessarily a multiple of its data size), so a reader must always use appropriate memory copy methods to access individual fields.

#### 'I': Information Message

The Information message defines a dictionary type definition `key` : `value` pair for any information, including but not limited to Hardware version, Software version, Build toolchain for the software, etc.

```c
struct ulog_message_info_header_s {
  struct message_header_s header; // msg_type = 'I'
  uint8_t key_len;
  char key[key_len];
  char value[header.msg_size-1-key_len]
};
```

- `key_len`: Length of the key value
- `key`: Contains the key string in the form`type name`, e.g. `char[value_len] sys_toolchain_ver`. Valid characters for the name: `a-zA-Z0-9_-/`. The type may be one of the [basic types including arrays](#data-types).
- `value`: Contains the data (with the length `value_len`) corresponding to the `key` e.g. `9.4.0`.

::: info
A key defined in the Information message must be unique. Meaning there must not be more than one definition with the same key value.
:::

Parsers can store information messages as a dictionary.

Predefined information messages are:

| key                                 | Description                                 | Example for value  |
| ----------------------------------- | ------------------------------------------- | ------------------ |
| `char[value_len] sys_name`          | Name of the system                          | "PX4"              |
| `char[value_len] ver_hw`            | Hardware version (board)                    | "PX4FMU_V4"        |
| `char[value_len] ver_hw_subtype`    | Board subversion (variation)                | "V2"               |
| `char[value_len] ver_sw`            | Software version (git tag)                  | "7f65e01"          |
| `char[value_len] ver_sw_branch`     | git branch                                  | "master"           |
| `uint32_t ver_sw_release`           | Software version (see below)                | 0x010401ff         |
| `char[value_len] sys_os_name`       | Operating System Name                       | "Linux"            |
| `char[value_len] sys_os_ver`        | OS version (git tag)                        | "9f82919"          |
| `uint32_t ver_os_release`           | OS version (see below)                      | 0x010401ff         |
| `char[value_len] sys_toolchain`     | Toolchain Name                              | "GNU GCC"          |
| `char[value_len] sys_toolchain_ver` | Toolchain Version                           | "6.2.1"            |
| `char[value_len] sys_mcu`           | Chip name and revision                      | "STM32F42x, rev A" |
| `char[value_len] sys_uuid`          | Unique identifier for vehicle (eg. MCU ID)  | "392a93e32fa3"...  |
| `char[value_len] log_type`          | Type of the log (full log if not specified) | "mission"          |
| `char[value_len] replay`            | File name of replayed log if in replay mode | "log001.ulg"       |
| `int32_t time_ref_utc`              | UTC Time offset in seconds                  | -3600              |

::: info
`value_len` represents the data size of the `value`. This is described in the `key`.
:::

- The format of `ver_sw_release` and `ver_os_release` is: 0xAABBCCTT, where AA is **major**, BB is **minor**, CC is patch and TT is the **type**.
  - **Type** is defined as following: `>= 0`: development, `>= 64`: alpha version, `>= 128`: beta version, `>= 192`: RC version, `== 255`: release version.
  - For example, `0x010402FF` translates into the release version v1.4.2.

This message can also be used in the Data section (this is however the preferred section).

#### 'M': Multi Information Message

Multi information message serves the same purpose as the information message, but for long messages or multiple messages with the same key.

```c
struct ulog_message_info_multiple_header_s {
  struct message_header_s header; // msg_type = 'M'
  uint8_t is_continued; // can be used for arrays
  uint8_t key_len;
  char key[key_len];
  char value[header.msg_size-2-key_len]
};
```

- `is_continued` can be used for split-up messages: if set to 1, it is part of the previous message with the same key.

Parsers can store all information multi messages as a 2D list, using the same order as the messages occur in the log.

Valid names and types are the same as for the Information message.

#### 'P': Parameter Message

Parameter message in the _Definitions_ section defines the parameter values of the vehicle when logging is started. It uses the same format as the [Information Message](#i-information-message).

```c
struct message_info_s {
  struct message_header_s header; // msg_type = 'P'
  uint8_t key_len;
  char key[key_len];
  char value[header.msg_size-1-key_len]
};
```

If a parameter dynamically changes during runtime, this message can also be [used in the Data section](#messages-shared-with-the-definitions-section) as well.

The data type is restricted to `int32_t` and `float`. Valid characters for the name: `a-zA-Z0-9_-/`.

#### 'Q': Default Parameter Message

The default parameter message defines the default value of a parameter for a given vehicle and setup.

```c
struct ulog_message_parameter_default_header_s {
  struct message_header_s header; // msg_type = 'Q'
  uint8_t default_types;
  uint8_t key_len;
  char key[key_len];
  char value[header.msg_size-2-key_len]
};
```

- `default_types` is a bitfield and defines to which group(s) the value belongs to.
  - At least one bit must be set:
    - `1<<0`: system wide default
    - `1<<1`: default for the current configuration (e.g. an airframe)

A log may not contain default values for all parameters.
In those cases the default is equal to the parameter value, and different default types are treated independently.

This message can also be used in the Data section, and the same data type and naming applies as for the Parameter message.

This section ends before the start of the first [Subscription Message](#a-subscription-message) or [Logging](#l-logged-string-message) message, whichever comes first.

### Data Section

The message types in the _Data_ section are:

1. [Subscription](#a-subscription-message)
2. [Unsubscription](#r-unsubscription-message)
3. [Logged Data](#d-logged-data-message)
4. [Logged String](#l-logged-string-message)
5. [Tagged Logged String](#c-tagged-logged-string-message)
6. [Synchronization](#s-synchronization-message)
7. [Dropout Mark](#o-dropout-message)
8. [Information](#i-information-message)
9. [Multi Information](#m-multi-information-message)
10. [Parameter](#p-parameter-message)
11. [Default Parameter](#q-default-parameter-message)

#### `A`: Subscription Message

Subscribe a message by name and give it an id that is used in [Logged data Message](#d-logged-data-message).
This must come before the first corresponding [Logged data Message](#d-logged-data-message).

```c
struct message_add_logged_s {
  struct message_header_s header; // msg_type = 'A'
  uint8_t multi_id;
  uint16_t msg_id;
  char message_name[header.msg_size-3];
};
```

- `multi_id`: the same message format can have multiple instances, for example if the system has two sensors of the same type. The default and first instance must be 0.
- `msg_id`: unique id to match [Logged data Message](#d-logged-data-message) data. The first use must set this to 0, then increase it.
  - The same `msg_id` must not be used twice for different subscriptions.
- `message_name`: message name to subscribe to.
  Must match one of the [Format Message](#f-format-message) definitions.

#### `R`: Unsubscription Message

Unsubscribe a message, to mark that it will not be logged anymore (not used currently).

```c
struct message_remove_logged_s {
  struct message_header_s header; // msg_type = 'R'
  uint16_t msg_id;
};
```

#### 'D': Logged Data Message

```c
struct message_data_s {
  struct message_header_s header; // msg_type = 'D'
  uint16_t msg_id;
  uint8_t data[header.msg_size-2];
};
```

- `msg_id`: as defined by a [Subscription Message](#a-subscription-message)
- `data` contains the logged binary message as defined by [Format Message](#f-format-message)

See above for special treatment of padding fields.

#### 'L': Logged String Message

Logged string message, i.e. `printf()` output.

```c
struct message_logging_s {
  struct message_header_s header; // msg_type = 'L'
  uint8_t log_level;
  uint64_t timestamp;
  char message[header.msg_size-9]
};
```

- `timestamp`: in microseconds
- `log_level`: same as in the Linux kernel:

| Name    | Level value | Meaning                          |
| ------- | ----------- | -------------------------------- |
| EMERG   | '0'         | System is unusable               |
| ALERT   | '1'         | Action must be taken immediately |
| CRIT    | '2'         | Critical conditions              |
| ERR     | '3'         | Error conditions                 |
| WARNING | '4'         | Warning conditions               |
| NOTICE  | '5'         | Normal but significant condition |
| INFO    | '6'         | Informational                    |
| DEBUG   | '7'         | Debug-level messages             |

#### 'C': Tagged Logged String Message

```c
struct message_logging_tagged_s {
  struct message_header_s header; // msg_type = 'C'
  uint8_t log_level;
  uint16_t tag;
  uint64_t timestamp;
  char message[header.msg_size-11]
};
```

- `tag`: id representing source of logged message string. It could represent a process, thread or a class depending upon the system architecture.
  - For example, a reference implementation for an onboard computer running multiple processes to control different payloads, external disks, serial devices etc can encode these process identifiers using a `uint16_t enum` into the `tag` attribute of struct as follows:

  ```c
  enum class ulog_tag : uint16_t {
    unassigned,
    mavlink_handler,
    ppk_handler,
    camera_handler,
    ptp_handler,
    serial_handler,
    watchdog,
    io_service,
    cbuf,
    ulg
  };
  ```

- `timestamp`: in microseconds
- `log_level`: same as in the Linux kernel:

| Name    | Level value | Meaning                          |
| ------- | ----------- | -------------------------------- |
| EMERG   | '0'         | System is unusable               |
| ALERT   | '1'         | Action must be taken immediately |
| CRIT    | '2'         | Critical conditions              |
| ERR     | '3'         | Error conditions                 |
| WARNING | '4'         | Warning conditions               |
| NOTICE  | '5'         | Normal but significant condition |
| INFO    | '6'         | Informational                    |
| DEBUG   | '7'         | Debug-level messages             |

#### 'S': Synchronization message

Synchronization message so that a reader can recover from a corrupt message by searching for the next sync message.

```c
struct message_sync_s {
  struct message_header_s header; // msg_type = 'S'
  uint8_t sync_magic[8];
};
```

- `sync_magic`: [0x2F, 0x73, 0x13, 0x20, 0x25, 0x0C, 0xBB, 0x12]

#### 'O': Dropout message

Mark a dropout (lost logging messages) of a given duration in ms.

Dropouts can occur e.g. if the device is not fast enough.

```c
struct message_dropout_s {
  struct message_header_s header; // msg_type = 'O'
  uint16_t duration;
};
```

#### Messages shared with the Definitions Section

Since the Definitions and Data Sections use the same message header format, they also share the same messages listed below:

- [Information Message](#i-information-message).
- [Multi Information Message](#m-multi-information-message)
- [Parameter Message](#p-parameter-message)
  - For the _Data_ section, the Parameter Message is used when the parameter value changes
- [Default Parameter Message](#q-default-parameter-message)

## Requirements for Parsers

A valid ULog parser must fulfill the following requirements:

- Must ignore unknown messages (but it can print a warning)
- Parse future/unknown file format versions as well (but it can print a warning).
- Must refuse to parse a log which contains unknown incompatibility bits set (`incompat_flags` of [Flag Bits Message](#b-flag-bits-message)), meaning the log contains breaking changes that the parser cannot handle.
- A parser must be able to correctly handle logs that end abruptly, in the middle of a message.
  The unfinished message should just be discarded.
- For appended data: a parser can assume the Data section exists, i.e. the offset points to a place after the Definitions section.
  - Appended data must be treated as if it was part of the regular Data section.

## Known Parser Implementations

- PX4-Autopilot: C++
  - [logger module](https://github.com/PX4/PX4-Autopilot/tree/main/src/modules/logger)
  - [replay module](https://github.com/PX4/PX4-Autopilot/tree/main/src/modules/replay)
  - [hardfault_log module](https://github.com/PX4/PX4-Autopilot/tree/main/src/systemcmds/hardfault_log): append hardfault crash data.
- [pyulog](https://github.com/PX4/pyulog): python, ULog reader and writer library with CLI scripts.
  The project also has tools to convert ULog to rosbag and other formats.
- [ulog_cpp](https://github.com/PX4/ulog_cpp): C++, ULog reader and writer library.
- [FlightPlot](https://github.com/PX4/FlightPlot): Java, log plotter.
- [MAVLink](https://github.com/mavlink/mavlink): Messages for ULog streaming via MAVLink (note that appending data is not supported, at least not for cut off messages).
- [QGroundControl](https://github.com/mavlink/qgroundcontrol): C++, ULog streaming via MAVLink and minimal parsing for GeoTagging.
- [mavlink-router](https://github.com/mavlink-router/mavlink-router): C++, ULog streaming via MAVLink.
- [MAVGAnalysis](https://github.com/ecmnet/MAVGCL): Java, ULog streaming via MAVLink and parser for plotting and analysis.
- [PlotJuggler](https://github.com/facontidavide/PlotJuggler): C++/Qt application to plot logs and time series. Supports ULog since version 2.1.3.
- [ulogreader](https://github.com/maxsun/ulogreader): Javascript, ULog reader and parser outputs log in JSON object format.
- [Foxglove](https://foxglove.dev): an integrated visualization and diagnosis tool for robotics data that supports ULog files.
- [TypeScript ULog parser](https://github.com/foxglove/ulog): TypeScript, ULog reader that outputs JS objects.
- [yule_log](https://crates.io/crates/yule_log): A streaming ULog parser written in Rust.

## File Format Version History

### Changes in version 2

- Addition of [Multi Information Message](#m-multi-information-message) and [Flag Bits Message](#b-flag-bits-message) and the ability to append data to a log.
  - This is used to add crash data to an existing log.
  - If data is appended to a log that is cut in the middle of a message, it cannot be parsed with version 1 parsers.
- Other than that forward and backward compatibility is given if parsers ignore unknown messages.
