# ULog 파일 포맷

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).

## 데이터 형식

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

| 형식                                                          | 바이트 크기 |
| ----------------------------------------------------------- | ------ |
| 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.

### 헤더 섹션

헤더는 고정 크기 섹션이며, 다음 형식(16바이트)을 갖습니다.

```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`.
:::

### 정의 섹션

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.
  이 비트는 향후 기존 파서와 호환되는 ULog 변경에 사용할 수 있습니다.
  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.
  이는 알 수 없는 비트 중 하나가 설정되어 있으면, 파서가 해당 비트를 무시할 수 있음을 의미합니다.

- `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.
  이것은 기존 파서가 처리할 수 없는 주요 변경 사항을 도입하는 데 사용할 수 있습니다. 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.
  데이터가 추가되지 않은 경우에는 모든 오프셋은 0이어야 합니다.
  이것은 메시지 중간에 멈출 수 있는 로그에 대한 데이터를 안정적으로 추가할 수 있습니다.
  For example, crash dumps.

  데이터를 추가하는 프로세스는 다음과 같습니다.

  - 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.

향후 ULog 사양에서 이 메시지 끝에 추가 필드가 존재할 수 있습니다.
이것은 파서가 이 메시지의 고정된 길이를 가정해서는 안 된다는 것을 의미합니다.
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).

- 유형은 정의되기 전에 사용할 수 있습니다.
  - 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`

일부 필드 이름은 특별합니다.

- `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.
  - 이 필드는 올바른 정렬을 보장하기 위하여 작성자가 삽입할 수 있습니다.
  - 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.
  - 그러나 메시지가 중첩 정의에서 사용될 때 패딩은 여전히 필요합니다.
- 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.
:::

파서는 정보 메시지를 사전으로 저장할 수 있습니다.

사전 정의된 정보 메시지는 다음과 같습니다.

| 키                                   | 설명                                                      | 예제 값                                                              |
| ----------------------------------- | ------------------------------------------------------- | ----------------------------------------------------------------- |
| `char[value_len] sys_name`          | 시스템 이름                                                  | "PX4"                                                             |
| `char[value_len] ver_hw`            | 하드웨어 버전 (보드)                         | "PX4FMU_V4"                                  |
| `char[value_len] ver_hw_subtype`    | 보드 하위 버전(변형판)                        | "V2"                                                              |
| `char[value_len] ver_sw`            | 소프트웨어 버전(git tag)                    | "7f65e01"                                                         |
| `char[value_len] ver_sw_branch`     | git branch                                              | "master"                                                          |
| `uint32_t ver_sw_release`           | 소프트웨어 버전 (아래 참고)                     | 0x010401ff                                                        |
| `char[value_len] sys_os_name`       | 운영체제 이름                                                 | "Linux"                                                           |
| `char[value_len] sys_os_ver`        | 운영체제 버전 (git tag)                    | "9f82919"                                                         |
| `uint32_t ver_os_release`           | 운영체제 버전 (아래 참고)                      | 0x010401ff                                                        |
| `char[value_len] sys_toolchain`     | 툴체인 이름                                                  | "GNU GCC"                                                         |
| `char[value_len] sys_toolchain_ver` | 툴체인 버전                                                  | "6.2.1"                           |
| `char[value_len] sys_mcu`           | 칩 이름과 버전                                                | "STM32F42x, rev A"                                                |
| `char[value_len] sys_uuid`          | 차량 고유 식별자(예: MCU ID) | "392a93e32fa3"... |
| `char[value_len] log_type`          | 로그 형식(지정하지 않으면 전체 기록)                | "mission"                                                         |
| `char[value_len] replay`            | 재생 모드인 경우 재생된 로그의 파일 이름                                 | "log001.ulg"                                      |
| `int32_t time_ref_utc`              | UTC 시간 오프셋(초)                        | -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.

이 메시지는 데이터 섹션에서도 사용할 수 있습니다(그러나 선호하는 섹션임).

#### '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.

파서는 다중 메시지를 로그에서 발생하는 메시지와 동일한 순서를 사용하여 2D 목록으로 저장할 수 있습니다.

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.
  - 최소한 하나의 비트가 설정되어야 합니다.
    - `1<<0`: system wide default
    - `1<<1`: default for the current configuration (e.g. an airframe)

로그에는 모든 매개변수에 대한 기본값이 포함되어 있지 않을 수 있습니다.
이러한 경우 기본값은 매개변수 값과 같고, 다른 기본 유형은 독립적으로 처리됩니다.

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.

### 데이터 섹션

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. 기본 및 첫 번째 인스턴스는 0이어야 합니다.
- `msg_id`: unique id to match [Logged data Message](#d-logged-data-message) data. 처음 사용할 때는 이것을 0으로 설정한 다음 증가시켜야 합니다.
  - 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)

패딩 필드의 특수 처리에 대해서는 위를 참고하십시오.

#### '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:

| 명칭      | 레벨  | 설명            |
| ------- | --- | ------------- |
| EMERG   | '0' | 시스템 사용 불가     |
| ALERT   | '1' | 즉시 조치         |
| CRIT    | '2' | 임계 조건         |
| ERR     | '3' | 오류 조건         |
| WARNING | '4' | 경고 조건         |
| NOTICE  | '5' | 정상적이지만 중요한 상태 |
| INFO    | '6' | 정보 제공         |
| DEBUG   | '7' | 디버그 수준 메시지    |

#### '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. 시스템 아키텍처에 따라 프로세스, 스레드 또는 클래스를 나타낼 수 있습니다.

  - 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:

| 명칭      | 레벨  | 설명            |
| ------- | --- | ------------- |
| EMERG   | '0' | 시스템 사용 불가     |
| ALERT   | '1' | 즉시 조치         |
| CRIT    | '2' | 임계 조건         |
| ERR     | '3' | 오류 조건         |
| WARNING | '4' | 경고 조건         |
| NOTICE  | '5' | 정상적이지만 중요한 상태 |
| INFO    | '6' | 정보 제공         |
| DEBUG   | '7' | 디버그 수준 메시지    |

#### '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.

장치가 충분히 빠르지 않은 경우에는 손실이 발생할 수 있습니다.

```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)

## 파서 요구 사항

유효한 ULog 파서는 요구 사항은 다음과 같습니다.

- Must ignore unknown messages (but it can print a warning)
- 미래의/알 수 없는 파일 형식 버전도 구문 분석합니다(하지만 경고를 인쇄할 수 있음).
- 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.
- 파서는 메시지 중간에 갑자기 끝나는 로그를 올바르게 처리할 수 있어야 합니다.
  완료되지 않은 메시지는 무시하여야 합니다.
- 추가된 데이터의 경우: 파서는 데이터 섹션이 존재한다고 가정할 수 있습니다. 즉 오프셋은 정의 섹션 뒤의 위치를 가리킵니다.
  - 추가된 데이터는 일반 데이터 섹션의 일부인 것처럼 처리하여야 합니다.

## Known Parser Implementations

- PX4-오토파일럿: 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. 버전 2.1.3부터 ULog를 지원합니다.
- [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.

## 파일 형식 버전 이력

### 버전 2의 변경 사항

- 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.
  - 기존 로그에 충돌 데이터를 추가하는 데 사용됩니다.
  - 메시지 중간에 잘린 로그에 데이터가 추가되면, 버전 1 파서로 파싱할 수 없습니다.
- 그 외의 파서가 알 수 없는 메시지를 무시하면, 순방향 및 역방향 호환성이 제공됩니다.