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ULog 文件格式

ULog is the file format used for logging messages. The format is self-describing, i.e. it contains the format and uORB 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 memory layout for all binary types (the least significant byte (LSB) of data type is placed at the lowest memory address).


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

int8_t, uint8_t1
int16_t, uint16_t2
int32_t, uint32_t4
int64_t, uint64_t8
bool, char1

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.

String comparisons are case sensitive, which should be taken into account when comparing message names when adding subscriptions.

ULog File Structure

ULog files have the following three sections:

|         头         |
|        定义        |
|        数据        |

A description of each section is provided below.



| 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:

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.

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


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
  2. Format Definition
  3. Information
  4. Multi Information
  5. Parameter
  6. Default Parameter

'B': Flag Bits Message

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.

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

    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: 不兼容的标志位。

    • 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. 这可用于引入现有解析器无法处理的重大更改。 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. 如果没有附加数据,则所有偏移量必须为零。 这可以用于消息中途暂停的情况下可靠的添加数据。 For example, crash dumps.


    • 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 message is longer than expected (currently 40 bytes), the exceeding bytes must just be ignored. 这意味着解析器必须不能假定此消息的长度是固定的。 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.

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).

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 or a message_name of another format definition (nested usage).

  • 一个类型可以在定义之前使用。
    • 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


  • timestamp: every message format with 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)
    • 它的类型可以是:uint64_t (目前唯一使用的),uint32_t, uint16_t 或者是 uint8_t
    • The unit is microseconds.
    • 对于具有相同 msg_id 报文的时间戳必须是单调递增的。
  • _padding{}: field names that start with _padding (e.g. _padding[3]) should not be displayed and their data must be ignored by a reader.
    • 写入器可以通过插入这个字段确保正确对齐。
    • If the padding field is the last field, then this field may not be logged, to avoid writing unnecessary data.
    • 这意味着 会因为填充大小而更短。
    • 但是当报文在嵌套定义中使用时任然需要填充。
  • 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.

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 formtype 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.
  • value: Contains the data (with the length value_len) corresponding to the key e.g. 9.4.0.


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



char[value_len] sys_name系统名称"PX4"
char[value_len] ver_hw硬件版本 (主板)"PX4FMU_V4"
char[value_len] ver_hw_subtype主办子版本 (变化的)"V2"
char[value_len] ver_sw软件版本 (git 标签)"7f65e01"
char[value_len] ver_sw_branchgit branch"master"
uint32_t ver_sw_release软件版本 (见下文)0x010401ff
char[value_len] sys_os_name操作系统名称"Linux"
char[value_len] sys_os_ver操作系统版本 (git 标签)"9f82919"
uint32_t ver_os_release操作系统版本 (见下文)0x010401ff
char[value_len] sys_toolchain工具链名称"GNU GCC"
char[value_len] sys_toolchain_ver工具链版本"6.2.1"
char[value_len] sys_mcu芯片名称和修订"STM32F42x, rev A"
char[value_len] sys_uuidUnique identifier for vehicle (eg. MCU ID)"392a93e32fa3"...
char[value_len] log_typeType of the log (full log if not specified)"mission"
char[value_len] replay重播日志的文件名如果处于重播模式"log001.ulg"
int32_t time_ref_utcUTC 时间的秒偏移量-3600

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.

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.

解析器可以将所有多报文信息存储为一个 2D 列表,使用与日志中报文相同的顺序。

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.

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 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.

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 or Logging message, whichever comes first.


The message types in the Data section are:

  1. Subscription
  2. Unsubscription
  3. Logged Data
  4. Logged String
  5. Tagged Logged String
  6. Synchronization
  7. Dropout Mark
  8. Information
  9. Multi Information
  10. Parameter
  11. Default Parameter

A: Subscription Message

Subscribe a message by name and give it an id that is used in Logged data Message. This must come before the first corresponding Logged data Message.

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 message_data_s data. The first use must set this to 0, then increase it. The same msg_id must not be used twice for different subscriptions, not even after unsubscribing. message_name: message name to subscribe to. Must match one of the message_format_s definitions. 默认值以及第一个实例一定是0.
  • msg_id: unique id to match Logged data Message data. 第一次使用一定要设置为 0,然后递增。
    • The same msg_id must not be used twice for different subscriptions.
  • msg_name:订阅的消息名称。 Must match one of the Format Message definitions.

R: Unsubscription Message

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

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

'D': Logged Data Message

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


'L': Logged String Message

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

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:

'C': Tagged Logged String Message

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:
    enum class ulog_tag : uint16_t {
  • timestamp: in microseconds

  • log_level: same as in the Linux kernel:


'S': Synchronization message

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

struct message_sync_s {
  struct message_header_s header; // msg_type = 'S'
  uint8_t sync_magic[8];
  • sync_magic: to be defined.

'O': Dropout message

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


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:


一个有效的 ULog 解析器必须满足以下要求:

  • Must ignore unknown messages (but it can print a warning)
  • 解析未来/未知的文件格式版本 (但可以打印警告) 。
  • Must refuse to parse a log which contains unknown incompatibility bits set (incompat_flags of 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 discarged. 未完成的报文应该丢弃。
  • 对于附加数据:解析器可以假设数据部分存在,即在定义部分之后的位置有一个偏移点。
    • 必须将附加数据视为常规数据部分的一部分。

Known Parser Implementations


版本 2 中的改变

  • Addition of Multi Information Message and Flag Bits Message and the ability to append data to a log.
    • 这被用来给现有的日志添加损坏的数据。
    • 如果从中间切开的报文数据被附加到日志中,这不能被版本 1 解析器解析。
  • 除此之外,如果解析器忽略未知消息,则提供向前和向后的兼容性。