Development Guide

ULog File Format

ULog is the file format used for logging system data. The format is self-describing, i.e. it contains the format and message types that are logged.

The format uses Little Endian for all binary types.

Data types

The following binary types are used. 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 all can be used as an array, eg. float[5]. In general all strings (char[length]) do not contain a '\0' at the end. String comparisons are case sensitive.

File structure

The file consists of three sections:

|       Header       |
|    Definitions     |
|        Data        |

Header Section

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

| 0x55 0x4c 0x6f 0x67 0x01 0x12 0x35 | 0x00         | uint64_t       |
| File magic (7B)                    | Version (1B) | Timestamp (8B) |

Version is the file format version, currently 0. Timestamp is an uint64_t integer, denotes the start of the logging in microseconds.

Definitions Section

Variable length section, contains version information, format definitions, and (initial) parameter values.

The Definitions and Data sections consist of a stream of messages. Each starts with 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 (hdr_size= 3 bytes). msg_type defines the content and is one of the following:

  • 'F': format definition for a single (composite) type that can be logged or used in another definition as a nested type.

    struct message_format_s {
      struct message_header_s header;
      char format[header.msg_size-hdr_size];

    format: plain-text string with the following format: message_name:field0;field1; There can be an arbitrary amount of fields (at least 1), separated by ;. A field has the format: type field_name or type[array_length] field_name for arrays (only fixed size arrays are supported). type is one of the basic binary types or a message_name of another format definition (nested usage). A type can be used before it's defined. There can be arbitrary nesting but no circular dependencies.

    Some field names are special:

    • timestamp: every logged message (message_add_logged_s) must include a timestamp field (does not need to be the first field). Its type can be: uint64_t (currently the only one used), uint32_t, uint16_t or uint8_t. The unit is always microseconds, except for uint8_t it's milliseconds. A log writer must make sure to log messages often enough to be able to detect wrap-arounds and a log reader must handle wrap-arounds (and take into account dropouts). The timestamp must always be monotonic increasing for a message serie with the same msg_id.
    • Padding: field names that start with _padding 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 will not be logged, to avoid writing unnecessary data. This means the will be shorter by the size of the padding. However the padding is still needed when the message is used in a nested definition.

  • 'I': information message.

    struct message_info_s {
      struct message_header_s header;
      uint8_t key_len;
      char key[key_len];
      char value[header.msg_size-hdr_size-1-key_len]

    key is a plain string, as in the format message, but consists of only a single field without ending ;, eg. float[3] myvalues. value contains the data as described by key.

    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 "PX4FMU_V4"
char[value_len] ver_sw Software version "7f65e01"
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
  • 'P': parameter message. Same format as message_info_s. If a parameter dynamically changes during runtime, this message can also be used in the Data section. The data type is restricted to: int32_t, float.

This section ends before the start of the first message_add_logged_s message.

Data Section

The following messages belong to this section:

  • 'A': subscribe a message by name and give it an id that is used in message_data_s. This must come before the first corresponding message_data_s.

    struct message_add_logged_s {
      struct message_header_s header;
      uint8_t multi_id;
      uint16_t msg_id;
      char message_name[header.msg_size-hdr_size-3];

    multi_id: the same message format can have multiple instances, given by multi_id. The default and first instance is 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.

  • 'R': 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;
      uint16_t msg_id;
  • 'D': contains logged data.

    struct message_data_s {
      struct message_header_s header;
      uint16_t msg_id;
      uint8_t data[header.msg_size-hdr_size];

    msg_id: as defined by a message_add_logged_s message. data contains the logged binary message as defined by message_format_s. See above for special treatment of padding fields.

  • 'S': synchronization message so that a reader can recover from a corrupt message by search for the next sync message (not used currently).

    struct message_sync_s {
      struct message_header_s header;
      uint8_t sync_magic[8];

    sync_magic: to be defined.

  • 'O': mark a dropout (lost logging messages) of a given duration in ms. Dropouts can occur eg if the device is not fast enough.

    struct message_dropout_s {
      struct message_header_s header;
      uint16_t duration;