Interface definition using .msg / .srv / .action files

Authors: Dirk Thomas

Date Written: 2019-03

Last Modified: 2021-02

Scope

This article specifies the file format coming from ROS 1 describing the data structures which are being used to exchange information between components. The data structures are defined in a programming language agnostic way. The format is based on the .msg format definition from ROS 1.

Below only the mapping to IDL types is described. Please see the Interface Definition and Language Mapping article for the mappings to programming language specific types and API.

Overview

A data structure is defined by a set of fields. The order of the fields is irrelevant. Each field is described by a type and a name.

Messages

A single data structure is called message. Each message has a name. Together with the name of the package a message can be uniquely identified.

Services

For request / reply style communication the two exchanged data structures are related. These pairs of data structures are called services. A service is identified by its name and the package it is in. Each service describes two messages, one for the request data structure, one for the reply data structure.

Actions

For longer running request / reply style communication with feedback about the progress the exchanged data structures are related. These triplets of data structures are called actions. An action is identified by its name and the package it is in. Each action describes three messages, one for the goal data structure, one for the result data structure, and one for the feedback data structure.

Field types

The type of a field can be either a primitive type or another data structure. Each of these can optionally be a dynamically or statically sized array.

Primitive field types

The following primitive types are defined:

• bool
• byte
• char
• float32, float64
• int8, uint8
• int16, uint16
• int32, uint32
• int64, uint64
• string

Non-primitive field types

Beside the primitive types other messages can be referenced to describe the type of a “complex” field. A complex field type is identified by a package and a message name.

Arrays with static size

A static array has exactly N elements of the specified type. N must be greater than 0.

Arrays with dynamic size

A dynamic array can have between 0 and N elements of the specified type. N might not have an upper bound and may only be limited by the memory or other system specific limitations.

Upper boundaries

The size of strings as well as dynamic arrays can be limited with an upper boundary. This enables the preallocation of memory for data structures which use dynamically sized data.

Default values

A field can optionally specify a default value. If no default value is specified a common default value is used:

• for bool it is false
• for numeric types it is a 0 value
• for string it is an empty string
• for static size arrays it is an array of N elements with its fields zero-initialized
• for bounded size arrays and dynamic size arrays it is an empty array []

Array default values

A field of type array can optionally specify a default value.

• default values for an array must start with an opening square bracket ([) and end with a closing square bracket (])
• each value within the array must be separated with a comma (,)
• all values in the array must be of the same type as the field
• there cannot be a comma , before the first value of the array
• a trailing comma after the last element of the array is ignored

Additional rule for string arrays:

• string arrays must contain only strings respecting the following rules:
  • a string value which can optionally be quoted with either single quotes (') or double quotes (")
  • a double-quoted (") string (respectively single-quoted (')) should have any inner double quotes (respectively single quotes) escaped

Constants

Constants are defined by a primitive type, a name, and a fixed value. If the primitive type is an integer type, the fixed value can be specified in one of the following ways:

• a decimal integer, e.g. 1
• a binary integer, e.g. 0b01 or 0B01
• an octal integer, e.g. 0o01 or 0O01
• a hexadecimal integer, e.g. 0x01 or 0X01

Conventions

Naming of messages and services

Each file contains a single message or service. Message files use the extension .msg, service files use the extension .srv.

Both file names must use an upper camel case name and only consist of alphanumeric characters.

Naming of fields

Field names must be lowercase alphanumeric characters with underscores for separating words. They must start with an alphabetic character, they must not end with an underscore and never have two consecutive underscores.

Naming of constants

Constant names must be uppercase alphanumeric characters with underscores for separating words. They must start with an alphabetic character, they must not end with an underscore and never have two consecutive underscores.

Syntax

The message and service definitions are text files.

Comments

The character # starts a comment, which terminates at the end of the line on which it occurs.

Message file format

A line can either contain a field definition or a constant definition. While a single space is mandatory to separate tokens additional spaces can be inserted optionally between tokens.

Field definition

A field definition has the following structure:

<type> <name> <optional_default_value>

Constant definition

A constant definition has the following structure:

<type> <name>=<value>

Types

A <type> is defined by its base type and optional array specifier.

The base type can be one of the following:

• a primitive type from the list above: e.g. int32

• a string with an upper boundary: string<=N to limit the length of the string to N characters

• a complex type referencing another message:

  • an absolute reference of a message: e.g. some_package/SomeMessage
  • a relative reference of a message within the same package: e.g. OtherMessage

The array specifier can be one of the following:

• a static array is described by the suffix [N] where N is the fixed size of the array
• an unbounded dynamic array is described by the suffix []
• a bounded dynamic array is described by the suffix [<=N] where N is the maximum size of the array

Values

Depending on the type the following values are valid:

• bool:

  • true, 1
  • false, 0

• byte:

  • an opaque 8-bit quantity with a numerical value in the following interval [0, 255]

• char:

  • an unsigned integer value in the following interval [0, 255]

• float32 and float64:

  • a decimal number using a dot (.) as the separator between the integer-part and fractional-part.

• int8, int16, int32 and int64:

  • an integer value in the following interval [- 2 ^ (N - 1), 2 ^ (N - 1) - 1] where N is the number of bits behind int

• uint8, uint16, uint32 and uint64:

  • an unsigned integer value in the following interval [0, 2 ^ N - 1] where N is the number of bits behind uint

• string:

  • a string value which can optionally be quoted with either single quotes (') or double quotes (")

  • a double-quoted (") string (respectively single-quoted (')) should have any inner double quotes (respectively single quotes) escaped:

    • string my_string "I heard \"Hello\"" is valid
    • string my_string "I heard "Hello"" is not valid
    • string my_string "I heard 'Hello'" is valid
    • string my_string 'I heard \'Hello\'' is valid
    • string my_string 'I heard 'Hello'' is not valid
    • string my_string 'I heard "Hello"' is valid

Service file format

A service file contains two message definitions which are separated by a line which only contains three dashes:

---

Conversion to IDL

Code is generated for defined interfaces to be usable by different client libraries. Interfaces described using the above format are first converted to IDL. Code generation uses the generated file.

Mapping to IDL types