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ROSframework~5 mins

Service definition (.srv files) in ROS

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Introduction

A service definition (.srv file) in ROS lets two parts of a robot program talk by sending a request and getting a reply.

When you want one part of your robot to ask another part for information.
When you need to perform an action and wait for the result before continuing.
When you want a simple question-answer style communication between nodes.
When you want to organize your robot's tasks into clear request and response steps.
Syntax
ROS
RequestType1 request_field1
RequestType2 request_field2
---
ResponseType1 response_field1
ResponseType2 response_field2

The '---' line separates the request fields from the response fields.

Each field has a type and a name, like variables.

Examples
This service takes two integers as input and returns their sum.
ROS
int32 a
int32 b
---
int32 sum
This service sends a command string and gets back a success flag and a message.
ROS
string command
---
bool success
string message
Sample Program

This service definition lets a node send two 64-bit integers and get back their sum as a 64-bit integer.

ROS
# Example .srv file: AddTwoInts.srv
int64 a
int64 b
---
int64 sum
OutputSuccess
Important Notes

Always use the '---' line to separate request and response parts.

Field types can be standard ROS types like int32, float64, string, or custom message types.

After creating a .srv file, you must build your ROS package to generate code for using the service.

Summary

.srv files define how two nodes communicate with a request and a response.

They have two parts separated by '---': request fields and response fields.

They help organize robot programs into clear question-answer interactions.

Practice

(1/5)
1. What is the main purpose of a .srv file in ROS?
easy
A. To define the structure of a service with request and response parts
B. To list all the topics a node subscribes to
C. To specify the launch configuration for nodes
D. To store parameters for the robot

Solution

  1. Step 1: Understand what a .srv file represents

    A .srv file defines a service interface in ROS, which includes a request and a response part.

  2. Step 2: Match the purpose to the options

    Only To define the structure of a service with request and response parts correctly states that .srv files define the structure of a service with request and response parts.

  3. Final Answer:

    To define the structure of a service with request and response parts -> Option A

  4. Quick Check:

    .srv files define service structure = C [OK]
Hint: Remember .srv files split request and response with --- [OK]
Common Mistakes:
  • Confusing .srv files with launch files
  • Thinking .srv files list topics
  • Assuming .srv files store parameters
2. Which of the following is the correct syntax to separate request and response fields in a .srv file?
easy
A. Using a colon ':' between request and response
B. Using a comma ',' between request and response
C. Using a semicolon ';' between request and response
D. Using three dashes '---' between request and response

Solution

  1. Step 1: Recall the .srv file format

    The .srv file separates request and response fields with three dashes '---'.

  2. Step 2: Identify the correct separator

    Only Using three dashes '---' between request and response correctly uses '---' to separate the two parts.

  3. Final Answer:

    Using three dashes '---' between request and response -> Option D

  4. Quick Check:

    Request and response separated by --- = A [OK]
Hint: Look for '---' as the separator in .srv files [OK]
Common Mistakes:
  • Using commas or semicolons instead of ---
  • Confusing with YAML or JSON syntax
  • Missing the separator line entirely
3. Given this .srv file content:
int32 a
int32 b
---
int32 sum

What will the response contain when the service is called with request values a=3 and b=4 if the service adds them?
medium
A. sum = 12
B. sum = 34
C. sum = 7
D. sum = 0

Solution

  1. Step 1: Understand the request and response fields

    The request has two integers a and b; the response has one integer sum.

  2. Step 2: Apply the service logic

    The service adds a and b, so sum = 3 + 4 = 7.

  3. Final Answer:

    sum = 7 -> Option C

  4. Quick Check:

    3 + 4 = 7 [OK]
Hint: Add request fields to get response sum [OK]
Common Mistakes:
  • Concatenating numbers as strings
  • Multiplying instead of adding
  • Returning zero by mistake
4. Identify the error in this .srv file:
float64 x
float64 y
float64 z
---
float64 result
medium
A. No error, this is a valid .srv file
B. Response field should be on the same line as request fields
C. Missing separator '---' between request and response
D. Request fields must be integers, not float64

Solution

  1. Step 1: Check the separator

    The separator '---' is present between request and response fields.

  2. Step 2: Validate field types and format

    Using float64 for request and response fields is valid in .srv files.

  3. Final Answer:

    No error, this is a valid .srv file -> Option A

  4. Quick Check:

    Correct separator and valid types = A [OK]
Hint: Look for '---' and valid types to confirm syntax [OK]
Common Mistakes:
  • Thinking separator is missing when it is present
  • Believing response must be on same line
  • Assuming only integers allowed
5. You want to define a service that takes a string command and an integer timeout as request, and returns a boolean success and a string message as response. Which of these .srv files correctly defines this?
hard
A. string command int32 timeout --- bool success, string message
B. string command int32 timeout --- bool success string message
C. string command int32 timeout bool success string message
D. string command int32 timeout --- string message bool success

Solution

  1. Step 1: Check for correct separator and field order

    The request fields are before '---' and response fields after. string command int32 timeout --- bool success string message correctly uses '---' and lists request then response fields.

  2. Step 2: Validate field types and syntax

    string command int32 timeout --- bool success string message lists request fields (string, int32) and response fields (bool, string) correctly without syntax errors.

  3. Final Answer:

    string command int32 timeout --- bool success string message -> Option B

  4. Quick Check:

    Request before --- and response after = D [OK]
Hint: Request fields above ---; response fields below --- [OK]
Common Mistakes:
  • Placing response fields before separator
  • Missing separator line
  • Using commas inside .srv files
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