Comments are Posted anonymously I am using Services Module in Drupal 7. When I try to POST comments, these comments are Posted anonymously.

I am sending POST request to ?q=endpoint/comment.json

with this Body 

{   "nid":"401","subject":"Comment Subject sdfsdf ",   "comment_body":{
    "und":[{
      "value":"Comments body sdfsnd fsd fds "
    }]   } }


Comments are getting posted but anonymously. I want to comments to be posted by the name of logged in user.
The default for an unprovided uid property to a comment entity is 0, so with your current code it will always be owned by an anonymous user. If you want a specific user, you have to be specific in the request.

It can just go straight in the payload though:

{
  "nid": "401",
  "uid": "123",
  "subject": "Comment Subject sdfsdf ",
  "comment_body": {
    "und": [
      {
        "value": "Comments body sdfsnd fsd fds "
      }
    ]
  }
}


You have access to uid from the return value from the login, so it's easy to get hold of.